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Procalcitonin (PCT) 🧪
Procalcitonin (PCT) is the 116–amino-acid precursor of calcitonin. In health, it is produced in thyroid C-cells and rapidly cleaved, so circulating levels are negligible. In systemic bacterial infection, however, PCT is synthesised ubiquitously by parenchymal tissues (lung, liver, kidney, adipose), making it a useful biomarker of bacterial-driven inflammation.
🧠 Physiology
Bacterial endotoxin and pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) upregulate CALC-1 gene expression outside the thyroid, leading to a rapid rise in circulating PCT. Crucially, interferon-γ—prominent in viral infections and autoimmune disease—suppresses PCT production. This physiological “brake” explains why PCT tends to remain low in viral illness and sterile inflammation, unlike CRP.
⏱️ Kinetics
- Rise: detectable within 4–6 hours of bacterial insult
- Peak: ~12–24 hours
- Half-life: ~24 hours (falls predictably if source control achieved)
- Clinical implication: serial measurements are more informative than a single value
🩺 Clinical Uses
- Sepsis & ICU: Supports early recognition of bacterial sepsis and antibiotic stewardship; falling PCT supports de-escalation.
- Respiratory infection: Helps distinguish bacterial pneumonia from viral/LRTI phenotypes when used with clinical assessment.
- Rheumatology: In conditions like SLE, CRP may rise during flares, but PCT usually remains low; a raised PCT should prompt infection work-up.
⚖️ Interpreting PCT with CRP
- High CRP + Normal PCT: Likely autoimmune or sterile inflammation
- High CRP + High PCT: Suggests bacterial infection
- Normal CRP + High PCT: Early bacterial infection or non-inflammatory context
🧬 Pathology & Pitfalls
- False negatives: Localised infections (e.g. abscess, osteomyelitis)
- False positives: Major surgery, trauma, cardiogenic shock, severe burns
- Renal impairment: Reduced clearance may modestly elevate baseline levels
🇬🇧 UK Context & Teaching Pearl
In the NHS, PCT is most useful when embedded in protocol-driven pathways (e.g. ICU sepsis bundles) rather than as a stand-alone test. It should never override clinical judgement, cultures, or imaging.
📌 Makindo pearl: CRP tells you there is “fire” (inflammation). PCT helps decide whether the “smoke” is bacterial. Use both, trend them, and always interpret in clinical context.
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