Procalcitonin (PCT)
Procalcitonin is a serum marker that rises rapidly in bacterial infection and sepsis but stays low in viral infection — useful for antibiotic-stewardship decisions.
What it measures
Procalcitonin is a precursor of the hormone calcitonin. In health it is negligible; during a severe bacterial infection multiple non-thyroid tissues start secreting PCT directly into blood within 4–6 hours of insult, often peaking 12–24 h before classic markers. Viral infections, autoimmune flares and most localised bacterial processes do NOT trigger this response. That separation makes PCT one of the best single biomarkers for distinguishing bacterial sepsis from viral or sterile inflammation, and for guiding antibiotic start/stop decisions in pneumonia, sepsis and post-operative fever.
What a high value can mean
- >0.5 ng/mL — bacterial infection likely; >2 strongly suggests sepsis.
- >10 ng/mL — severe sepsis or septic shock — emergency.
- Major surgery / trauma — transient PCT rise in first 24–48 h without infection.
- End-stage renal disease — mildly elevated baseline (clearance reduced).
- Some neuroendocrine tumours (medullary thyroid cancer) — chronic elevation.
What a low value can mean
- Viral infection — PCT typically stays <0.25 even with high fever.
- Localised bacterial infection — UTI, skin abscess often <0.5.
- Healthy state — <0.1 ng/mL.
When to discuss with a doctor
PCT is an inpatient / emergency marker, not a routine outpatient panel. A PCT >0.5 on a discharged patient's report should be reviewed by the prescribing clinician within 24 h — most often it reflects an infection that's already on antibiotics. Mediora.AI flags the value with its bacterial-sepsis threshold; do not stop or start antibiotics based on PCT alone.