Atypical hemolytic uremic syndrome: clinical guidelines

List of Abbreviations

aHUS – atypical hemolytic-uremic syndrome

APAF – alternative complement pathway

ATIII – antithrombin III (name of medical service according to Order of the Russian Ministry of Health No. 804n dated October 13, 2017: "Determination of antithrombin III activity in blood")

APS – antiphospholipid syndrome

P-HUS – pregnancy-associated HUS

HUS – hemolytic-uremic syndrome

DIC – disseminated intravascular coagulation

LDH – lactate dehydrogenase (name of service according to Order of the Russian Ministry of Health No. 804n dated October 13, 2017: "Determination of lactate dehydrogenase activity in blood")

MAHA – microangiopathic hemolytic anemia

MAC (C5b-9) – membrane attack complex

AKI – acute kidney injury

PE – plasma exchange (name of service according to Order of the Russian Ministry of Health No. 804n dated October 13, 2017: "Plasma exchange")

FFP – fresh frozen plasma

SLE – systemic lupus erythematosus

TT – thrombin time (name of service according to Order of the Russian Ministry of Health No. 804n dated October 13, 2017: "Determination of thrombin time in blood")

TMA – thrombotic microangiopathy

TTP — thrombotic thrombocytopenic purpura

CKD – chronic kidney disease

CNS – central nervous system

CFH – complement factor H

CKD-EPI – formula for calculating glomerular filtration rate (CKD-Epidemiology Collaboration 2009)

STEC-HUS – typical hemolytic uremic syndrome

THBD – thrombomodulin

Terms and Definitions

Atypical hemolytic uremic syndrome (aHUS) is a systemic disease belonging to the group of thrombotic microangiopathies with a progressive course and poor prognosis, which is based on uncontrolled activation of the alternative complement pathway of hereditary or acquired origin, leading to generalized thrombosis in the microvasculature.

Complement-mediated thrombotic microangiopathy is a thrombotic microangiopathy caused by hyperactivation of the alternative complement pathway due to dysregulation caused by mutations in regulatory protein genes or autoantibodies against complement factor H, the main regulatory protein of the alternative complement pathway. Synonym: aHUS.

Complement-blocking therapy is therapy aimed at suppressing the activity of the complement system and thereby contributing to the cessation of microcirculatory thrombosis processes. In these clinical guidelines, it is carried out using two drugs – eculizumab** and ravulizumab.

Microangiopathic hemolysis is intravascular non-immune (Coombs-negative) mechanical hemolysis resulting from erythrocyte damage during circulation in narrowed microvascular vessels due to thrombi and endothelial edema.

Thrombotic microangiopathy is a clinical and morphological syndrome characterized by a specific type of microvascular injury, based on endothelial damage followed by thrombosis.

Thrombotic thrombocytopenic purpura is a systemic form of thrombotic microangiopathy caused by ultra-large multimers of von Willebrand factor due to acquired (autoantibodies) or hereditary (gene mutations) ADAMTS13 deficiency.

ADAMTS13 is a metalloprotease that cleaves ultra-large multimers of von Willebrand factor, serving as a diagnostic marker for thrombotic thrombocytopenic purpura.

STEC-HUS is an infection-mediated hemolytic uremic syndrome developing as a result of infection with Shiga toxin-producing bacteria: enterohemorrhagic or enteroaggregative E. coli or Shigella dysenteriae type I. Synonym: typical hemolytic uremic syndrome.

CFH (complement factor H) is the main regulatory factor of the alternative complement pathway, controlling its activity.

1. Brief information on the disease or condition

1.1 Definition of the disease or condition

Atypical hemolytic uremic syndrome (aHUS) is a systemic disease from the group of thrombotic microangiopathies (TMAs) with a progressive course and poor prognosis, which is based on uncontrolled activation of the alternative complement pathway (ACP) of hereditary or acquired nature, leading to generalized thrombosis in the vessels of the microvasculature [1,2].

TMA is a heterogeneous group of syndromes/diseases sharing a similar clinical and morphological phenotype [1-4]. Clinically, TMA manifests as consumptive thrombocytopenia, microangiopathic hemolytic anemia (mechanical hemolysis), fever, and ischemic damage to various organs, mainly the kidneys and the central nervous system (CNS). The histological picture of acute TMA is represented by endothelial cell swelling with their detachment from the basement membrane, expansion of the subendothelial space with accumulation of amorphous membrane-like material in it, and the formation of thrombi containing platelets and fibrin, often causing complete occlusion of the vessel lumen.

1.2 Etiology and pathogenesis of the disease or condition (group of diseases or conditions)

aHUS is caused by genetic disorders or, less commonly, autoimmune reactions leading to pathology of the complement system, which gave rise to the term "complement-mediated TMA," which is a synonym for the term "aHUS." In most cases, aHUS is associated with genetically determined defects in ACP regulation, resulting in its chronic uncontrolled activation [1,5]. In addition, in almost 20% of patients, predominantly children, autoantibodies to complement factor H (CFH) – the main regulatory protein of the ACP – are detected, the presence of which leads to the same consequences as its mutations. "Antibody-mediated" aHUS is prone to frequent relapses [1,6,7]. The following defects leading to excessive activation of the ACP can be identified in patients with aHUS [6-9]:

• Mutations in the genes of complement regulatory proteins that disrupt their functions (loss-of-function): CFH (factor H), CFI (factor I), MCP (membrane cofactor protein), THBD (thrombomodulin). Among all mutations associated with the development of aHUS, CFH gene mutations are detected most frequently (about 30% of cases).

• Mutations in the genes of factor B (CFB) and complement component C3 that provide a significant increase in their activity (gain-of-function).

• Antibodies to CFH and associated deletions in the genes of CFH-related proteins – CFHR1, CFHR3, or CFHR4 [8,10].

In addition to genetic or acquired defects in the complement system, mutations in genes related to the blood coagulation system – THBD, diacylglycerol kinase epsilon (DGKE), plasminogen (PLG) – which are included in the gene panel tested when aHUS is suspected, can also lead to the development of aHUS [127].

Mutations can be identified in only 50-70% of patients with aHUS [7], and the absence of a family history does not exclude the genetic nature of the disease (gene penetrance is about 50%).

Mutations present in patients with aHUS lead to impaired protection of endothelial cells from complement activation due to deficiency or, more often, functional impairments of regulatory proteins. Normally, ACP activity regulators and factors enhancing its activity (CFB and C3) are in a strictly balanced state. A genetic defect in regulatory proteins – CFH, CFI, MCP, and THBD – causes a decrease in their functional activity. Conversely, mutations in the CFB and C3 genes enhance their activity. Complement dysregulation leads to continuous cleavage of component C5 into the potent proinflammatory anaphylatoxin C5a and C5b, which initiates the formation of the membrane attack complex C5b-9, possessing prothrombotic and cytolytic action. Thus, in patients with aHUS, uncontrolled complement activation is caused either by the reduced function of factors regulating ACP action or by increased activity of factors activating it. In both cases, an imbalance develops between control and activation factors towards the predominance of the latter [8,11]. The result of excessive ACP activation on the surface of endothelial cells is the increasing formation of the membrane attack complex, causing their damage with exposure of the subendothelial matrix, transformation of the antithrombotic phenotype into a prothrombotic and proinflammatory one, and subsequent thrombus formation [1,2,11]. An additional contribution to thrombosis in patients with CFH mutations can also be made by complement activation on the platelet surface, leading to enhanced functional activity. The predominant renal lesion in aHUS is apparently associated with the special sensitivity of fenestrated glomerular endothelium to damage caused by impaired complement regulation [1,12,13].

The development of aHUS fully corresponds to the two-hit theory. Genetic complement anomalies should be considered not as the cause, but as a factor predisposing to the development of TMA. It has been established that complement gene mutations or autoantibodies against CFH, which were previously considered the main cause of aHUS, only predispose to the development of TMA – the "first hit". To realize this predisposition, additional factors ("second hit") are required, which can influence the development or progression of aHUS [4,14]. Such factors causing additional complement activation in predisposed individuals and therefore called complement-activating conditions are infections, primarily of the respiratory and gastrointestinal tracts (30% of cases), pregnancy (7-17%), solid organ and bone marrow transplantation (in 5%), autoimmune diseases, surgical interventions, etc. However, in almost half of patients with aHUS, trigger factors are not obvious and cannot be identified [2,15].

1.3 Epidemiology of the Disease or Condition

aHUS is a rare (orphan) disease that can occur at any age, though it most frequently affects children and young adults. According to various sources, the global incidence is 0.23–1.9 new cases per 1 million population overall, and the prevalence is approximately 4.9 per 1 million, ranging from 2.2 to 9.4 per 1 million among children and adolescents [16, 17]. The incidence and prevalence in our country are unknown. aHUS develops with equal frequency in men and women. When manifesting at an older age, the disease affects women slightly more often.

1.4 Features of Coding the Disease or Condition

N08.2* – Glomerular disorders in diseases classified elsewhere

D59.3 – Haemolytic-uraemic syndrome

M31.1 – Thrombotic microangiopathy

N17.0 – Acute kidney failure with tubular necrosis

N17.1 – Acute kidney failure with acute cortical necrosis

1.5 Classification of the disease or condition

Historically, TMAs were classified as primary and secondary. Primary TMAs included thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) [18]. A frequent cause of the latter is Shiga toxin-producing Escherichia coli (STEC) infection, and this variant of the disease is defined as STEC-HUS. TTP is caused by acquired or genetic deficiency in the activity of the ADAMTS-13 metalloprotease (less than 5%), which cleaves ultra-large von Willebrand factor multimers that induce thrombus formation in the microvasculature under conditions of insufficient enzyme activity [19,20]. STEC-HUS is the most frequent form of HUS, accounting for nearly 90% of its structure. About 10% of HUS cases unrelated to STEC infection are classified as aHUS. In the 1980s, the association of aHUS with hereditary or acquired abnormalities of the alternative complement pathway (ACP) was established, which were detected in 50–60% of cases, providing the basis to classify aHUS as complement-mediated. Other TMAs associated with pregnancy, autoimmune and oncological diseases, medications, malignant arterial hypertension, and solid organ and bone marrow transplantation formed the group of secondary TMAs [18]. Currently, attempts are being made to update this classification. In particular, a KDIGO conference decision proposed replacing the term "aHUS" with "primary aHUS," designating cases with clearly established genetic defects of the ACP or anti-CFH antibodies, and referring to secondary TMAs according to their underlying cause, such as "drug-induced aHUS" or "pregnancy-associated aHUS" [21,22]. Another proposal, which appears more substantiated, is the division of aHUS into complement-mediated HUS (proper aHUS or primary aHUS) and secondary HUS, the latter referring to situations previously designated as secondary TMAs [23,24]. In such cases, secondary complement activation is postulated as a result of endothelial damage caused by various factors and mechanisms in a patient without genetic abnormalities of the complement system [25]. A new TMA classification is also proposed, according to which TMAs should be divided into hereditary, acquired, infection-associated, and secondary. According to this classification, aHUS caused by anti-CFH antibodies belongs to the group of primary acquired TMAs, whereas aHUS associated with genetic defects of the ACP is included in the group of primary hereditary TMAs [25,26]. Thus, today there are no universally accepted classifications of TMA and aHUS. In this regard, these guidelines use a classification according to which aHUS (complement-mediated TMA) is classified as a primary TMA, while all other TMAs listed above are considered secondary. aHUS is also subdivided into familial and sporadic forms. In the structure of aHUS, familial aHUS (diagnosed in at least two family members) accounts for only 10–20% according to various data, whereas sporadic aHUS, in which there is no family history, occurs in 80–90% of patients with this pathology. It should be remembered that the absence of the disease in relatives does not exclude its hereditary nature [1,16].

