What this test measures
Procalcitonin is a precursor of calcitonin normally produced by thyroid C-cells but, during severe bacterial infection, released ectopically by all parenchymal tissues under the influence of bacterial endotoxin and inflammatory cytokines. PCT rises within 2–6 hours of bacterial infection, peaks at 12–48 hours, and falls with recovery. It is less elevated in viral infections, localised bacterial infections, and most non-infectious inflammation.
Why it matters
Indian critical care and respiratory medicine increasingly use PCT for two purposes: (1) Antibiotic stewardship — low / falling PCT supports stopping antibiotics in pneumonia and sepsis, reducing antibiotic days, resistance, and side effects (PRORATA, ProHOSP, ProACT trials). (2) Risk stratification in sepsis — high PCT predicts more severe disease. PCT also helps distinguish bacterial pneumonia from viral pneumonia and exacerbation of COPD or asthma where antibiotics are often over-prescribed.
How to prepare
No fasting required. Random sample. Disclose recent surgery, trauma, burns, or thyroid medullary carcinoma (raises PCT independent of infection). For antibiotic stewardship, serial PCT (admission, day 3, day 5) is more useful than a single value.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Serum Procalcitonin (ng/mL (µg/L))[1][2] | < 0.05 healthy adults | < 0.25: bacterial infection unlikely — antibiotics often unnecessary in respiratory infections; consider viral aetiology. In sepsis context, very low PCT suggests low likelihood of bacterial sepsis. | 0.25 – 0.5: possible bacterial infection; clinical correlation. 0.5 – 2: likely systemic bacterial infection; consider antibiotics. 2 – 10: bacterial sepsis likely; start broad-spectrum antibiotics. > 10: severe sepsis / septic shock; high mortality risk. |
PCT thresholds in respiratory infection and sepsis
| PCT (ng/mL) | Interpretation | Antibiotic decision |
|---|---|---|
| < 0.1 | Bacterial infection very unlikely | Strongly discourage antibiotics |
| 0.1 – 0.25 | Bacterial infection unlikely | Discourage; observe |
| 0.25 – 0.5 | Possible bacterial infection | Consider antibiotics |
| 0.5 – 2 | Likely bacterial | Encourage antibiotics |
| > 2 | Strong bacterial sepsis evidence | Strongly encourage; consider broad spectrum |
| Falling 50% from peak | Recovery | Consider stopping antibiotics |
Frequently asked questions
Can PCT distinguish bacterial from viral pneumonia?
Yes, modestly. Low PCT (< 0.25) strongly favours viral aetiology and supports withholding antibiotics. High PCT supports bacterial — but neither cut-off is perfect; clinical judgement matters.
Does PCT rise in COVID-19?
COVID-19 typically produces low PCT (< 0.5) unless there is bacterial co-infection. Rising PCT during COVID admission suggests secondary bacterial infection.
Will PCT help me decide when to stop antibiotics?
Yes — major RCTs (PRORATA, ProHOSP) showed PCT-guided antibiotic stopping shortens treatment by 2–3 days without increasing mortality. Surviving Sepsis 2021 endorses PCT for stewardship.
What non-infection causes raise PCT?
Major surgery, severe trauma, burns, cardiogenic shock, thyroid medullary carcinoma, paraneoplastic syndromes, and certain immunotherapy reactions.
Why was CRP not enough?
CRP rises in any inflammation — bacterial, viral, autoimmune, malignancy, post-surgery — making it less specific than PCT for bacterial infection. PCT is preferred when the question is "antibiotics or not".
How fast does PCT respond to treatment?
PCT typically halves every 24 hours with effective antibiotic therapy. A failure to fall by day 3 suggests inadequate source control or wrong drug choice.
Related Cardiac Markers tests
Tests commonly ordered alongside PROCALCITONIN, or that help interpret an unexpected result.
Sources & references
- Surviving Sepsis Campaign 2021 · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Procalcitonin · accessed 2026-05-30T00:00:00.000Z
- IDSA — Procalcitonin in LRTI · accessed 2026-05-30T00:00:00.000Z
- FDA — Procalcitonin clearance · accessed 2026-05-30T00:00:00.000Z
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