What the procalcitonin test shows
Procalcitonin (PCT) is the precursor of the hormone calcitonin, made by the CALC-1 gene. Normally only the thyroid’s C-cells make it, converting nearly all to calcitonin, so blood levels stay tiny — under 0.05 ng/mL. In serious bacterial infection that flips: bacterial toxins and signals such as interleukin-6 and TNF-alpha switch the gene on across many tissues, which pour procalcitonin into the blood. The level then tracks how bacterial and how systemic the illness is, as MedlinePlus explains.
What makes it useful is what does not raise it: viral infections trigger interferon-gamma, which suppresses procalcitonin, so PCT stays low in viral illness and autoimmune flares. That specificity sets it apart from CRP and ESR, which rise with almost any inflammation; PCT also moves faster, rising within 6–12 hours and easing within a day once controlled. Do not confuse it with calcitonin itself, a thyroid-tumor marker: the sepsis surge is never converted to calcitonin, so your calcium is unaffected.
Procalcitonin normal range
Procalcitonin is reported in ng/mL, numerically identical to the SI unit µg/L (1 ng/mL = 1 µg/L), so US and European reports agree. It does not split by adult sex or age — a healthy 25- and 75-year-old share the same low baseline; newborns are the exception, surging in their first 2–3 days. What matters is the band your value falls into:
| Level, ng/mL (= µg/L) | What it usually suggests |
|---|---|
| < 0.10 | Normal; systemic bacterial infection very unlikely |
| 0.10–0.25 | Bacterial infection unlikely |
| 0.25–0.50 | Local bacterial infection possible |
| 0.50–2.0 | Systemic infection or sepsis possible — read in context |
| 2.0–10 | High risk of sepsis or severe bacterial infection |
| > 10 | Almost always severe sepsis or septic shock |
A value under 0.10 ng/mL has a high negative predictive value for bacterial infection; Cleveland Clinic Laboratories reads under 0.5 ng/mL as low sepsis risk and over 2.0 ng/mL as high, and StatPearls notes most decisions hinge on the 0.1–0.5 ng/mL window. Cut-offs shift with the setting and assay — read your result against your own report.
Why procalcitonin is high
A raised procalcitonin points above all to bacterial infection, and its height tracks how systemic it is. Roughly by frequency:
- Bacterial infection and sepsis — the reason the test exists. A localized infection nudges it up; bacteremia and sepsis reach single or double digits, and septic shock the highest values.
- Major tissue insult: major surgery (especially cardiac bypass), severe trauma, extensive burns, pancreatitis and cardiogenic shock, for a few days.
- Severe kidney impairment: slower clearance lifts the baseline, so values are read cautiously in advanced kidney disease and dialysis.
- Rare causes: newborns in their first 2–3 days, medullary thyroid carcinoma, small-cell lung cancer, severe malaria and heat stroke.
When is it urgent? A high procalcitonin with signs of sepsis — fever, fast heart rate or breathing, confusion or low blood pressure — is a medical emergency, because sepsis is time-critical and antibiotics work best within the first hour. Above 10 ng/mL strongly suggests septic shock and needs immediate hospital care.
Why procalcitonin is low
A low or undetectable procalcitonin is the normal, healthy result — here reassuring, not a problem to fix. Its value is what it rules out:
- No systemic bacterial infection. A low PCT makes sepsis unlikely, so it is used to withhold or safely stop antibiotics — one tool in antibiotic-stewardship protocols. Whether it should be used routinely for this is still debated: NICE has so far found the evidence insufficient to recommend routine adoption, and large trials have been mixed.
- A likely viral cause. Because viruses keep procalcitonin low, a low value during a fever favors a virus and supports not prescribing antibiotics.
A low result does not fully exclude infection, though: it can read low in the first hours before PCT rises, or when infection is walled off, such as an abscess or early endocarditis. So it is weighed against symptoms; the NEJM ProACT trial is a reminder that procalcitonin alone did not cut antibiotic use without solid protocols around it.
What to test alongside
Procalcitonin is read as part of an infection work-up, not alone:
- CRP — the everyday inflammation marker; slower and less specific, but ordered alongside PCT.
- Interleukin-6 — the upstream cytokine that drives procalcitonin; it rises earliest and features in some sepsis protocols.
- Creatinine — kidney function, which affects the PCT baseline and is hit early in sepsis.
- ALT and AST — liver enzymes that flag organ involvement in severe infection.
- Ferritin — another acute-phase protein; very high values raise concern for a cytokine storm.
The panel’s autoimmune companions — rheumatoid factor, anti-CCP and ASO — answer a different question, and PCT stays low when autoimmune disease, not infection, is the cause.
What to do about an abnormal result
- Read it in context. Procalcitonin gauges how likely and how severe a bacterial infection is; it is not a diagnosis on its own.
- Act fast on sepsis signs. A high value with the sepsis signs above needs same-day emergency care — call for urgent help, don’t wait for a repeat.
- A high result with no clear infection prompts a search for a non-infectious cause — surgery, trauma, burns, kidney impairment — and a repeat, since a falling value suggests resolution.
- A low or normal result is reassuring, but keep watching symptoms and seek review if you feel worse, since early or localized infection can read low.
- See the right clinician: the hospital team, or your primary-care doctor, who decides whether the picture is bacterial and needs antibiotics or viral and does not — never start or stop antibiotics yourself.
Mini-FAQ
What does a high procalcitonin level mean?
A raised procalcitonin usually points to a bacterial infection, and the higher it climbs the more likely that infection is systemic — above about 0.5 ng/mL raises concern for sepsis and above 2 ng/mL for severe sepsis or septic shock. Surgery, major trauma, burns and severe kidney impairment can also push it up without infection.
How is procalcitonin different from CRP?
Both rise with inflammation, but procalcitonin is fairly specific for bacterial infection and stays low in viral illness and autoimmune flares, whereas CRP rises with all of them. Procalcitonin also climbs and falls faster, which makes it useful for deciding when antibiotics can be stopped.
What is a normal procalcitonin level?
In healthy people procalcitonin is very low, usually under 0.1 ng/mL and often under 0.05. A value under 0.1 ng/mL makes a systemic bacterial infection unlikely, though it does not completely rule out an early or walled-off infection.
Can procalcitonin tell viral from bacterial infection?
It helps but is not perfect. Viral infections usually keep procalcitonin low, so a low value during a fever favors a viral cause and supports holding antibiotics, while a high value points toward bacteria — always read alongside your symptoms and other tests.
Does a low procalcitonin mean I don’t need antibiotics?
Often it supports not starting antibiotics, or safely stopping them, which is how procalcitonin is used in antibiotic-stewardship programs. But the decision is your doctor’s and depends on the whole picture, because very early or localized infections can still show a low level.