1.6 Clinical presentation of the disease or condition

The clinical presentation of aHUS is characterized by significant polymorphism. The main manifestations of the disease are thrombocytopenia, microangiopathic hemolytic anemia (MAHA), and acute kidney injury (AKI), which constitute the classic TMA triad [1, 2, 15]. In nearly half of patients with aHUS, the disease develops after exposure to triggers, the most common of which are infections: primarily respiratory tract infections (in 18% of patients) and gastrointestinal infections (23–30% of cases). aHUS frequently develops or relapses following viral infections such as H1N1 influenza and chickenpox. An important factor contributing to the development or relapse of aHUS is pregnancy, which precedes the disease in 7–17% of patients, and organ transplantation in 5% of patients [1, 16].

In most cases, aHUS begins suddenly. The disease often manifests with non-specific symptoms such as weakness, fatigue, general malaise, and a flu-like syndrome. In adult patients, a blurred onset with slow progression is noted in 20% of cases. In such instances, mild anemia with minimal or absent thrombocytopenia and preserved renal function may persist for a long time, up to several months. The development of "incomplete" TMA without thrombocytopenia or, more rarely, without MAHA is also possible. In some cases, the development of only renal signs of TMA in the absence of hematological disorders has been described [27].

In most patients, the clinical presentation includes thrombocytopenia, non-immune MAHA, and AKI [1, 2, 16]:

• Thrombocytopenia (less than 150,000/mm³ or a rapid platelet count drop of more than 25% from baseline) develops as a result of platelet consumption in microcirculatory thrombus formation processes.

• MAHA (hemoglobin generally less than 100 g/L) is the result of mechanical hemolysis due to damage to red blood cell membranes upon contact with thrombi and the effects of shear stress in narrowed small-caliber vessels. The presence of hemolysis is confirmed by low haptoglobin and high lactate dehydrogenase (LDH) levels in the blood, and the microangiopathic nature of hemolysis is indicated by the presence of schistocytes in the peripheral blood smear and a negative Coombs test.

• Kidney involvement in most cases manifests as AKI with or without oliguria/anuria. In patients with preserved diuresis, proteinuria of varying severity is observed. With the gradual development of the disease, nephrotic syndrome may develop. Hematuria may also appear. AKI is not always the first manifestation of the disease: about 17% of patients exhibit only a moderate urinary syndrome without impaired renal function. In rare cases, aHUS may debut with isolated proteinuria and a moderate, non-progressing increase in blood creatinine. Most adult patients with aHUS and AKI require hemodialysis at the time of hospitalization. End-stage renal disease in aHUS develops regardless of the nature of the disease manifestation, frequently as an outcome of the first TMA episode [1, 2, 16].

• Most patients with aHUS develop arterial hypertension due to volume overload in cases of oliguria/anuria and/or hyperreninemia due to renal tissue ischemia caused by TMA.

• aHUS is a systemic TMA characterized by damage not only to the kidneys but also to other vital organs, including the brain, heart, lungs, digestive tract, and visual system [4, 28–31]. Extrerenal manifestations of the disease are detected in at least 20–30% of patients, with two-thirds of them having more than one organ involvement besides the kidneys.

• Most patients with aHUS have massive peripheral edema up to anasarca and cavity edema (hydrothorax, hydropericardium, ascites), the main cause of which is the action of anaphylatoxins C3a and C5a generated during complement activation, which induce the release of histamine that enhances vascular permeability [32].

• CNS involvement of varying severity (drowsiness, irritability, seizures, visual disturbances, hemiparesis or hemiplegia, stupor, coma) develops in nearly half of patients with aHUS. In some cases, brain edema due to increased vascular permeability may develop [33].

• Myocardial TMA develops in 40% of patients with aHUS, manifesting as symptoms of dilated cardiomyopathy with progressively worsening or acute heart failure, acute myocardial infarction, and severe rhythm and conduction disorders that can lead to sudden death [30, 31].

• Pulmonary TMA can cause hemorrhagic alveolitis or acute respiratory distress syndrome in adults. Respiratory failure resulting from these causes in some cases requires mechanical ventilation. Patients with aHUS frequently develop bilateral lung infiltrates, which complicates diagnosis verification and necessitates differential diagnosis with vasculitis and infectious pathology [28].

• Gastrointestinal tract involvement occurs in approximately 30% of patients with aHUS. Ischemic bowel disease is most frequently observed, often manifesting as diarrhea (including bloody diarrhea), nausea, and vomiting, although abdominal pain syndrome may also develop. The development of acute pancreatitis with characteristic clinical signs up to pancreatic necrosis is frequently noted. The acute onset of diabetes mellitus has been described. Ischemic necrosis of the liver is observed less frequently [28, 29, 34].

• Skin involvement, presented by the development of extensive areas of necrosis, is rare in adult patients. Digital ischemic gangrene leading to the amputation of fingers and toes has also been described [28].

• Approximately 5% of patients, most commonly in obstetric aHUS, develop multi-organ failure caused by generalized TMA with microcirculatory vessel involvement in vital organs [1].

2. Diagnostics, medical indications and contraindications for diagnostic methods

To date, there are no specific diagnostic markers for aHUS. The diagnosis of aHUS is a diagnosis of exclusion. It is established based on the characteristic clinical picture after ruling out other forms of TMA, both primary and secondary. Because all TMAs, regardless of their pathogenesis, share similar clinical and laboratory manifestations and a common histological picture, differential diagnosis between the main forms of primary TMA—TTP, STEC-HUS, and aHUS—is extremely important. In adult patients with TMA, it is also necessary to exclude a significant number of diseases and conditions that can cause secondary TMAs, primarily those associated with pregnancy and childbirth, systemic diseases (systemic lupus erythematosus (SLE), antiphospholipid syndrome (APS), scleroderma), malignancies, HIV infection, sepsis, malignant arterial hypertension, drug therapy, and disseminated intravascular coagulation (DIC) syndrome. Thus, the diagnosis of aHUS can be represented as a two-level process, with diagnostic tests at both levels performed simultaneously rather than sequentially. The first level is intended to verify the TMA syndrome, and the second to verify the nosological diagnosis, for which differential diagnosis between primary and secondary TMAs and among primary TMAs (TTP, STEC-HUS, and aHUS) is necessary [1, 2, 15, 16, 35, 36].

Diagnostic criteria for aHUS

1. Presence of acute TMA symptom complex: thrombocytopenia, microangiopathic Coombs-negative hemolytic anemia, acute kidney injury

2. Exclusion of primary TMAs: TTP and STEC-HUS

3. Exclusion of secondary TMAs

Algorithms for differential diagnosis pathways for aHUS when TMA is suspected are presented in Appendix B, items 1 (TMA) and 2 (aHUS).

2.1 Complaints and Anamnesis

Clinical data obtained from the analysis of complaints and medical history that indicate the likely presence of aHUS are provided in section 1.6.

• We recommend paying attention to the following complaints when examining patients with suspected aHUS for the diagnosis of renal and extrarenal manifestations of the disease: peripheral edema, decreased urine output, changes in urine color; shortness of breath; hemorrhagic skin rashes; weakness, lethargy, fatigue, loss of appetite, dizziness; headaches, visual impairments of varying severity (up to complete blindness); nausea, vomiting, abdominal pain, frequent loose stools [1,2].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: the complaints of patients with aHUS are non-specific. At onset, the disease may present with mild or subclinical manifestations [2].

2.2 Physical Examination

Physical examination findings indicating the probable presence of aHUS are given in section 1.6.

• We recommend assessing the presence of the following during physical examination in patients with suspected aHUS to diagnose renal and extrarenal manifestations of the disease: fever; skin changes (pallor, jaundice, hemorrhages); peripheral edema; moist rales in the lungs; cardiomegaly; elevated blood pressure; tachycardia or cardiac arrhythmias [1,2,3].

GRC CLEL 4Grade of Recommendation C (Level of Evidence – 4)

Comments: The main purpose of the physical examination is to detect renal and extrarenal manifestations of the disease [1-3].

2.3 Laboratory Diagnostics

No specific diagnostic markers for aHUS have been developed yet; therefore, the latter is a diagnosis of exclusion and can be established only after a thorough patient evaluation that rules out diagnoses of any other primary (STEC-HUS, TTP) and secondary TMAs (associated with infection, medications, pregnancy and childbirth, autoimmune diseases, tumors, malignant arterial hypertension, bone marrow and solid organ transplantation) [4,16,25,35,37-49].

• We recommend performing a complete blood count (CBC) with differential, including platelet count and schistocyte evaluation, a general biochemical blood test with serum creatinine, blood LDH activity, free (unconjugated) bilirubin levels, indirect antiglobulin test (Coombs test), direct antiglobulin test (direct Coombs test), and blood haptoglobin level in all patients with suspected TMA for its diagnosis [1,14-16,35,37-39,50].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: TMA diagnosis is based on the presence of thrombocytopenia and MAHA combined with signs of renal and/or extrarenal involvement. Thrombocytopenia is defined as a platelet count <150,000/mm³. If the platelet count exceeds this value, platelet consumption can be inferred from a decrease in platelet count >25% from the baseline level (if known). In rare cases, MAHA may develop without thrombocytopenia. The presence of MAHA is established in patients with anemia based on elevated LDH levels and/or schistocytosis (schistocyte count in the peripheral blood smear greater than 1%) [1,14-16,35,37-39].

When TMA is suspected, all three specified hemolysis markers (LDH, schistocytes, haptoglobin) must be determined, since in the absence of changes in one of them and failure to perform tests for the other two, a diagnosis of TMA cannot be established (false-negative result!) [37]. All patients with suspected TMA must also undergo an indirect antiglobulin test (Coombs test) to rule out an immune nature of hemolysis. In patients with MAHA and thrombocytopenia, the presence of AKI or other signs of renal involvement, either isolated or combined with symptoms of other organ damage, provides the basis for diagnosing TMA.

• We recommend diagnosing aHUS in patients with a TMA symptom complex only after ruling out STEC-HUS, TTP, and secondary TMAs [1,16].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

• We recommend that all patients with suspected aHUS and signs of gastrointestinal tract involvement, especially those with diarrhea, undergo the necessary laboratory tests to rule out STEC-HUS (see comments) upon admission to the hospital before the start of antibacterial therapy [2,39,51].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: since about 30% of patients with aHUS present with gastrointestinal symptoms such as nausea, vomiting, diarrhea, and abdominal pain at disease onset, screening for STEC-HUS is necessary. The diagnosis of STEC-HUS is based on the identification of the causative agent (shiga toxin-producing bacteria) via microbiological (culture) stool examinations: Determination of E. coli O157:H7 antigen in feces, determination of enterohemorrhagic E. coli (EHEC) toxins in stool samples) or molecular genetic methods (polymerase chain reaction in stool or rectal swab). Determination of antibodies to the pathogen in blood serum by enzyme-linked immunosorbent assay (determination of antibodies to Shiga toxin in blood serum) can detect a recently resolved infection [2,35,39,51,128].

