Procalcitonin (PCT) is used as a biomarker to guide antibiotic stewardship in respiratory infections. What is the biological basis for elevated PCT in bacterial sepsis compared to viral respiratory infections?

• A PCT is a direct cleavage product of bacterial exotoxins and is absent in viral infections
• B PCT is produced exclusively by C-cells of the thyroid in response to calcitonin gene stimulation by bacterial LPS
• C Viral infections trigger IL-6-mediated acute phase response that downregulates PCT synthesis in the liver
• D In bacterial infection, lipopolysaccharide (LPS) and other bacterial PAMPs induce systemic PCT production by multiple tissues (liver, lung, kidney), whereas type I interferons (IFN-α/β) produced in viral infections specifically suppress PCT production ✓

Explanation

Procalcitonin (PCT, the precursor of calcitonin) is normally synthesised only by C-cells of the thyroid. In bacterial infection, microbial cell wall components (LPS, LTA) and cytokines (TNF-α, IL-6, IL-1β) induce ubiquitous expression of PCT in nearly all tissues (liver, lung, kidney, adipocytes, monocytes). PCT levels rise dramatically within 4–6 hours of bacterial infection (half-life ~24 hours). Critically, type I interferons (IFN-α/β), which are robustly produced in viral infections, specifically suppress PCT induction — explaining why PCT remains low (<0.1 ng/mL) in uncomplicated viral infections. PCT-guided algorithms using thresholds of 0.25–0.5 ng/mL allow safe antibiotic withholding/early discontinuation in lower respiratory tract infections, reducing antibiotic exposure.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.