Procalcitonin

Function

Procalcitonin (PCT) is a peptide precursor of the hormone calcitonin. In healthy individuals, PCT is produced only in the C-cells of the thyroid gland. However, during a severe systemic bacterial infection, inflammatory cytokines trigger the nearly universal expression of the PCT gene in various tissues throughout the body, leading to a rapid rise in blood levels. Interestingly, viral infections typically stimulate interferon-gamma, which actually inhibits the production of PCT, making this test a highly specific marker for bacterial vs. viral etiology.

Why it is Ordered

PCT is a critical biomarker in acute care settings (Emergency Department and ICU). It is ordered to:

• Assist in the diagnosis of bacterial sepsis and septic shock.
• Distinguish between bacterial and viral pneumonia.
• Guide 'Antibiotic Stewardship'—deciding when to start or, more importantly, when to safely discontinue antibiotics in respiratory infections or sepsis.
• Monitor the response to treatment in patients with severe infections.

Associated Conditions

• Sepsis: High levels (>2.0 ng/mL) indicate a high risk of systemic infection and organ dysfunction.
• Bacterial Pneumonia: Elevated PCT helps clinicians decide if a patient with a cough and fever needs antibiotics.
• Meningitis: Used to differentiate bacterial meningitis (high PCT) from viral meningitis (low/normal PCT).
• Secondary Infections: Monitoring patients after major surgery or trauma for the onset of bacterial complications.