• We recommend determining plasma ADAMTS-13 metalloproteinase activity in all patients with suspected aHUS to rule out TTP [2,37,39,52,53,54].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: ADAMTS-13 metalloproteinase plays a key role in the pathogenesis of TTP; therefore, determining its activity allows for a clear differentiation between TTP and aHUS. Normal ADAMTS-13 activity ranges from 50–150% depending on the assay method. An ADAMTS-13 activity deficiency with a reduction below 10% is a diagnostic marker for TTP. In patients with enzyme activity below 10%, plasma antibodies to ADAMTS-13 metalloproteinase must be determined. If present, idiopathic (autoimmune) TTP is diagnosed; if absent, congenital TTP is diagnosed. In any other TMA, including aHUS, ADAMTS-13 activity may be reduced, but it always exceeds 10% [2,35,37,39,52,53,54]. Once secondary TMAs are ruled out, ADAMTS-13 values above 10% allow for the diagnosis of aHUS. Plasma ADAMTS-13 metalloproteinase activity should be determined prior to initiating plasma therapy.

• For the differential diagnosis of aHUS and TTP when rapid assay of plasma ADAMTS-13 metalloprotease activity is unavailable in TMA patients with life-threatening clinical manifestations and/or a high risk of severe renal and/or extrarenal complications requiring prompt initiation of therapy, we recommend considering the diagnosis of TTP unlikely if the blood creatinine level is >150 µmol/L (1.7 mg/dL) combined with a blood platelet count >30,000/µL [52-57].

GRC CLEL 4Grade of Recommendation C (Level of Evidence – 4)

• We recommend that all patients with a suspected diagnosis of aHUS undergo an evaluation aimed at ruling out secondary TMAs most commonly encountered in clinical practice (Table 1) for differential diagnosis purposes [16,38-49].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: The development of TMA during pregnancy and postpartum requires immediate verification of the diagnosis, which determines the management strategy and prognosis for both mother and child. "Obstetric TMA" is represented in the majority of cases by specific gestational pathologies—preeclampsia and HELLP syndrome—which should be ruled out first. Systemic diseases such as SLE, scleroderma, and APS must be ruled out in patients with the TMA symptom complex. The latter can develop both within the context of SLE (secondary APS) and as an independent disease (primary APS). The combination of clinical and laboratory manifestations of TMA with the presence of antiphospholipid antibodies unquestionably points toward the diagnosis of "catastrophic APS", regardless of whether the patient has clinical and immunological signs of SLE. Therefore, serological markers for both SLE and APS must be determined in patients with signs of TMA, since the detected spectrum of markers determines the therapeutic strategy. In addition to systemic diseases, HIV infection must be ruled out, as the incidence of TMA is higher among HIV-infected patients than in the general population and increases as the disease progresses. Other types of pathologies that should be ruled out before diagnosing aHUS are listed in Table 1.

Table 1. Most frequent secondary TMAs [38-49]

• We recommend performing a panel of tests required to diagnose disseminated intravascular coagulation (DIC) in all patients with suspected aHUS, especially in cases of multiple organ dysfunction syndrome, to rule out DIC [2,39,41,58,59].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: DIC is a pathological condition based on microvascular thrombosis caused by the activation of the plasma coagulation cascade followed by the secondary activation of the fibrinolysis system. This is its main difference from other TMAs, primarily aHUS, which is characterized by primary endothelial injury leading to platelet activation and consumption [58]. The diagnostic panel for DIC includes: activated partial thromboplastin time (APTT), prothrombin time (thromboplastin time) in blood or plasma, thrombin time (TT) in blood, blood fibrinogen level, blood antithrombin III (ATIII) activity, and blood D-dimer concentration or soluble fibrin monomer complexes in blood. In most cases of aHUS, coagulation test results (screening hemostasis assays) remain unchanged [2,39,41,58,59]. However, in some patients, aHUS can be complicated by DIC, which is manifested by prolonged clotting times, decreased fibrinogen and ATIII levels, and elevated D-dimer levels. The combination of aHUS and DIC is possible due to the close interaction between the hemostatic and complement systems, which are capable of mutually activating each other [60].

• To detect pathological complement system activation, we recommend, at a minimum, testing for serum complement component C3 and serum complement component C4 levels in all patients with a suspected or confirmed diagnosis of aHUS [1,38,39,41,42].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: genetic defects in the alternative complement pathway (ACP) predispose to the development of aHUS; therefore, measuring blood levels of complement components and factors regulating ACP activity is an important additional diagnostic method in aHUS [1,38,39,41,42]. A normal C3 level does not rule out a diagnosis of aHUS, but a decrease in this complement component can serve as an additional argument in favor of this diagnosis.

In addition to complement C3 and C4 levels, it is advisable to measure total hemolytic complement activity (CH50, AP50), as well as complement regulatory factors CFH, CFI, CFB, if these tests are available. In routine clinical practice, testing blood levels of complement C3 and C4 is currently the main available option. A decrease in C3 with normal C4 levels, indicating ACP activation, is observed in no more than 50% of patients with aHUS [61-67].

For the differential diagnosis between aHUS and TTP in patients with an acute TMA episode, it is advisable to measure blood levels of complement fragment C5a and the soluble membrane attack complex (MAC, C5b-9), which reflect the activation of the alternative and terminal complement pathways. These factors have been found to be elevated in acute complement-mediated TMA, providing a rationale to consider them as biological markers of aHUS [35,64,65]. However, further studies are required to confirm their diagnostic value.

• We recommend measuring anti-factor H (CFH) antibodies in all patients with suspected aHUS to rule out or confirm the autoimmune (antibody-mediated) variant of aHUS, drawing blood for this test prior to initiating plasma therapy [1,39,42,67].

GCR ALOE 2Grade of Recommendation A (Level of Evidence – 2)

Comments: autoantibodies against CFH are detected in 5–20% of patients with aHUS [7,61,66]. Although most commonly identified in children aged 5 to 10 years, they are also frequently detected in adults [5]. The action of anti-CFH antibodies, similar to CFH mutations, leads to excessive ACP activation. The presence of such antibodies is often associated with a deficiency of CFH-related proteins 1 and 3 (CFHR1 and CFHR3) resulting from mutations in the respective genes. It has been established that 90% of patients with anti-CFH antibodies have a complete absence of CFHR1 and CFHR3 due to homozygous deletion in the genes encoding these proteins. The relatively high frequency of detection of anti-CFH antibodies provides a rationale for distinguishing an autoimmune type of aHUS. Timely detection of anti-CFH antibodies is essential for selecting treatment tactics.

• We recommend using KDIGO 2012 criteria for the diagnosis of AKI in patients with suspected aHUS [39,68].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

2.4 Instrumental Diagnostics

Specific instrumental diagnostics for aHUS are not provided.

• We recommend additional imaging workup based on indications determined by clinical signs of organ involvement in patients with extrarenal manifestations of the disease [2,3].

GDR CLE 5Grade of recommendation C (level of evidence – 5)

2.5 Other Procedures

• We recommend performing an ultrasound-guided renal biopsy with pathomorphological examination of the biopsy (surgical) specimen in patients with suspected or known aHUS and signs of renal dysfunction to confirm the diagnosis of TMA in ambiguous and unclear cases [26,45,49,69,70].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: renal biopsy is not mandatory for the diagnosis of aHUS, as the disease lacks specific morphological features, and the invasive procedure carries an increased risk of bleeding even in patients with mild thrombocytopenia [35,69]. On the other hand, the invasive procedure can trigger additional complement activation, which in patients with aHUS will contribute to the generalization of microangiopathic thrombosis with the development of extrarenal involvement and, ultimately, multiple organ dysfunction syndrome. The decision to perform a renal biopsy and the timing thereof for each individual patient is strictly individual. Renal biopsy should not be performed in patients with familial aHUS and relapses of acute TMA, as the diagnosis is obvious in these cases [69]. However, it can help verify the diagnosis in the following situations [26,45,49,69,70]:

− Need to determine the cause of AKI when the diagnosis of aHUS is uncertain;

− Absence of the full laboratory symptom complex of TMA (most commonly thrombocytopenia);

− Massive proteinuria in patients with hematological manifestations of TMA;

− Suspicion of secondary forms of TMA;

− Suspected chronic TMA.

• We recommend genetic testing in patients with a clinically established diagnosis of aHUS to determine the genetic variant of the disease, prognosis, and long-term management strategy [42,59,69,129-137].

GCR BLEL 2Grade of Recommendations B (Level of Evidence – 2)

Comments: Molecular genetic testing should include mutation analysis of genes for factor H (CFH), I (CFI), B (CFB), C3, thrombomodulin (THBD), membrane cofactor protein (MCP), CFH-related structural proteins (CFHR1-R5), as well as non-complement genes related to the hemostatic system—diacylglycerol kinase epsilon (DGKE), plasminogen (PLG), and the ADAMTS13 gene.

Genetic testing is not required for the initial diagnosis of aHUS and plays no role in deciding patient management tactics during an acute episode; however, it is essential for determining prognosis, treatment duration, and treatment cessation. The rationale for omitting genetic screening in the initial diagnosis of aHUS is that mutations in complement regulatory protein genes are detected in 60–70% of patients with hereditary aHUS and in 30% of those with the sporadic form [1,15,35,62,63]. Thus, a negative genetic screening result in a patient with a definitive TMA symptom complex does not rule out aHUS. Performing genetic testing takes at least two months, and the prognosis is similar in patients with both identified and unidentified mutations [61,62]. Therefore, genetic testing is not required to diagnose aHUS or prescribe treatment. This testing is of particular importance when planning a kidney transplantation, since the post-transplant risk of aHUS recurrence—which determines the prognosis—depends on the type of mutation (Table 2) [61,62,71,72]. In this case, in addition to guiding post-operative management, it can help select a donor, especially when living-related transplantation is being considered [35].

Table 2. Prognosis in aHUS according to the type of complement gene mutations

Note: ESRD – end-stage renal disease.

It should be noted that genetic screening is necessary to confirm or rule out the association of the disease with complement pathway pathology, which is important when end-stage renal disease of unclear etiology develops, particularly in young patients, as well as for determining prognosis, recurrence risk, and the progression of chronic kidney disease (CKD) (Table 2) [35]. Currently, genetic testing is performed using next-generation sequencing [35,41,42,59,62,63,69].

3. Treatment, including drug and non-drug therapies, diet therapy, and pain management

The goals of aHUS therapy, in addition to ensuring better patient survival, are to inhibit uncontrolled complement activation, halt microcirculatory thrombosis, relieve the clinical and laboratory manifestations of TMA, preserve and improve the function of affected organs (including preventing the development of end-stage renal disease, eliminating the need for dialysis, and preventing damage to internal organs other than the kidneys), and improve patients' quality of life. The patient management algorithm for aHUS is presented in Appendix B, Section 3.

3.1 Plasma therapy

• We recommend plasma therapy in the form of plasma exchange (PE) as first-line therapy for all patients with a suspected diagnosis of aHUS while awaiting the results of workup for STEC-HUS and TTP, in order to reduce the severity of life-threatening clinical manifestations of the disease [39,41,42,59,69,73–76].

GCR BLoE 1Grade of Recommendation B (Level of Evidence – 1)

Comments: Plasma therapy should be initiated before obtaining the results of the workup for STEC-HUS and TTP (determination of plasma ADAMTS-13 metalloprotease activity, hereinafter referred to as ADAMTS13), and if these diagnoses are ruled out, complement-blocking therapy should be considered.

• For patients with a suspected diagnosis of aHUS, we recommend administering plasma therapy in the form of fresh frozen plasma (FFP) transfusions only if immediate initiation of PE is impossible or unavailable [39,41,42,59,69,73,74,75,77].

