Procalcitonin in Laboratory Diagnostics

Place in the inflammatory marker panel

In modern laboratory practice, procalcitonin is evaluated in conjunction with other acute-phase reactants, which include C-reactive protein (CRP) and interleukins (IL-6, IL-8). The main feature of these biomarkers is that they can significantly increase with the development of a severe infection, primarily bacteremia.

However, it is important for the practicing physician to remember that an increase in levels does not always occur. In situations where a viral or fungal infection causes the systemic response, inflammatory biomarkers may show weak dynamics or fail to respond to the pathological process altogether. It is this phenomenon that underlies the use of this indicator as a differential criterion.

Standard admission diagnostics

To obtain an objective clinical picture, all key tests must be performed directly upon the patient's admission to the hospital, and regularly evaluated dynamically.

Procalcitonin is part of the general therapeutic biochemical blood test. The mandatory diagnostic panel includes the following parameters:

• Assessment of renal function: creatinine with mandatory calculation of the glomerular filtration rate (GFR).
• Assessment of hepatic function: aspartate aminotransferase (AST) and alanine aminotransferase (ALT).
• Inflammatory markers: C-reactive protein and procalcitonin.
• Electrolyte status: potassium, sodium, chlorides.

In addition to extended biochemistry, comprehensive laboratory diagnostics is always supplemented by a complete (clinical) blood count and a general (clinical) urinalysis.

Differential diagnosis of lung lesions

One of the most important tasks of pulmonology and critical care medicine is the differential diagnosis between primary viral lung injury and the development of secondary, i.e., bacterial, pneumonia.

A specific laboratory picture develops in viral pneumonias (including SARS-CoV-2) and acute interstitial pneumonia (AIP), which frequently occurs against the background of systemic connective tissue diseases. Such underlying pathologies include rheumatoid arthritis, systemic lupus erythematosus, and so-called "methotrexate lung".

These clinical situations are characterized by marker dissociation:

• There is a pronounced leukocytosis.
• A high CRP level is diagnosed.
• Important: the procalcitonin level fundamentally remains within normal limits.

The absence of an increase in this indicator argues against the addition of a bacterial component.

Clinical significance during neutropenia

Special caution is required when interpreting laboratory data in patients with neutropenia. Immunosuppression imposes significant limitations on decision-making algorithms.

First, an isolated increase in biomarker levels (including PCT) is not an independent basis for prescribing new antibiotics or changing current antibacterial therapy, provided the clinical situation remains stable.

Secondly, in clinical practice, it is not recommended to order procalcitonin and associated markers during the first few hours after the onset of fever. The main reason for this limitation lies in management protocols: in the event of febrile neutropenia, empiric antibiotics are prescribed by the physician immediately in any case. Consequently, the blood biomarker concentration at this acute moment cannot influence the initial choice of antibacterial drug.

Limitations of laboratory diagnostics

Despite the high significance of inflammatory markers in systemic infections, there are diseases in which their routine use is ineffective.

For example, in coxarthrosis (CA), the "gold standard" of laboratory diagnostics is absent. This is due to the extremely low concentration of specific biochemical compounds in blood and urine, as well as the overlap of parallel pathological processes in other joints. In such cases, only a standard panel is recommended, including a complete blood count (with ESR assessment) and C-reactive protein testing (recommendation grade C). The purpose of these tests is to assess the condition of internal organs and perform differential diagnosis, rather than to search for an infectious agent.

Additionally, specific tasks require targeted studies. For instance, patients with lymphoproliferative diseases and those on long-term glucocorticoid therapy are indicated for direct determination of cytomegalovirus (CMV) DNA (CMV) in the blood, since general markers do not reflect viral activity.

Class and exam questions

What else to study on this topic

• Pathogenesis of the systemic inflammatory response in severe bacteremia.
• Differential diagnosis of acute interstitial pneumonia in systemic connective tissue diseases.
• The role of interleukins (IL-6, IL-8) in the development of cytokine storm.
• Principles of empirical antibacterial therapy in febrile neutropenia.
• Laboratory monitoring of renal function: dynamic calculation of the glomerular filtration rate (GFR).

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.