What is procalcitonin?
Procalcitonin (PCT) is the precursor to the hormone calcitonin (which regulates calcium). In healthy people, PCT levels in the blood are very low. During bacterial infections (especially systemic ones), bacterial toxins and cytokines stimulate PCT production throughout the body — causing levels to rise rapidly within 2-4 hours and peak at 24-48 hours. Viral infections cause only mild or no rise in PCT because viral infections do not trigger the same bacterial-specific immune pathways. This makes PCT the most useful biomarker for distinguishing bacterial from viral infections in clinical practice.
Procalcitonin normal range and interpretation
| PCT Level (ng/mL) | Interpretation | Action |
|---|---|---|
| < 0.1 | Normal | Bacterial infection very unlikely |
| 0.1 – 0.25 | Slightly elevated | Localised bacterial infection possible; antibiotic decision individualised |
| 0.25 – 0.5 | Moderately elevated | Bacterial infection likely; consider antibiotics |
| 0.5 – 2.0 | High | Bacterial infection probable; antibiotics recommended |
| > 2.0 | Very high | Sepsis likely; urgent treatment |
| > 10 | Severely elevated | Severe sepsis / septic shock |
Procalcitonin in sepsis
Sepsis is a life-threatening organ dysfunction caused by the body's dysregulated response to infection. Procalcitonin is one of the most useful sepsis biomarkers. High PCT (>2 ng/mL) combined with clinical signs of infection and organ dysfunction supports a sepsis diagnosis. PCT is also used to guide antibiotic de-escalation: a falling PCT (more than 80% reduction from peak) indicates the infection is responding and antibiotics can be stopped sooner — reducing antibiotic resistance and side effects.
Conditions that can cause false elevation
- Major surgery or trauma (non-infectious PCT rise)
- Severe burns
- Prolonged cardiogenic shock
- Medullary thyroid cancer (rare source of calcitonin/PCT)
- Newborns in the first 48 hours (normal physiological elevation)
Questions to ask your doctor
- Is my infection bacterial or viral?
- Do I actually need antibiotics based on my PCT level?
- Is my PCT falling with treatment?
- Can antibiotics be stopped safely now?