GCR BLoE 1Grade of Recommendation B (Level of Evidence – 1)

Comments: in patients with suspected aHUS during an acute TMA episode, empirical plasma therapy has remained the first-line treatment for several decades. It should be started as soon as possible after establishing the diagnosis of TMA and continued until the diagnosis is confirmed, i.e., until STEC-HUS and TTP are ruled out. Two regimens of plasma therapy are used: PE, which is preferred, and FFP transfusions if PE is unavailable [39,41,42,69,73,74,77]. FFP transfusions introduce functionally active complement regulatory proteins into the bloodstream, compensating for the deficiency of the patients' own regulators (factors CFH and CFI), as well as the natural plasma component ADAMTS13, which limits microvascular thrombosis by cleaving von Willebrand factor ultra-large multimers [78]. PE additionally eliminates modified endogenous soluble complement inhibitors and, in cases of antibody-mediated aHUS, circulating anti-CFH antibodies. The PE regimen also helps minimize the risk of volume overload, which is particularly important in patients with oliguria, CNS involvement, and cardiac injury [69]. The use of plasma therapy has reduced mortality during an acute TMA episode in aHUS by 50–60%; however, it does not prevent disease relapses or the progression of CKD to end-stage renal disease [61,79]. The response to plasma therapy varies widely and depends on which complement factor harbors the mutations [42]. Most frequently—in 55–80% of cases—a complete response is observed in patients with mutations in the C3 and thrombomodulin (THBD) genes [42,61]. However, although restoration of hematological parameters, primarily the platelet count, is observed in the majority of cases, fewer than 50% of patients demonstrate a complete hematological and renal response, and among patients with CFH and CFI mutations, the complete response rate does not exceed 15–25% [61,69].

The advent of effective pathogenetic drugs for aHUS—eculizumab** and ravulizumab—has reduced the role of PE in patient management [42,80]. At the same time, PE retains its role as a first-line treatment for patients with aHUS while awaiting conversion to complement-blocking therapy.

• For patients with a first episode of aHUS or a relapse who are not receiving eculizumab** or ravulizumab, we recommend PE therapy with an extraction and FFP replacement volume equal to 1.5 circulating plasma volumes (approximately 60–75 mL plasma/kg) to reduce the severity of clinical disease manifestations [39,41,42,59,69,73,74,91,138].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

• We recommend administering FFP transfusions at a rate of 25–30 mL/kg/day if immediate PE is not feasible, in order to reduce the severity of clinical manifestations of the disease [39,41,42,59,69,73,74].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: the efficacy of plasma therapy in aHUS can only be judged by strictly following the treatment protocol, which initially prescribes PE sessions for 5 consecutive days [42,59,69,81], after which a decision on further management is made based on the presence or absence of a response to plasma therapy. If a response to therapy is observed, defined as an increase in the platelet count and a decrease in LDH, plasma therapy should be continued in the previous regimen with 5 PE sessions for the next 2 weeks, followed by 3 sessions per week for another 2 weeks [15,59]. The criteria for the efficacy of plasma therapy are normalization of the platelet count and cessation of hemolysis, as evidenced by LDH normalization. The duration of FFP treatment is not defined, but the indication for discontinuing plasma therapy is a normal platelet count and LDH level for 2 consecutive days [80]. It should be noted that in some cases, LDH levels remain elevated even after the cessation of hemolysis, since the main source of LDH in the body is not only erythrocytes, but also the tissues of internal organs, the ischemia of which during TMA development also contributes to the elevation of LDH [82]. Therefore, it is also proposed to use a decrease in blood creatinine by at least 25% from baseline after 5 PE sessions as a criterion for the efficacy of plasma therapy [81,83].

• We recommend discontinuing FFP treatment and initiating pathogenetic complement-blocking therapy with eculizumab** or ravulizumab in patients with confirmed aHUS who have an inadequate response to plasma therapy, or in patients exhibiting plasma dependence, to achieve remission and improve prognosis [39,41,42,59,69,81,84,91,99,138].

GCR ALOE 2Grade of Recommendation A (Level of Evidence – 2)

Comments: the absence of a trend toward an increase in platelet count, a decrease in LDH, and/or a reduction in blood creatinine during FFP treatment (after several full-volume PE procedures) should be considered as resistance to plasma therapy, which is an absolute indication to discontinue it and prescribe eculizumab** or ravulizumab [42,59]. Plasma dependence is defined as the normalization of platelet counts during plasma therapy followed by their re-drop and an increase in creatinine levels during interruptions in PE sessions, which necessitates the resumption of procedures [42].

• For patients with autoimmune aHUS identified by the presence of anti-factor H antibodies (anti-CFH antibodies), we recommend plasma exchanges in combination with glucocorticoid induction therapy and/or immunosuppressants to reduce the risk of adverse clinical events [42,59,69,85,86].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: treatment of autoimmune aHUS requires a combination of plasmatherapy with glucocorticoids and antineoplastic agents [35,42,59,66,82,86]. The addition of glucocorticoids to plasmatherapy, either as monotherapy or in combination with #rituximab** (intravenously at a dose of 375 mg/m² once weekly, 4 administrations [125]) or #cyclophosphamide** (intravenously, 6 administrations of 500 mg over 3 months [126]), can reduce the risk of death or reaching stage 4 CKD within the first year from 59% to 24%. Subsequent maintenance immunosuppressive therapy further reduced the 1-year risk of relapse of this form of aHUS from 21% to 8% [86].

3.2 Pathogenetic Complement-Blocking Therapy

• For patients with confirmed aHUS, we recommend prescribing eculizumab** or ravulizumab in accordance with the drug's prescribing information to reduce the severity of clinical manifestations, the risk of death, and to improve renal outcomes [20,39,41,42,69,78,84,87-91,138].

GCR ALOE 2Grade of Recommendation A (Level of Evidence – 2)

Comments: the therapeutic effect of eculizumab** is based on the cessation of microcirculatory thrombosis due to the blockade of uncontrolled AP activation. Treatment leads to a decrease in blood levels of inflammatory markers, complement activation, endothelial damage and thrombosis, as well as markers of renal injury [87]. Studies have demonstrated high efficacy of eculizumab** regarding both hematological (normalization of platelet count and cessation of microangiopathic hemolysis) and renal manifestations of aHUS [88,89]. Patients treated with eculizumab** showed recovery or improvement in renal function and became independent of hemodialysis, with renal function parameters improving as the duration of therapy increased: for instance, in the study by C.M. Legendre et al., an increase in the estimated glomerular filtration rate of ≥15 mL/min/1.73 m² at 26 weeks of therapy was observed in 5% of patients, at 1 year in 15%, and at 2 years in 40%. A shift in CKD stage toward earlier stages and by ≥1 stage was demonstrated in 35%, 45%, and 60% of patients, respectively [88]. Notably, this effect was observed regardless of the presence or absence of mutations in complement genes [88]. With long-term use, eculizumab** prevents relapses of acute complement-mediated TMA [92].

During the induction phase (initial cycle), eculizumab** is administered at a dose of 900 mg as an intravenous infusion once weekly for the first 4 weeks of therapy (4 infusions in total). At week 5 of therapy, 1200 mg of the drug is administered intravenously (5th infusion), followed by 1200 mg every 2 weeks for an indefinite duration (Table 3). The route of administration corresponds to the prescribing information ("Appendix A3", cl. 1).

An alternative to eculizumab** is ravulizumab, which has demonstrated comparable efficacy [91]. The latter is also a humanized monoclonal antibody with a human kappa light-chain constant region and an IgG2/4 heavy-chain constant region. Similar to eculizumab**, the drug binds specifically and with high affinity to the C5 complement component, thereby inhibiting its cleavage into C5a (a pro-inflammatory anaphylatoxin) and C5b (the initiating subunit of the terminal complement complex [C5b-9]) and preventing the formation of the C5b-9 membrane attack complex.

Genetic engineering modifications of the molecule significantly extended its half-life, exceeding that of eculizumab** by 4-fold, which ensures continuous C5 suppression and an extended dosing interval (every 4–8 weeks). Unlike eculizumab**, ravulizumab dosing in adult patients depends on body weight (Table 3). The route of administration corresponds to the prescribing information ("Appendix A3", cl. 2).

Complete and sustained inhibition of complement C5 led to rapid improvement in hematological and renal parameters within 26 weeks and a further increase in response rates over a longer period without any unexpected safety concerns in adult patients with aHUS who had not previously received complement inhibitors (belonging to group L04AA – selective immunosuppressants). Upon switching from eculizumab** to ravulizumab, the latter maintained stable renal and hematological parameters for 1 year.

Table 3. Dosing regimen of eculizumab** and ravulizumab in patients with aHUS

• We recommend vaccination against Neisseria meningitidis with the polysaccharide, conjugated** meningococcal polysaccharide vaccine covering serogroups A, C, W, Y for all patients with aHUS 2 weeks prior to the first administration of eculizumab** or ravulizumab [2,69,93,138].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: blockade of the C5 complement component with eculizumab** impairs the immune response to certain encapsulated bacteria, primarily Neisseria species, specifically the meningococcus. This creates an increased risk of meningococcal infection during eculizumab treatment, to prevent which patients with aHUS must be immunized prior to initiating therapy [2,69,93].

• We recommend that patients with aHUS who have indications for urgent initiation of treatment with eculizumab** or ravulizumab that precludes prior meningococcal vaccination receive antibiotic prophylaxis with drugs that cross the blood-brain barrier (broad-spectrum penicillins, third-generation cephalosporins, ciprofloxacin**) to prevent meningococcal infection [2,69,93,138].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: systemic antibacterial therapy should be initiated concurrently with vaccination and administered for at least 2 weeks if vaccination is performed on the day of the first infusion of eculizumab** or ravulizumab. The choice of broad-spectrum systemic antibacterial drug, duration of use, and dose are determined by the attending physician based on the specific clinical situation. If vaccination is not feasible by the start of treatment, antibiotic prophylaxis may be extended for up to several months depending on the timing of vaccination. In some cases, systemic antibacterial drugs may be prescribed even to timely vaccinated patients. Indications for their use are determined by the attending physician. The need for prophylactic administration of systemic antibacterial drugs in patients with aHUS is due to the lack of a vaccine against the N. meningitidis serotype B, meaning vaccination does not completely rule out the development of meningitis [69].

• We recommend making the decision to discontinue eculizumab** or ravulizumab no earlier than 3 months after achieving complement-mediated TMA remission and renal function stabilization to reduce the risks of therapy complications and achieve economic benefits [133-136].

GCR BLoE 3Level of Evidence B (Level of Confidence of Evidence – 3)

• We recommend making the decision to discontinue eculizumab** or ravulizumab after achieving remission of complement-mediated TMA and obtaining genetic test results identifying mutations in complement factor genes CFH, MCP, CFI, CFB, C3 [133].

GCR BLoE 3Level of Evidence B (Level of Confidence of Evidence – 3)

Comments: In an analysis of a cohort of 280 cases of intentional discontinuation of complement-blocking therapy upon achieving hematological remission and stable renal function not requiring renal replacement therapy [133], the overall rate of TMA relapse after therapy cessation was 29.6% (n=83).

Among patients for whom follow-up time information after eculizumab discontinuation** was available (n=218), the median follow-up was 23 months (95% CI, 20–24). The highest relapse rate in the cohort was observed among patients with single CFH variants (57.4%), especially those affecting exon 22 (90.9%); patients with single MCP/CD46 variants (51.4%); and patients with multiple concurrent genetic variants (50%). Among patients with single MCP/CD46 variants, the probability of relapse was higher than in patients without splice-region variants (83.3% vs. 35.7%; P=0.021). Mutations in the CFH, MCP, and C3 genes require more frequent monitoring after drug discontinuation for the timely diagnosis of disease relapse and resumption of therapy [133].

