Erythrocyte sedimentation rate, C - reactive protein and procalcitonin: When, who and why

Abstract

Inflammatory biomarkers are an integral part of daily clinical practice in paediatric primary care. Although erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and procalcitonin (PCT) are frequently used together, they differ in biological function, response kinetics, and clinical relevance. Awareness of these differences allows a more rational diagnostic approach, reduces unnecessary antibiotic prescribing, and supports optimal management of inflammatory conditions in children.

The distinct kinetics of individual inflammatory markers underlie their clinical utility. Figure 1 presents the characteristic time course (rise and fall) of the most common inflammatory markers after the onset of the inflammatory process and during recovery.

The most common inflammatory markers used in clinical practice are the total and differential leukocyte count (L), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and procalcitonin (PCT).

The values of leukocytes, neutrophils, and lymphocytes are strongly age dependent. Children physiologically have higher leukocyte values, especially up to 5 years of age, and they must always be compared with age-related reference values. Diagnostic algorithms commonly used to evaluate newborns, infants, or children with fever include interpreting total and differential leukocyte counts. Among febrile children, a total leukocyte count < 5 or ≥ 15 x 10⁹/L represents an increased risk of serious infection, although some studies have shown that both the total and differential L counts are not reliable or accurate indicators of serious infection in febrile infants (1).

Erythrocyte Sedimentation Rate

Erythrocyte sedimentation rate (ESR) is the oldest laboratory indicator of inflammation, but it still has its significance in everyday practice. It is blood test that measures how fast red blood cells settle in a blood test tube over one hour and which depends primarily on the concentration of plasma proteins, especially fibrinogen and immunoglobulins (2).

An elevated sedimentation rate indicates the presence of an inflammatory process but is not specific for any condition. It may be elevated in infections, autoimmune diseases, malignancies, anemia and pregnancy, and decreased in polycythemia, hypofibrinogenemia and heart failure. Any condition that affects red blood cells and fibrinogen will affect the ESR value.

The ESR increases within 24-48 hours of the onset of inflammation and slowly decreases as the inflammation resolves. However, an ESR >100 mm/hour requires a search for an underlying etiology. A high ESR has a high specificity: 0.96 for malignant neoplasms, 0.97 for infections and 0.99 as an indicator of disease (3,4).

The main advantage of ESR is its simplicity and availability, while its main limitation is its slow increase and slow normalization after the end of inflammation. Therefore, it has a greater diagnostic value in chronic and rheumatological diseases, where it helps in monitoring disease activity.

C - reactive protein

C-reactive protein (CRP) is the principal acute-phase reactant, synthesized in the liver under the influence of interleukin-6 and other pro-inflammatory cytokines. Following the onset of inflammation, CRP begins to rise within 4–6 hours, doubles approximately every 8 hours, and reaches its peak within 36–50 hours. Its half-life is approximately 19 hours. This rapid kinetic profile makes CRP the most useful biomarker of acute inflammation in primary care settings (5).

Normal CRP values are generally below 2 mg/L, although values up to 10 mg/L may still be considered normal. Levels above 40 mg/L usually suggest a bacterial etiology, whereas lower values are more commonly associated with viral infections or mild inflammatory conditions. Very high CRP concentrations (>200 mg/L) are typically observed in severe bacterial infections, sepsis, or extensive tissue inflammation (5).

In non-hospitalized children, the sensitivity and specificity of CRP for diagnosing serious bacterial infection are approximately 77% and 79%, respectively. Serial CRP measurements provide greater predictive value: an elevated CRP increases the probability of infection by 11%, whereas a normal CRP reduces it by 33%. Serial CRP monitoring may also indicate the appropriateness of antibiotic therapy; for example, if CRP remains unchanged or increases after 48 hours of antibiotic treatment, therapeutic failure should be suspected. Additionally, serial CRP values are useful in neonates with sepsis for assessing antibiotic response within the first 48 hours of therapy, as a decline in CRP during this period shows a sensitivity of 89% and specificity of 90%. CRP levels do not reliably decrease in immunosuppressed patients; therefore, its diagnostic accuracy for severe infection in patients with malignancy and febrile neutropenia is lower compared with biomarkers such as procalcitonin (6).

