Complete blood count

CBC in infectious processes (using COVID-19 as an example)

In clinical practice, the complete blood count (CBC) serves as the primary tool when a severe viral infection is suspected. This test is ordered for absolutely all patients with a presumed diagnosis.

The main goal of testing is not merely to document the fact of inflammation, but to comprehensively assess the severity of the condition in vivo. This enables the clinician to timely notice the development of dangerous complications or emergency conditions. To form a complete picture, the physician requires the following mandatory parameters:

• Total erythrocyte count and hematocrit.
• Platelet count.
• Total leukocyte count.
• Detailed differential count, known as the leukocyte differential.

Interestingly, at the peak of the disease, classic inflammatory markers behave specifically. For example, the erythrocyte sedimentation rate (ESR) often remains within the reference range or paradoxically decreases. The general nonspecific picture may show a tendency toward lymphocytosis; however, a detailed clinical analysis of patients with COVID-19 demonstrates a different pattern.

In the majority of infected individuals, the total leukocyte count does not exceed reference values, but approximately one-third of patients develop leukopenia. The most characteristic sign of the pathology is considered to be lymphopenia, which is recorded in 83.2% of examined patients. Moderate thrombocytopenia is also observed in the analysis — its progression is an extremely alarming sign, as a more pronounced drop in blood platelet levels is typical for severe disease courses and is often found in deceased patients.

The role of the hemogram in pathologic fractures

In addition to infectious diseases, the cellular composition of blood plays a crucial differential-diagnostic role in traumatology and endocrinology. This is especially relevant when examining patients with pathologic bone fractures. In this situation, a routine test helps draw the line between classic osteoporosis and other, more dangerous metabolic osteopathies.

The key rule for interpreting results: in uncomplicated osteoporosis, complete blood count parameters, as well as urinalysis, show no deviations from the norm.

However, if during the analysis interpretation the physician detects an elevated ESR or any other, even minor, deviations in the leukocyte differential, the initial diagnosis is called into question. Such hematological shifts require the immediate exclusion of severe bone marrow pathology. In such cases, the clinician is obliged to broaden the diagnostic search so as not to miss a hidden oncological process or secondary bone tissue involvement.

CBC in the context of comprehensive monitoring

A clinical blood count is rarely evaluated in isolation. To complete the picture, its results are always compared with biochemical general therapeutic blood test indicators.

In severe infectious processes against the background of changes in the leukocyte pool, protein-synthesizing function often suffers. In laboratory findings, this manifests as a decreased albumin concentration and a drop in the prothrombin index (PTI), although the thymol turbidity test usually remains normal. Additionally, physicians monitor the levels of glucose, ferritin, urea, liver transaminases (ALT, AST), and key electrolytes (sodium, potassium, chlorides).

In the case of a differential search for the causes of fractures, the baseline hemogram is supplemented by an assessment of renal function. For this purpose, the creatinine level is determined with mandatory calculation of the glomerular filtration rate (GFR). In parallel, the parathyroid hormone (PTH) level is investigated to exclude hypo- or hyperparathyroidism of various origins, and daily calcium and phosphorus excretion is evaluated. The detection of hypocalcemia forces the physician to rule out osteomalacia, and hypercalcemia — hyperparathyroid osteodystrophy.

It is important to remember the key postulate of therapy: upon completion of any treatment course, a follow-up clinical blood count is performed for the patient. All blood parameters of a recovering individual must strictly return to physiological limits.

Class and exam questions

What else to study on this topic

• Assessment of the liver's protein-synthesizing function: albumin and prothrombin index.
• Differential diagnosis of metabolic osteopathies: the role of PTH and GFR calculation.
• Personalized laboratory monitoring of calcium and vitamin D therapy.
• Assessment of calcium homeostasis: blood ionized calcium and daily urinary excretion.
• Expanded protein profile: albumin-globulin ratio and capillary electrophoresis.

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