Procalcitonin (PCT) Test: A Biomarker for Bacterial Infection and Antibiotic Stewardship
The Procalcitonin (PCT) test is a biomarker that helps distinguish bacterial infections from viral or non-infectious causes of inflammation, and guides antibiotic therapy decisions in critically ill patients. It has superior diagnostic accuracy for sepsis compared to other inflammatory markers like C-reactive protein (CRP) 1.
What is Procalcitonin?
Procalcitonin is a precursor protein of calcitonin that can be produced throughout the body. Key characteristics include:
- Released 3-4 hours after an inflammatory stimulus
- Peaks within 6-24 hours
- Half-life of 22-35 hours 2
- Normal values in healthy individuals: <0.05 ng/mL 1
Clinical Interpretation of PCT Levels
PCT levels correlate with infection severity and decrease rapidly after effective antibiotic treatment 1:
| PCT Level | Clinical Interpretation |
|---|---|
| <0.1 ng/mL | High probability of viral infection or non-infectious condition |
| 0.1-0.25 ng/mL | Low probability of bacterial infection |
| 0.25-0.5 ng/mL | Possible bacterial infection |
| >0.5 ng/mL | High probability of bacterial infection |
| >2.0 ng/mL | High probability of sepsis or severe bacterial infection |
Diagnostic Accuracy
PCT demonstrates better diagnostic accuracy for sepsis than CRP 2:
PCT: Overall area under SROC curve 0.85 (95% CI, 0.82-0.88)
- Sensitivity: 0.80 (95% CI, 0.69-0.87)
- Specificity: 0.77 (95% CI, 0.60-0.88)
CRP: Overall area under SROC curve 0.73 (95% CI, 0.69-0.77)
- Sensitivity: 0.80 (95% CI, 0.63-0.90)
- Specificity: 0.61 (95% CI, 0.50-0.72)
Clinical Applications
When to Use PCT Testing
- In critically ill patients with new fever and no clear focus of infection when probability of bacterial infection is deemed low to intermediate 2
- For monitoring response to antibiotic therapy 1
- For guiding antibiotic discontinuation decisions 3
When NOT to Use PCT Testing
- To rule out bacterial infection when clinical probability is high 2
- As the sole determinant for initiating antibiotics in suspected sepsis 2
- For routine use in all sepsis cases due to cost and availability issues 2
Advantages Over Other Biomarkers
PCT offers several advantages over CRP 1, 4:
- Earlier rise after infection onset (4 hours vs. 12-24 hours for CRP)
- More rapid return to normal levels
- Higher specificity for bacterial infections
- Less affected by neutropenia, immunodeficiency, and NSAIDs 2
Limitations and Considerations
- PCT can be elevated during severe viral illnesses, including influenza and COVID-19 1
- Levels are influenced by renal function and renal replacement therapy 2
- Should always be interpreted in conjunction with clinical assessment 1
- Serial measurements showing trends are more valuable than single measurements 1
Antibiotic Stewardship Applications
PCT-guided antibiotic therapy has been shown to:
- Reduce antibiotic exposure in hospitalized patients 3
- Improve survival and decrease antibiotic treatment duration in ICU patients 2
- Support safe antibiotic discontinuation when levels decrease by ≥80% from peak or to <0.25 ng/mL 1
PCT testing represents a valuable tool in the clinician's arsenal for diagnosing bacterial infections and optimizing antibiotic therapy, particularly in critically ill patients, but should always be used as a complement to thorough clinical assessment rather than as a standalone test.