Procalcitonin (PCT) Test

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

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?

Frequently Asked Questions

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Procalcitonin. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK539794

References

Sources cited on this page. PubMed links open the original abstract.

1. Schuetz P, Müller B, Christ-Crain M, et al. Procalcitonin to initiate or discontinue antibiotics in acute respiratory tract infections. Cochrane Database Syst Rev. 2012;9:CD007498. PMID 22512993 · doi:10.1002/14651858.CD007498.pub2

Why procalcitonin is more specific for bacterial infection than CRP

C-reactive protein (CRP) rises in both bacterial and viral infections, as well as in non-infectious inflammation (trauma, surgery, autoimmune disease). Procalcitonin (PCT) has better specificity for bacterial infection because of its production pathway: normally, PCT is produced exclusively in C-cells of the thyroid gland. During bacterial sepsis, however, virtually every cell type in the body can produce PCT in response to bacterial endotoxin (lipopolysaccharide) and inflammatory cytokines – particularly IL-1β and TNF-α. Viral infections trigger interferon-gamma, which actively suppresses PCT production. This divergent regulation means PCT rises dramatically in bacterial infection (often above 10 µg/L in severe sepsis) while remaining below 0.25 µg/L in most viral infections.

PCT thresholds and their clinical applications

• PCT below 0.1 µg/L: Bacterial infection very unlikely. Consider withholding antibiotics or stopping them in patients who are improving. Particularly useful in community-acquired respiratory tract infections – NICE recommends PCT-guided antibiotic prescribing in LRTI (lower respiratory tract infection) to reduce unnecessary antibiotic use.
• PCT 0.1–0.25 µg/L: Bacterial infection unlikely. Clinical context determines antibiotic decision.
• PCT 0.25–0.5 µg/L: Bacterial infection possible. Consider antibiotics based on full clinical picture.
• PCT above 0.5 µg/L: Bacterial infection likely. Antibiotic treatment recommended.
• PCT above 2 µg/L: Systemic bacterial infection or sepsis probable. Urgent treatment and source identification required.
• PCT above 10 µg/L: Severe sepsis or septic shock. High mortality risk – escalate care immediately.

Monitoring antibiotic therapy – the PCT kinetics approach

In confirmed sepsis, serial PCT measurement guides antibiotic de-escalation. A PCT that falls by more than 80% from its peak, or falls to below 0.5 µg/L, is a reliable indicator that the infection is controlled and antibiotics can be safely stopped. The PRORATA and SAPS trials demonstrated that PCT-guided antibiotic cessation significantly reduces antibiotic duration without worsening outcomes – an important strategy in reducing antibiotic resistance and adverse drug effects. Most intensive care units now incorporate PCT into sepsis management bundles.