• We recommend careful monitoring of patients when discontinuing eculizumab** or ravulizumab, using all necessary tests (see the "Diagnostics" section) for the timely diagnosis of a potential aHUS relapse and resumption of complement-blocking therapy [69,92,95,96,133–136].

Level of Evidence AGrade of Evidence 2Level of Recommendations A (Grade of Evidence – 2)

Comments: careful monitoring after discontinuation of eculizumab** or ravulizumab is indicated for patients for one year. In addition to self-monitoring of proteinuria and hematuria using urine test strips, it is necessary to monitor blood pressure, hematological parameters including signs of MAHA, and levels of proteinuria and blood creatinine. During the first month after discontinuing complement-blocking therapy, tests should be performed once a week; from the second to the sixth month inclusive, once a month; and starting from the seventh month, every 2–3 months. In addition, extrarenal manifestations should be monitored, and clinical signs of infection should be watched for [139,140].

Since aHUS relapses are most frequently observed during the first 3 months, and especially the first 30 days, after discontinuation of pathogenetic therapy, patients require particularly careful monitoring and control of the main laboratory parameters characterizing TMA during this period. This regime allows the detection of a disease exacerbation at its very onset, preventing the development of pronounced thrombocytopenia, MAHA, and renal damage [69,92,95,96]. At the same time, with a median time to relapse of 3.60 months (range 0.50–31.75), the probability of relapse persisted over a longer-term period [133]. It is important to note that among patients with a relapse who resumed eculizumab** therapy, the majority recovered their baseline renal function, and progression to renal replacement therapy (RRT) was rare (observed in a few cases). This suggests that careful monitoring and the guarantee of immediate therapy resumption can allow drug discontinuation even in patients at risk. The results of studies highlight the feasibility and social responsibility of controlled eculizumab** discontinuation in patients with aHUS [133–136].

• In patients with aHUS presenting with renal dysfunction requiring hemodialysis, we recommend continuing treatment with eculizumab** or ravulizumab for at least 3 months after achieving disease remission in order to achieve the maximum renal response to complement-blocking therapy [69,91,138].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: eculizumab** or ravulizumab suppress complement after the very first infusion, and its reduced activity persists throughout the entire treatment period. Therefore, the functional activity of complement (CH50) steadily approaches zero during treatment. The criteria for the clinical efficacy of eculizumab** or ravulizumab are the cessation of microangiopathic hemolysis (decrease in LDH levels to normal values), normalization of the platelet count, and improvement in renal function. At the same time, the hematological response significantly outpaces the renal response. As a rule, LDH and platelet counts normalize by the end of the induction course of therapy, whereas blood creatinine decreases significantly slower, and the patient may remain dialysis-dependent for a longer period. A gradual increase in diuresis by the end of the 3rd month serves as an important indication for continuing complement-blocking therapy, as improvement in renal function is frequently delayed. In these cases, performing a kidney biopsy can help decide whether to continue or discontinue therapy. If acute kidney injury (AKI) has not resolved by the specified period and the need for dialysis persists, eculizumab** or ravulizumab should be discontinued. The only exception is patients with multiorgan involvement, in which cases the decision to continue therapy is made individually.

3.3 Concomitant therapy

• We do not recommend platelet concentrate transfusion in patients with aHUS who do not have severe thrombocytopenia (platelet count >20,000/µL) and hemorrhagic complications (bleeding) due to the risk of worsening thrombocytopenia and generalized microthrombus formation [2,36,37,42,69,74].

GDR CLE 5Grade of recommendation C (level of evidence – 5)

Comments: platelet concentrate transfusions can increase the manifestations of TMA in patients with aHUS, as they provoke further platelet consumption, potentiating thrombus formation in the microvasculature. Platelet concentrate transfusion is indicated only upon the development of bleeding or when using invasive diagnostic or therapeutic interventions with a high bleeding risk [2,36,37,42,69,74].

• We recommend transfusing red blood cell mass in patients with aHUS to correct severe MAHA (hemoglobin <75 g/L) [2,69].

GDR CLE 5Grade of recommendation C (level of evidence – 5)

• We recommend prescribing antihypertensive drugs provided by the guidelines for the treatment of hypertension by the All-Russian Public Organization "Russian Society of Cardiology" and the All-Russian Public Organization "Russian Scientific Medical Society of Therapists": Clinical Guidelines. Arterial Hypertension in Adults. 2024 https://cr.minzdrav.gov.ru/viev-cr/62_4 [97].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments:. In the vast majority of cases, patients with aHUS have severe hypertension, and almost a quarter of them develop malignant arterial hypertension, a characteristic feature of which is resistance to multi-drug antihypertensive therapy. It should be remembered that the main pathogenetic mechanism of hypertension in patients with aHUS is the activation of the renin-angiotensin system (RAS) due to ischemic kidney damage. Therefore, first-line agents in the treatment of such hypertension should be agents acting on the renin-angiotensin system: angiotensin-converting enzyme (ACE) inhibitors or angiotensin II receptor blockers (ARBs). These drugs should be prescribed even with severe renal impairment, starting with a minimum dose and gradually increasing it under the control of blood creatinine and potassium levels [165.]. We recommend treating AKI in patients with aHUS in accordance with the AKI treatment guidelines [68].

• We recommend treating AKI in patients with aHUS in accordance with the AKI treatment guidelines [68].

  Level of recommendation C (evidence level – 5)

Comments: see AKI guidelines.

• We recommend identifying potential disease triggers in all patients with aHUS to conduct treatment aimed at their elimination, correction, and prevention [39,42,59,69,71,74].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: even in patients with a genetic defect of the complement system, manifestation of predisposition to aHUS requires exposure to a trigger acting as a "second hit." The most frequent triggers are infections, active inflammation, pregnancy, surgical interventions, contrast agents, and even stress. Any identified trigger should be eliminated as quickly as possible. Of particular importance is the timely diagnosis of infections most frequently preceding aHUS, followed by adequate antibacterial therapy [39,42,59,69,74].

3.4 Kidney transplantation

• We recommend considering kidney transplantation for patients with aHUS who have reached end-stage renal disease (CKD stage 5) and have no extrarenal clinical manifestations of TMA, with the goal of increasing life expectancy and improving quality of life [37,39,69].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: for patients who have reached end-stage renal disease, kidney transplantation should be considered if the period from the start of hemodialysis is at least 6 months [98]. The time from the start of renal replacement therapy to inclusion in the transplantation "Waitlist" may be extended. If renal failure has developed acutely, there are prospects for the recovery of renal function and the discontinuation of hemodialysis as a result of targeted treatment, patients should not be considered as candidates for kidney transplantation until the final evaluation of the response to therapy due to the high risk of graft loss. Limitations to performing kidney transplantation in patients with aHUS include both general contraindications to transplantation (malignant neoplasms, active infections, severe chronic extrarenal diseases, etc.) and those specific to aHUS: ongoing complement activation with extrarenal manifestations and the lack of possibility for prophylactic administration of eculizumab** or ravulizumab in high- and medium-risk patients.

• For patients with aHUS who are potential candidates for kidney transplantation, we recommend taking into account genetic test data and the level of anti-CFH antibodies (see section "Diagnostics") to assess the risk of disease recurrence in the post-transplantation period [66, 72].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: patients with aHUS may have a high, medium, or low risk of recurrence after kidney transplantation, while the degree of risk should be determined both on the basis of genetic testing results and taking into account the clinical course of the disease, the frequency of TMA recurrences, and family history (Table 4). It should be remembered that patients in whom genetic variants associated with the development of aHUS have not been identified should be classified into the medium-risk group for recurrence, while patients with a familial form of aHUS and/or a history of TMA recurrences, including those who previously lost a renal graft due to TMA, have a high risk of recurrence after kidney transplantation regardless of the nature of the identified mutations. It is advisable to perform molecular genetic testing for mutations in the genes for factor H (CFH), factor I (CFI), factor B (CFB), C3, thrombomodulin (THBD), membrane cofactor protein (MCP), CFH-related structural proteins (CFHR1-R5), diacylglycerol kinase epsilon (DGKE), and plasminogen (PLG) also in candidates for kidney transplantation with CKD stage G5 and an established diagnosis of STEC-HUS, since it has been shown that in some cases STEC infection can be a trigger for aHUS recurring after kidney transplantation [141-143]. Although the overall risk is determined primarily by the type of mutations, it partially depends on other peritransplantation factors as well. These include severe ischemia-reperfusion injury of the graft, high blood concentrations of calcineurin inhibitors in the recipient, acute graft rejection—especially antibody-mediated rejection—and viral infections (cytomegalovirus and other herpesvirus infections, influenza, BKV infection, parvovirus B19 infection, infectious gastroenteritis, etc.) [20, 83].

Table 4. Assessment of the risk of aHUS recurrence after kidney transplantation

• We recommend performing kidney transplantation against the background of prophylactic use of eculizumab** or ravulizumab in patients with an established diagnosis of aHUS who have a medium and high risk of recurrence in the post-transplantation period in order to prevent disease recurrence [91, 99, 144].

GCR ALOE 2Grade of Recommendation A (Level of Evidence – 2)

Comments: A recent MA has demonstrated the efficacy of such therapy [144]. Prophylactic use of eculizumab** or ravulizumab in high-risk groups for aHUS recurrence is more effective than treating manifested recurrence: while the rate of graft loss due to TMA in the prophylactic eculizumab** group was 5.5%, in recipients who received the drug for recurrent aHUS, this figure reached 22.5% [99]. Rescue therapy with eculizumab** or ravulizumab (emergency initiation of treatment for manifested aHUS recurrence after kidney transplantation) halted aHUS recurrences after transplantation, but was less effective compared to prophylaxis, failing to prevent progressive renal allograft dysfunction in a number of patients [145]. As shown by a meta-analysis including 18 studies, the use of eculizumab** after kidney transplantation in patients with aHUS reduced the need for hemodialysis, decreased the rate of graft rejection, and contributed to the improvement of renal function and quality of life [144].

Considering the similar mechanism of action of eculizumab** and ravulizumab, as well as data from studies on the efficacy and safety of the latter, ravulizumab may also be used for the prophylaxis of aHUS recurrence after kidney transplantation, although experience with the drug in this clinical setting is currently smaller compared to eculizumab** [146, 147].

In recipients previously treated with eculizumab**, conversion to ravulizumab can be performed to improve quality of life in accordance with the drug's prescribing information [148].

Performing kidney transplantation without targeted prophylaxis should only be done in patients with a confirmed low risk of aHUS recurrence. All others require prophylactic complement-blocking therapy, the duration of which has not yet been definitively determined. However, experts suggest that the duration of eculizumab** or ravulizumab administration after transplantation may also be determined based on the risk of recurrence: high-risk patients should receive the drug throughout the entire period of renal allograft function, while in medium-risk patients, in the absence of even subclinical recurrences in the post-transplantation period 1 year after kidney transplantation, a controlled attempt to withdraw complement-blocking therapy can be undertaken [20, 83, 98] (Table 5).

Table 5. Duration of eculizumab** treatment after kidney transplantation depending on the risk of aHUS recurrence

When analyzing data from the Global aHUS Registry, it was shown that initiating eculizumab** administration prior to kidney transplantation, compared to starting therapy after surgery (with a delay of more than 14 days), is associated with improved renal allograft function and a reduced risk of the need for hemodialysis [100]. On the day of transplantation, regardless of whether the patient received eculizumab** previously or not, an infusion of the drug must be performed at least one hour before the reperfusion of the donor organ. The dose of #eculizumab** does not depend on body weight and is 900 mg per administration. The next dose of #eculizumab** is administered 24 hours after kidney transplantation [160]; the need to administer an additional dose of #eculizumab 1 day after surgery is a specific feature of patient management during kidney transplantation and is associated with the presence of a large number of complement-activating conditions in the early post-transplantation period. Subsequent administration of the drug is carried out one week later (900 mg) with a subsequent transition to a regimen of once every 2 weeks (1200 mg) long-term (Table 6) [98, 161].

