Procalcitonin (PCT): The Bacterial Infection Blood Test Explained
Procalcitonin rises specifically in bacterial infections and sepsis, and helps decide when antibiotics can be stopped. Learn cut-offs, uses, and limits.
Procalcitonin below 0.25 ng/mL usually excludes clinically significant bacterial infection; between 0.25 and 0.5 ng/mL suggests possible localised infection; above 0.5 ng/mL supports bacterial infection; above 2.0 ng/mL is highly suggestive of sepsis. Trending PCT is used to safely stop antibiotics in respiratory infections and sepsis (a 80% drop or fall below 0.5 ng/mL).
Procalcitonin is a peptide precursor of calcitonin. In healthy individuals it is virtually undetectable, but bacterial toxins and endotoxin drive systemic PCT release. Viral infections and non-infectious inflammation raise it far less than bacterial infections — making PCT more bacteria-specific than CRP. PROCAP, PRORATA and STOP-IT trials showed PCT-guided antibiotic therapy safely reduces duration. Renal failure and burns raise PCT independently of infection; some tumours (medullary thyroid) release it constitutively. PCT is NOT a screening test for occult sepsis — it complements clinical assessment.
- Bacterial infection unlikely
- < 0.25 ng/mL
- Possible infection
- 0.25 – 0.5 ng/mL
- Bacterial infection likely
- > 0.5 ng/mL
- Sepsis suggestive
- > 2.0 ng/mL
- Best clinical use
- Antibiotic duration guidance
Where PCT changes practice
Respiratory tract infection: PCT can safely reduce antibiotic prescriptions by 25–40% without worsening outcomes. Guidance: withhold antibiotics if PCT < 0.25 ng/mL and clinical picture allows; stop antibiotics when PCT drops by 80% or below 0.5 ng/mL.
Sepsis and septic shock: baseline PCT + serial values guide antibiotic de-escalation and cessation. Persistent high PCT despite appropriate therapy points to inadequate source control.
PCT categories and typical action
| PCT (ng/mL) | Category | Action |
|---|---|---|
| < 0.1 | Very low | Bacterial infection very unlikely |
| 0.1 – 0.25 | Low | Bacterial infection unlikely; consider withholding antibiotics |
| 0.25 – 0.5 | Grey zone | Localised bacterial infection possible |
| 0.5 – 2.0 | Elevated | Bacterial infection likely |
| > 2.0 | High | Sepsis suggestive; treat aggressively |
| > 10 | Very high | Severe sepsis / septic shock |
Non-infective causes of raised PCT
Renal failure raises baseline PCT modestly. Major surgery, trauma, cardiac arrest, and burns produce transient PCT elevation without infection. Medullary thyroid carcinoma and small-cell lung cancer can release PCT ectopically. Autoimmune flares raise PCT less than bacterial infection.
- PCT > 2 ng/mL with fever, tachycardia, hypotension, or confusion — sepsis workup.
- Rising PCT despite antibiotics — inadequate source control (abscess, empyema, endocarditis).
- Very high PCT + no obvious source — search for endocarditis, deep abscess, meningitis.
- Persistent high PCT in ICU — reassess antibiotic spectrum and search for complications.
PCT workup
- 1Respiratory infection, PCT < 0.25?Consider withholding antibiotics; reassess clinically.
- 2PCT > 2 with clinical sepsis?Sepsis bundle: cultures, broad-spectrum antibiotics, fluid resuscitation, source control.
- 3PCT rising on antibiotics?Look for abscess, empyema, endocarditis, resistance.
- 4PCT falling ≥ 80% or < 0.5?Consider stopping antibiotics if clinically improving.
Related questions people ask
- Is PCT more accurate than CRP for infection?
- What is the PCT cut-off for sepsis?
- Does viral infection raise PCT?
- How does WBC compare?
- What is ESR's role?
- Can PCT stop antibiotics?
- Does kidney disease affect PCT?
Frequently asked questions
- PCT is more bacteria-specific than CRP.
- < 0.25 = usually no bacterial infection.
- > 2 ng/mL suggests sepsis.
- Falling PCT guides antibiotic cessation.
- Renal failure, surgery, burns raise baseline PCT.
- PCT complements — not replaces — clinical assessment.
References
3 sources- SSCSurviving Sepsis Campaign Guidelines
Sepsis management standard.
sccm.org
- IDSADiagnosis and Treatment of Adults With CAP
CAP guideline with PCT stewardship.
idsociety.org
- NICE NG51Sepsis: recognition, diagnosis and early management
UK sepsis pathway.
nice.org.uk
Evidence Snapshot
This snapshot shows verified evidence records matched to this article. It is not a diagnosis or personal medical advice.
Serum procalcitonin tends to rise in bacterial systemic infections and remains relatively lower in most viral infections, so it is used as one input in evaluating suspected bacterial infection.
Limitation. Procalcitonin can also be elevated in non-infectious conditions such as major surgery, trauma and some cancers.
Procalcitonin is not used on its own to start or cease antibiotic therapy; treatment decisions integrate clinical assessment, microbiology, imaging and other laboratory data.
Limitation. The performance of procalcitonin-guided algorithms varies between settings, populations and specific infection syndromes.
A blood culture is a laboratory test in which a blood sample is incubated in nutrient-containing media to detect the presence and identity of bacteria or fungi in the bloodstream.
Limitation. Blood cultures take time to become positive and can be affected by prior antibiotic exposure.
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