CRP is the most widely used biomarker of inflammation. It is easy to perform, available in most laboratories, inexpensive, and straightforward to interpret.

Procalcitonin

Procalcitonin (PCT) is the precursor of the hormone calcitonin, which under physiological conditions is produced by the parafollicular (C) cells of the thyroid gland. In the presence of systemic bacterial infection, PCT is also released from other tissues (lungs, intestines, pancreas) under the influence of bacterial endotoxins and cytokines such as TNF-α and IL-6. In contrast, during viral infections, its production is suppressed by interferon-γ (7).

Unlike CRP, PCT does not increase significantly in viral infections or non-infectious inflammatory conditions, making it a highly specific marker for bacterial infections and sepsis.

Normal serum PCT concentrations are below 0.05 ng/mL. PCT levels rise within 3–4 hours after infection onset and peak within 6–24 hours. Elevated PCT values are not observed in inflammatory conditions such as systemic lupus erythematosus, gout, juvenile rheumatoid arthritis, or inflammatory bowel disease, but transient increases may occur following massive trauma, such as severe burns. PCT values below 0.05 ng/mL are considered normal, whereas levels above 0.5 ng/mL suggest bacterial infection. Concentrations exceeding 2 ng/mL strongly indicate sepsis or severe systemic infection, while values above 10 ng/mL are typically associated with septic shock (2).

The kinetic profile of PCT is characterized by a rapid increase (within 3–4 hours) following exposure to bacterial endotoxins and a short elimination half-life of approximately 24 hours. This makes PCT particularly useful for (a) early identification of bacterial sepsis, (b) assessment of infection severity, and (c) guiding antibiotic therapy and monitoring treatment response (6,8).

A daily reduction of PCT levels by approximately 50% indicates a favorable therapeutic response (8).

Special Considerations in the Pediatric Population

In children, ESR demonstrates (a) lower reference values than in adults (Table 2), (b) less sensitive to inflammation than CRP, and (c) ESR is strongly influenced by hematologic parameters—particularly anemia and hematocrit (10).

CRP does not show major age-related differences except during the first 24–48 hours of life (Table 3). Neonates may exhibit falsely elevated CRP levels due to delivery, stress, or perinatal asphyxia; therefore, a single CRP measurement is insufficient for therapeutic decision-making. In pediatric patients, CRP effectively differentiates bacterial from viral infections but is less specific than PCT (11).

For PCT, age specificity is particularly important (Table 4). PCT should not be interpreted as a marker of sepsis within the first 72 hours of life without serial measurements. Beyond the neonatal period, PCT represents the most reliable marker of bacterial infection and sepsis in children, as it rises and falls more rapidly than CRP and is not elevated in viral infections (12).

Clinical Application of Inflammatory Biomarkers in Primary Care

Mild respiratory infections: CRP alone is usually sufficient; values below 40 mg/L generally suggest a viral etiology.

Bronchitis and pneumonia: CRP and, if necessary, PCT may aid in decisions regarding antibiotic therapy.

Urinary tract infections: In febrile patients, PCT may help differentiate pyelonephritis from cystitis.

Autoimmune diseases: A combination of ESR and CRP is optimal for monitoring disease activity.

Chronic infections (e.g., endocarditis, tuberculosis): ESR often remains elevated even after CRP normalization.

Suspected sepsis: Combined measurement of CRP and PCT provides the greatest diagnostic and prognostic value.

Conclusion

ESR, CRP, and PCT are not competing but complementary inflammatory markers. ESR remains useful in the assessment of chronic and systemic diseases, CRP is the gold standard for rapid detection and monitoring of acute inflammation, while PCT is the most valuable marker for bacterial sepsis and rational antibiotic therapy.

Appropriate indication and interpretation of these markers, particularly in the context of clinical presentation and symptom duration, enable more rational antibiotic prescribing, earlier detection of serious infections, and improved collaboration between primary care and laboratory medicine.