Table 6. Regimen for prophylactic administration of #eculizumab** during kidney transplantation in patients with aHUS

At the same time, in individual high-risk patients with a complicated postoperative course accompanied by additional complement-activating conditions (severe reperfusion injury, graft rejection, surgical complications, additional surgical interventions), a more intensive eculizumab** administration regimen in the early post-transplantation period can be used, with a transition to a 2-week dosing interval of 1200 mg after day 36 (Table 7). If there is a vital indication for TPE or FFP transfusions during targeted therapy with #eculizumab**, the administration of additional doses of the drug is necessary in accordance with its instructions for use [98].

Table 7. Intensified regimen for prophylactic administration of #eculizumab** during kidney transplantation in patients with aHUS

When using ravulizumab, the dosage depends on body weight: an IV loading dose is administered on day 0, a maintenance dose on day 15, and then a maintenance dose every 8 weeks (Table 8) [138,146].

Table 8. Ravulizumab dosing schedule in kidney transplantation in patients with aHUS

• We recommend vaccination against meningococcal, pneumococcal, and Haemophilus influenzae type b infections as part of the preparation for kidney transplantation and the prophylactic use of eculizumab** or ravulizumab in patients with aHUS, if they have not been vaccinated previously, to minimize the risk of infectious complications [39,91,101,138, 159].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: the mechanism of action of eculizumab** and ravulizumab involves an increased risk of meningococcal infection and other infections caused by encapsulated bacteria; therefore, all patients receiving this drug should be vaccinated against meningococcal infection, and candidates for kidney transplantation should also be vaccinated against pneumococcal and Haemophilus influenzae infections, preferably before initiating complement-blocking therapy. Since kidney transplantation is scheduled electively in most cases, vaccination should be carried out before inclusion in the waiting list. In patients with aHUS, general recommendations for infection prevention in solid organ recipients should also be applied [102,103]. In addition, practical recommendations for the primary and secondary prevention of infections in adults and children with aHUS were developed by Russian experts in 2016 and published as the document "Resolution of the Interdisciplinary Expert Council on the Prevention of Severe Infections in Patients with Genetic Disorders of Complement Regulation Receiving Eculizumab Therapy" [104]. It should be remembered that after kidney transplantation, vaccination with any live vaccines is strictly contraindicated.

• We do not recommend using organs from expanded criteria donors, performing the surgery in the presence of a high percentage of preformed anti-HLA antibodies in the potential recipient, or ABO incompatibility between the donor and recipient during kidney transplantation in patients with aHUS to prevent disease recurrence in the renal allograft [69,81].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: this recommendation is due to the high risk of aHUS recurrence in the setting of severe ischemia-reperfusion injury of the renal allograft and antibody-mediated rejection.

• We do not recommend performing living-related kidney transplantation in patients with aHUS without molecular genetic testing for mutations in the genes for factor H (CFH), I (CFI), B (CFB), C3, thrombomodulin (THBD), membrane cofactor protein (MCP), CFH-related structural proteins (CFHR1-R5), diacylglycerol kinase epsilon (DGKE), and plasminogen (PLG) in both the recipient and the potential donor to prevent post-transplant disease recurrence [65,69,72,81].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: a diagnosis of aHUS is, in most cases, a contraindication to living-related kidney transplantation due to the almost inevitable recurrence of TMA in the graft. However, kidney transplantation may be performed if molecular genetic testing for mutations in the genes for factor H (CFH), I (CFI), B (CFB), C3, thrombomodulin (THBD), membrane cofactor protein (MCP), CFH-related structural proteins (CFHR1-R5), diacylglycerol kinase epsilon (DGKE), and plasminogen (PLG) reveals mutations associated with the development of aHUS in the kidney transplant candidate, while none are found in the potential donor. At the same time, the donor and recipient must be fully informed about the potential complications of kidney transplantation. Conversely, if no mutations responsible for the development of aHUS are detected in either the donor or the potential recipient, or if genetic variants of uncertain clinical significance are found, living-related kidney transplantation should be abandoned.

• In renal allograft recipients suffering from aHUS, we recommend selecting induction and maintenance immunosuppressive therapy taking into account the risk of rejection, avoiding high blood concentrations of calcineurin inhibitors to reduce the likelihood of premature loss and the development of drug-induced TMA of the transplanted kidney [69,81,98].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: in patients with aHUS, induction therapy with antilymphocyte antibodies—antithymocyte globulin** (rabbit) or antilymphocyte globulin equine—is highly desirable as it significantly reduces the risk of allograft rejection. The use of cyclosporine** and tacrolimus** after kidney transplantation can be a trigger for the development of allograft TMA; however, these drugs currently remain the backbone of immunosuppression. Furthermore, immunosuppressive regimens without cyclosporine**/tacrolimus** do not lead to a significant reduction in the risk of aHUS recurrence after kidney transplantation [105]. In recipients diagnosed with aHUS, excessively high blood concentrations of calcineurin inhibitors should be avoided: the concentration of tacrolimus** should not exceed 12 ng/mL, and cyclosporine** should not exceed 200 ng/mL (C0). In most cases, mTOR inhibitors (sirolimus or everolimus**) should not be included in the post-transplant immunosuppressive regimen for patients with aHUS, because the use of these drugs can impair the repair of damaged endothelium due to antiproliferative and procoagulant effects [106,107]; however, in individual patients with indications and a careful benefit/risk assessment, sirolimus or everolimus** may be used.

• In patients with aHUS after kidney transplantation, we recommend careful monitoring of clinical and laboratory parameters that facilitate the detection of TMA, and performing graft biopsies (ultrasound-guided kidney biopsy with morphological examination of the biopsy (surgical) material) for the timely diagnosis and treatment of allograft TMA [69,81,98].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: monitoring of TMA laboratory parameters (blood platelet count, total blood hemoglobin level, blood LDH activity, blood creatinine level) after kidney transplantation should be performed once a week during the first month, once every 2 weeks for the subsequent 2–4 months, and thereafter once a month. More frequent monitoring is necessary during intercurrent infections, the development of renal allograft dysfunction, and after the withdrawal of eculizumab**. Patients with aHUS after kidney transplantation who have discontinued eculizumab** treatment must be under the close supervision of a nephrologist to ensure control over signs of potential TMA activation and the development of its severe complications. Signs of aHUS recurrence after eculizumab** withdrawal include (1) any two or repeatedly detected changes in one of the following parameters: a decrease in platelet count by 25% or more from the baseline value or the maximum platelet count during eculizumab** treatment; an increase in serum creatinine concentration by 25% or more compared to the baseline value or the minimum level during eculizumab** therapy, or an increase in serum LDH activity by 25% or more compared to the baseline or minimum value during eculizumab** therapy, or (2) any of the following symptoms: altered mental status, seizures, angina pectoris, dyspnea, thrombosis, severe pancreatitis, sudden vision loss. Particular attention should be paid to impaired renal function even in the absence of extrarenal manifestations. These symptoms can be attributed to aHUS after ruling out a number of causes that can also be accompanied by changes in the listed laboratory parameters, especially in the early post-transplant period (e.g., surgical complications, infections, hematological toxicity of immunosuppressants, etc.). In the case of an isolated increase in serum creatinine level or an incomplete symptom complex of TMA, morphological verification of the cause of graft dysfunction is mandatory, since TMA localized exclusively in the graft cannot be ruled out [108,109].

• For the treatment of aHUS recurrence in patients after kidney transplantation who have not received complement-blocking therapy, we recommend the use of eculizumab** or ravulizumab as first-line therapy [69,81,98,110,111,147,148].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: in the event of aHUS recurrence after kidney transplantation or the development of de novo aHUS (TMA with complement gene mutations first identified after kidney transplantation), eculizumab** has demonstrated high efficacy, especially with early initiation of therapy [110,111]. The renal graft is even more susceptible to complement-mediated injury than native kidneys; therefore, even a minor delay in starting targeted therapy can lead to incomplete recovery or loss of the transplanted organ's function. While awaiting the delivery of eculizumab** (if immediate administration is not possible), therapeutic plasma exchange (TPE) may be performed with an adequate volume of exchange and replacement using fresh frozen plasma (40–60 ml/kg of the patient's body weight). A single dose of eculizumab** for the treatment of aHUS recurrence after kidney transplantation is not supported. The duration of complement-blocking therapy in patients who have experienced an aHUS recurrence after kidney transplantation has not been definitively established, but it is clear that these patients fall into the high-risk group for repeat recurrence if eculizumab** is discontinued.

3.5 Monitoring Treatment Efficacy

In many cases, aHUS requires long-term complement-blocking therapy. Such a treatment regimen may be necessary for patients with identified mutations in the complement factor genes CFH, CFI, CFB, and C3, as they belong to the high-risk category for aHUS recurrence [42,69,92,93].

We do not recommend laboratory monitoring of the degree of complement blockade to assess the efficacy of eculizumab** during complement-blocking therapy in all patients with aHUS [88].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: complement blockade occurs within one hour after the first infusion of eculizumab** and is maintained at the achieved level throughout the interval between administrations [88]. The only available marker for assessing the degree of blockade is total hemolytic complement activity (CH50) [112]. As a rule, it remains low for 12–14 days, which justifies the two-week interval during maintenance therapy. Complement blockade is achieved if CH50 values are <10%. The vast majority of patients, as shown in studies, achieve this level immediately after the first infusion of eculizumab**; therefore, there is no need to monitor CH50 during treatment if the patient experiences normalization of the platelet count, a decrease in LDH, and a reduction in creatinine levels by more than 25% from baseline during the induction period [69]. The complement C3 fraction level is not suitable for monitoring the efficacy of ongoing therapy, as it is initially reduced in patients with CFH and C3 mutations and cannot be expected to increase during eculizumab** treatment [65]. At the same time, the introduction of more precise methods for assessing complement activity into clinical practice may be useful for the personalized management of patients with aHUS.

• In the absence of a positive trend in hematological parameters during induction therapy in patients receiving eculizumab** or ravulizumab, we recommend determining total hemolytic complement activity (CH50) to assess the efficacy of complement blockade [91,113,138].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: a CH50 value greater than 10% indicates the absence of complete complement blockade, which is the cause of ineffective therapy. Today, several causes associated with insufficient complement blockade are known, and all of them are extremely rare. These may include individual dose insufficiency (more common in children, but also possible in very large adult patients), and the excretion of eculizumab** in the urine in patients with massive proteinuria and nephrotic syndrome. With an adequate drug dose, the cause of the lack of complement blockade may be a genetic variant of the C5 component, which prevents eculizumab** from binding to it and thus confers resistance to the drug. During complement-blocking therapy, the appearance of antibodies against eculizumab** is also possible, which likewise leads to treatment failure.

3.6 Management of Pregnancy-Associated aHUS

Pregnancy-associated aHUS (P-aHUS) is aHUS that develops from the confirmation of pregnancy up to 12 weeks postpartum or following pregnancy termination [47,114]. In more than 50% of female patients, aHUS develops during subsequent pregnancies. The disease can manifest at any stage of pregnancy; however, in the vast majority of cases (74–94%), aHUS occurs in the third trimester and the early postpartum period, whereas the first trimester accounts for only 11% of cases, and the second trimester accounts for up to 15%. P-aHUS is characterized by an aggressive course and an extremely poor prognosis: maternal mortality reaches 42% [47,115]. The detection rate of genetic defects in the complement system in obstetric aHUS is comparable to that in the general aHUS population, providing grounds to consider this condition not as a secondary form of aHUS, but as a true complement-mediated TMA triggered by pregnancy. Studies of the genetic profile of the complement system in women with obstetric aHUS have revealed mutations associated with the development of aHUS in 46–56% of patients [116,117,149]. Furthermore, pregnancy and delivery complications—such as preeclampsia, hemorrhage, and operative delivery—play an important role in the manifestation of P-aHUS, representing additional complement-activating conditions considered as triggers of acute TMA [118–120]. Thus, the treatment of obstetric aHUS, which is unequivocally a complement-mediated pathology, must follow the same principles as the treatment of aHUS.

• We recommend initiating plasma therapy in all female patients with a TMA symptom complex and suspected P-aHUS within 24 hours after pregnancy termination/delivery to prevent disease progression and the development of life-threatening complications [47,80,114,115,118,121].

GCR BLEL 3Grade of Recommendation B (Level of Evidence – 3)

Comments: Taking into account the particularly severe, often catastrophic course of obstetric aHUS, it is necessary to initiate plasma therapy, preferably in the TPE mode, immediately after identifying signs of TMA. It should be noted that the effect of plasma therapy is observed in only half of patients; according to a retrospective study, the risk of end-stage renal disease (ESRD) in patients who suffered from obstetric HUS was similarly high (about 50%) both in the 56 women who received TPE and in the 16 who did not undergo this therapy [114].

• In the event of failure of plasma therapy administered for at least 3 consecutive days, we recommend initiating complement-blocking therapy with eculizumab** or ravulizumab to prevent disease progression, the development of life-threatening complications, and to achieve remission in patients with P-aHUS [114,115,118,122,150].

LeRR CLoE 4Level of recommendation confidence C (level of evidence – 4)

Comments: the efficacy and safety of eculizumab** during pregnancy were initially established in patients with paroxysmal nocturnal hemoglobinuria, and subsequently in aHUS [122,151–153]. The treatment protocol for obstetric aHUS is completely analogous to the treatment protocol for patients with aHUS regarding drug dosages and dosing intervals if the disease developed in the postpartum period. The maximum efficacy of eculizumab**, manifested not only by the resolution of hematological parameters but also by the complete recovery of renal function in the majority of patients, was noted when therapy was initiated early—within the first week from disease onset: thus, among patients receiving early, delayed, and late eculizumab** therapy, the rates of complete renal function recovery were 100% (28/28), 78.6% (11/14), and 54.5% (6/11), respectively, while the rates of dialysis-dependent chronic kidney disease were 0% (0/28), 21.4% (3/14), and 45.5% (5/11) [115,154]. When aHUS develops during pregnancy, higher doses of eculizumab** and/or shortened dosing intervals may be required for complete C5 blockade due to an increase in circulating blood volume, increased C5 synthesis, or drug loss in the urine in the presence of massive proteinuria [149,156]. The duration of complement-blocking therapy in patients with obstetric aHUS has not been defined. It depends on the presence of a genetic defect, the severity of multiorgan failure, the observed response to eculizumab**, and must be determined individually [149,156].

• In cases of clinical suspicion of infectious complications, we recommend promptly prescribing systemic broad-spectrum antibacterial therapy to patients with P-aHUS in order to resolve them [123,124].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: the development of TMA should be considered as a risk factor for systemic infection, since ischemic organ damage predisposes to infection. Another risk factor for infection is a Cesarean section, since in the overwhelming majority of cases, preeclampsia preceding the development of aHUS is an indication for operative delivery. Thus, the early administration of systemic broad-spectrum antibacterial drugs in patients with aHUS serves as a means of sepsis prevention [123,124].

Of course, to identify the causative agent, a microbiological (cultural) blood culture for sterility is necessary before initiating empirical antibacterial therapy.

In cases where a systemic infection complicates aHUS or, conversely, obstetric sepsis is its trigger, complement-blocking therapy can be used. However, eculizumab** or ravulizumab should be prescribed only after the manifestations of the infection have been resolved with systemic antibacterial drugs; otherwise, complement blockade and the resulting generalization of the infectious process can lead to patient death [157,158].

4. Medical Rehabilitation and Sanatorium-Resort Treatment

Rehabilitation methods for patients who have undergone aHUS have not been developed. With adequate and timely initiated therapy, some patients not only experience resolution of the hematological manifestations of the disease, but also fully recover renal function. In these cases, there is no need for medical rehabilitation methods. However, in a number of patients, despite the resolution of hematological symptoms, renal function may not recover or may recover incompletely, resulting in them remaining dialysis-dependent, or developing CKD as an outcome of AKI. It should be remembered that the vast majority of patients who have experienced acute aHUS are young adults. Therefore, the goal of their rehabilitation should be the maximum preservation and, if possible, enhancement of physical activity. In these cases, rehabilitation methods indicated for patients with CKD, regardless of its cause, should be used.

• In patients with aHUS who have CKD G3b-G5D, we recommend drawing up an individualized plan of activities to maintain or increase physical activity, involving a physical therapy doctor to determine the volume and methodology of physical exercises, taking into account contraindications, in order to improve general health, musculoskeletal status, and increase tolerance to aerobic loads [163]

GCR ALEL 1Grade of Recommendation A (Level of Evidence – 1)

Comments: The overall goal of rehabilitation is to provide patients with maximum opportunities to reduce the severity of any limitations imposed on their daily life due to impairments caused by CKD. Clinicians should inform CKD patients with functional limitations about the benefits of regular exercise, as there is sufficient evidence that regular exercise is beneficial for rehabilitation in terms of general health, reducing the severity or preventing the progression of functional impairments in CKD patients, including CKD G5D [163]. There is strong evidence of a significant favorable effect of regular exercise on physical fitness, walking, cardiovascular parameters (blood pressure and heart rate), health-related quality of life, and some nutritional parameters in adults with CKD [163].

• In patients with aHUS who have CKD G3b-G5D and impairments in daily activities, we recommend consultations with a medical rehabilitation doctor and other necessary specialists to develop an individualized rehabilitation plan [164].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: Specific rehabilitation interventions should take into account comorbidities. An individualized approach should consider patient preferences and take into account the complex relationship between many different factors, not just the underlying disease. The rehabilitation approach should include measures to reduce the adverse consequences of CKD. The main type of physical activity is aerobics or endurance exercise. Proper rehabilitation interventions should be planned taking into account the functional capacities of CKD patients and based on interdisciplinary approaches. Interdisciplinary care, including a physical therapy doctor, a medical rehabilitation doctor, a physical therapist, a geriatrician, a nephrologist, and other necessary specialists, can be more effective than a standard approach focused solely on CKD. Involving individuals with knowledge in elderly care (nurses, social workers, and physical therapy assistants) and using simple assistive devices (dressing sticks, long-handled shoehorns, long-handled sponges, button hooks, shower seats, and three-in-one commodes) increases the likelihood of achieving rehabilitation goals [164].

5. Prevention and Dispensary Observation

Methods for the primary prevention of aHUS have not been developed. In patients with an established diagnosis who are receiving pathogenetic therapy, a number of measures aimed at preventing adverse effects of therapy should be implemented, as outlined in section 3.6. Taking into account the role of external factors (primarily infection, as well as stress, trauma, intravenous contrast administration, pregnancy complications) in the development of an acute TMA episode in predisposed individuals, the prevention of aHUS relapses is possible.

• We recommend educating patients who have survived an acute episode of complement-mediated TMA with an obvious or probable triggering factor on how to reduce the risk of its development to prevent disease relapse [39].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

• We recommend performing dynamic monitoring of all patients who have undergone aHUS, regardless of whether they are receiving complement-blocking therapy with eculizumab** or ravulizumab, for the timely detection and treatment of probable relapses of the disease [2,69,91,138].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

Comments: during the first 4-6 months, patients need to be examined every 2-4 weeks. Subsequently, the patient should be monitored quarterly for the first 3 years, and thereafter once every 6 months.

• We recommend individualized dynamic monitoring of all patients who have undergone aHUS, including peripheral blood pressure measurement (with 24-hour ambulatory blood pressure monitoring indicated), blood creatinine testing with subsequent calculation of the glomerular filtration rate, blood albumin testing, determination of urinary protein, quantification of 24-hour urinary protein, microscopic examination of urine sediment, and a complete blood count to monitor disease relapses and assess the severity of renal dysfunction [2,69].

GCR CLEL 5Grade of Recommendation C (Level of Evidence – 5)

• We recommend providing all female patients who have had aHUS and are planning a pregnancy with pre-pregnancy counseling regarding the potential risks of relapse and adverse pregnancy outcomes, and developing an individualized pregnancy management plan from conception to delivery, including the early postpartum period [114,115,116].

GCR BLEL 3Grade of Recommendation B (Level of Evidence – 3)

Comments: pregnant women with a history of aHUS require careful blood pressure monitoring; in the first and second trimesters of gestation, a complete blood count with mandatory platelet count, urinalysis, blood creatinine level, and blood aspartate aminotransferase, alanine aminotransferase, and LDH activity must be performed monthly. Once per trimester, the Reberg test (renal function study using creatinine clearance) should be performed to determine the glomerular filtration rate, along with a microbiological (cultural) urine test for bacterial pathogens. In the third trimester, complete blood and urine counts, blood creatinine testing, and blood aspartate aminotransferase and alanine aminotransferase activity determinations should be performed every 2 weeks, and microbiological (cultural) urine testing for bacterial pathogens monthly. Starting from the 12th week of pregnancy, preeclampsia prevention with acetylsalicylic acid** preparations is indicated. In case of proteinuria ≥1 g/day, low-molecular-weight heparins (heparin group) are indicated. If a urinary tract infection is detected, the timely initiation of adequate antibacterial therapy according to pathogen sensitivity is necessary. All these therapeutic measures are aimed at preventing obstetric complications, the combination of which can lead to the development of obstetric aHUS.

6. Organization of Medical Care

Form – emergency, urgent; setting – inpatient

Indications for emergency hospitalization in a medical organization:

1. development of severe organ- and life-threatening disease manifestations – acute kidney injury requiring renal replacement therapy (hemodialysis), acute heart and respiratory failure, severe gastrointestinal, central nervous system, and ocular lesions, hemolytic anemia, hemorrhagic syndrome, for the purpose of primary diagnosis, plasma therapy, and complement-blocking therapy;

2. need for emergency initiation of complement-blocking therapy in patients with a relapse of acute TMA.

Type: elective; setting: inpatient, day hospital

Indications for scheduled hospitalization in a medical organization:

1. initiation of complement-blocking therapy for non-life-threatening disease variants;

2. need to clarify the cause of inefficacy of complement-blocking therapy in a non-life-threatening course of the disease;

3. need to perform a kidney biopsy;

4. scheduled repeated administration of a complement-blocking medicinal product (selective immunosuppressants) (eculizumab** or ravulizumab) in the absence of life-threatening disease manifestations.

Indications for discharging a patient from a medical organization:

1. persistent improvement in condition when the patient can continue treatment in an outpatient clinic or at home without detriment to health;

2. if necessary to transfer the patient to another healthcare organization;

3. gross violation of the hospital regime;

4. upon the written request of the patient or their legal representative, if discharge does not threaten the patient's life and is not dangerous to others. In this case, discharge may be carried out only with the permission of the chief physician of the hospital or their deputy for medical work.

7. Additional Information

The presence of mutations in genes of a particular complement factor directly affects the clinical course of aHUS, the overall and renal prognosis, and also determines the prognosis in kidney transplant patients (see Table 2). Thus, in patients with aHUS, the prognosis varies depending on the genotype. Specifically, patients with CFH mutations have the worst prognosis, whereas patients with MCP mutations have the best. Mortality during the first disease episode with CFH mutations is 4%. End-stage renal disease as an outcome of an acute episode occurs in 48% of survivors. No deaths were recorded during the acute disease episode in any patient with MCP mutations, regardless of age. End-stage renal disease developed in 25% of patients with these mutations [1,35,42,69]. In the natural course of aHUS, the disease is characterized by an unfavorable prognosis regardless of which regulatory proteins and complement components harbor identified mutations, as well as in cases where they could not be detected. The use of complement-blocking therapy has radically changed the prognosis of patients with aHUS. Early initiation of treatment with eculizumab**—within the first 7 days from disease onset—not only allows for the resolution of the hematological manifestations of aHUS, but also restores renal function, freeing patients from the need for dialysis therapy [35,39,42,69,81,93].

Criteria for Evaluating the Quality of Medical Care

References

Appendix A1. Working Group Composition

1. Bobrova Larisa Aleksandrovna – PhD, Assistant, Department of Internal, Occupational Diseases and Rheumatology, Institute of Clinical Medicine, Sechenov First Moscow State Medical University of the Russian Ministry of Health (Sechenov University).

2. Vatazin Andrey Vladimirovich – MD, Prof., Head of the Department of Kidney Transplantation Surgery, Head of the Department of Transplantology, Nephrology and Artificial Organs, Moscow Regional Research and Clinical Institute named after M.F. Vladimirsky, Moscow. Chief Freelance Nephrologist of the Ministry of Health of the Russian Federation for the Central Federal District. President of the National Association of Nephrologists. Honored Doctor of the Russian Federation. Honored Scientist of the Russian Federation.

3. Demyanova Ksenia Andreevna – PhD, Assistant, Department of Internal Diseases with Courses of Cardiology and Functional Diagnostics named after Academician V.S. Moiseev, Peoples' Friendship University of Russia (RUDN University), Nephrologist, Center for Care of Pregnant Women with Renal and Urinary Tract Pathology, Eramishantsev City Clinical Hospital of the Moscow Healthcare Department.

4. Dobronravov Vladimir Alexandrovich (Co-Chair of the Working Group) – MD, Prof., Director of the Research Institute of Nephrology, Professor, Department of Propaedeutics of Internal Diseases with Clinic, Pavlov First Saint Petersburg State Medical University, Saint Petersburg. Board Member of the National Association of Nephrologists.

5. Drapkina Oksana Mikhailovna – MD, Prof., Corresponding Member of the Russian Academy of Sciences, Director of the National Medical Research Center for Therapy and Preventive Medicine, Chief Freelance Specialist in Therapy and General Practice of the Russian Ministry of Health, Honored Doctor of the Russian Federation.

6. Karunnaya Anna Viktorovna (Secretary of the Working Group) – Nephrologist, Head of the Chronic Hemodialysis Unit, Clinic of the Research Institute of Nephrology, Pavlov First Saint Petersburg State Medical University, Saint Petersburg. Member of the National Association of Nephrologists.

7. Kirsanova Tatiana Valerievna – PhD, Senior Researcher, Department of Reproductive Hematology and Clinical Hemostaseology, Kulakov National Medical Research Center for Obstetrics, Gynecology and Perinatology of the Ministry of Health of the Russian Federation.

8. Kozlovskaya Natalia Lvovna (Head of the Working Group) – MD, Professor, Head of the Center for Care of Pregnant Women with Renal and Urinary Tract Pathology, Eramishantsev City Clinical Hospital of the Moscow Healthcare Department, Professor, Department of Internal Diseases with Courses of Cardiology and Functional Diagnostics named after Academician V.S. Moiseev, Medical Institute of the Patrice Lumumba Peoples' Friendship University of Russia

9. Prokopenko Elena Ivanovna – MD, Professor, Department of Transplantology, Nephrology and Artificial Organs, MONIKI named after M.F. Vladimirsky, Chair of the Association of Nephrologists of the Central Federal District of Russia.

Conflict of interest: none of the working group members had a conflict of interest during the development of these clinical guidelines, specifically a personal interest in receiving, personally or through a company representative, material gain or other advantage that would or could affect the proper performance of their professional duties.

Appendix A2. Methodology for Developing Clinical Guidelines

The instructions provided in these guidelines are based on a systematic literature review performed by the working group members. All articles published up to 2021 were selected for study. The experts reviewed these data and individual articles with the highest level of evidence.

To assess the quality and strength of the recommendations, the levels of evidence validity and recommendation grades were used (Tables 1–3).

Target audience for these clinical guidelines:

1. Nephrologist.

2. Hematologist.

3. Internist.

4. General practitioner / Primary care physician.

5. General practitioner (family physician).

6. Obstetrician-gynecologist.

Table 1.Scale for assessing levels of evidence (LE) for diagnostic methods (diagnostic interventions)

Table 2. Levels of evidence (LoE) rating scale for prevention, treatment, and medical rehabilitation methods, including those based on natural therapeutic factors (preventive, therapeutic, and rehabilitative interventions)

Table 3. Grades of recommendation (GoR) rating scale for prevention, diagnostics, treatment, and medical rehabilitation methods, including those based on natural therapeutic factors (preventive, diagnostic, therapeutic, and rehabilitative interventions)

Procedure for updating clinical guidelines.

The update mechanism for clinical guidelines provides for their systematic revision—at least once every three years, as well as upon the emergence of new evidence-based data regarding the diagnosis, treatment, prevention, and rehabilitation of specific conditions, or in the presence of justified additions/comments to previously approved clinical guidelines, but no more frequently than once every 6 months.

Appendix A3. Reference Materials

1. Route of administration of eculizumab**

Bolus administration of the drug is strictly prohibited. The drug must be administered only via intravenous infusion in accordance with the description below.

The prepared drug solution must be administered via intravenous infusion over 25–45 minutes (35±10 minutes) in adult patients.

Special infusion systems with controlled delivery must be used for intravenous administration of the prepared drug solution. There is no need to protect the prepared drug solution from light during administration.

The infusion solution should be used immediately after preparation.

Unused solution must be discarded.

After the infusion is completed, the patient should be monitored for 1 hour. If adverse events develop during drug administration, the infusion rate may be reduced down to complete cessation at the physician's discretion. When reducing the infusion rate, the total infusion time must not exceed 2 hours for adult patients.

Recommendations for solution preparation and infusion procedure

Do not mix the drug with any other drug in the same syringe or vial during intravenous administration!

Using a sterile syringe with a needle (not supplied), withdraw the entire contents of the drug vial and transfer the recommended dose into a vial containing one of the following injection solutions: 0.9% sodium chloride solution**, 0.45% sodium chloride solution, or 5% dextrose solution** to obtain an infusion solution with a concentration of 5.0 mg/mL.

The volume of the prepared infusion solution containing the drug at a concentration of 5.0 mg/mL should be: 60 mL (for a 300 mg dose), 120 mL (for a 600 mg dose), 180 mL (for a 900 mg dose), and 240 mL (for a 1200 mg dose).

The prepared infusion solution should be clear and colorless.

The temperature of the infusion solution should be 20–25 °C.

Detailed information is provided in the instructions for medical use of the drug (see State Register of Medicines, http://grls.rosminzdrav.ru).

2. Route of administration of ravulizumab

The drug must be prepared for administration by a qualified medical professional observing aseptic techniques.

Visually inspect the drug solution for particulate matter and discoloration.

Using a sterile syringe, extract the required amount of solution from the vial(s).

Transfer the recommended dose into an infusion bag.

Dilute the drug to a final concentration of 50 mg/mL (initial concentration divided by 2) by adding the appropriate amount of 0.9% sodium chloride solution** to the infusion bag in accordance with the instructions.

Gently swirl the contents of the infusion bag containing the diluted drug to ensure thorough mixing of the medicinal product and the diluent. The product must not be shaken.

Before administration, the temperature of the diluted drug solution should be brought to room temperature (18–25 °C) by keeping it indoors for approximately 30 minutes.

Do not heat the diluted solution in a microwave oven or using any heat source other than prevailing room temperature.

Discard any unused portion of the drug remaining in the vial, as the medicinal product contains no preservatives.

The prepared solution should be administered immediately after preparation. The infusion must be performed through a 0.2 µm pore-size filter.

If the medicinal product is not used immediately after dilution, storage time must not exceed 24 hours at 2–8 °C or 4 hours at room temperature, taking into account the expected infusion time.

Intravenous push or bolus administration of the drug is prohibited.

The drug solution should be administered only as an intravenous infusion.

The diluted drug solution should be administered via intravenous infusion over approximately 45 minutes using a syringe pump or infusion pump. There is no need to protect the diluted drug solution from light during administration to the patient.

Following drug infusion, the patient should be monitored for one hour. If an adverse reaction occurs during drug infusion, the infusion rate may be reduced or the infusion stopped at the physician's discretion.

Detailed information is provided in the instructions for medical use of the drug (see State Register of Medicines, http://grls.rosminzdrav.ru).

Appendix B. Physician Action Algorithms

1. Algorithm of actions in case of suspected TMA

Note: sCr – blood creatinine level; GCs – glucocorticoids; ravulizumab is not included in the list of vital and essential drugs for medical use (Order of the Government of the Russian Federation No. 2406-r dated October 12, 2019 (as amended on April 16, 2024) "On Approval of the List of Vital and Essential Drugs, as well as Lists of Drugs for Medical Use and Minimum Assortment of Drugs Required for Medical Care")

2. Diagnostic algorithm for aHUS

3. Management algorithm for a patient with atypical hemolytic uremic syndrome

Appendix C. Patient Information

Atypical hemolytic uremic syndrome (aHUS) is a condition belonging to the group of thrombotic microangiopathies, most commonly of hereditary and more rarely of acquired origin, associated with uncontrolled complement system activation. This leads to thrombus formation in small vessels and is accompanied by thrombocytopenia, hemolytic anemia, and damage to vital organs, primarily the kidneys. In most cases, the disease develops following a respiratory or gastrointestinal tract infection, during pregnancy, or upon the use of certain medications. Sometimes the onset of the disease may be preceded by severe arterial hypertension. As a rule, the disease develops rapidly and in some cases can lead to disability or even death of the patient. Therefore, prompt hospitalization in a medical facility is crucial to provide necessary modern therapy and determine further treatment tactics, since after the resolution of the acute episode, the patient requires long-term follow-up by a nephrologist (and other specialists if necessary) and maintenance therapy. Because the disease has a hereditary nature, it is very important to provide the attending physician with information regarding relatives' illnesses, including kidney diseases, cases of myocardial infarction and stroke, and sudden death in the family, especially at a young age. In cases of delayed diagnosis or ineffective therapy during an acute episode, progressive renal failure may develop, leading to the need for renal replacement therapy—hemodialysis and kidney transplantation. When planning a kidney transplantation, genetic testing of the complement system (molecular genetic testing for mutations in factors H, I, B, C3, thrombomodulin, and MCP) is mandatory. Its results are necessary to assess the risk of aHUS recurrence in the transplanted kidney and, if the risk is high, to take appropriate preventive measures.

Appendix G. Assessment Scales, Questionnaires, and Other Evaluation Tools

Specific scales, questionnaires, and other assessment tools for the condition of a patient with aHUS have not been developed.