What is procalcitonin (PCT) used for?

Procalcitonin: Clinical Applications

Procalcitonin (PCT) is primarily used to differentiate bacterial from viral infections and to guide antibiotic therapy decisions—specifically for determining when to discontinue antibiotics in patients with respiratory infections and sepsis. 1

Primary Clinical Uses

1. Distinguishing Bacterial from Viral Infections

• PCT rises within 4 hours of bacterial infection exposure, reaching peak levels at 6-8 hours, making it superior to other inflammatory markers for distinguishing bacterial from non-bacterial causes of inflammation. 2, 3
• PCT has 77% specificity for bacterial infections compared to CRP's 61%, with normal values <0.05 ng/mL in healthy individuals. 1
• The Society of Critical Care Medicine suggests measuring PCT in critically ill patients with new fever and no clear infection focus when bacterial infection probability is low-to-intermediate. 1
• However, PCT sensitivity ranges from 38-91% for detecting bacterial infection, meaning it cannot be used alone to withhold antibiotics from patients with suspected bacterial infections like community-acquired pneumonia. 4

2. Antibiotic Stewardship and Duration Guidance

• The American College of Critical Care Medicine recommends PCT-guided antibiotic therapy, with PCT levels <0.5 μg/L or decreases of ≥80% from peak levels guiding antibiotic discontinuation in stabilized ICU patients. 1
• The Surviving Sepsis Campaign suggests PCT can support shortening antimicrobial therapy duration in sepsis patients, though this is a weak recommendation with low-quality evidence. 1
• PCT-guided strategies have demonstrated both reduced antibiotic exposure and improved outcomes in critically ill patients. 1
• Serial PCT measurements are more valuable than single determinations for monitoring treatment response. 1

3. Severity Assessment in Sepsis

PCT levels correlate with infection severity along a continuum: 3

• 0.6-2.0 ng/mL: Systemic inflammatory response syndrome (SIRS)
• 2.0-10 ng/mL: Severe sepsis
• >10 ng/mL: Septic shock

4. Specific Clinical Scenarios

Respiratory Infections:

• The European Society of Clinical Microbiology and Infectious Diseases suggests using PCT in the emergency department to guide antibiotic initiation for patients with suspected lower respiratory tract infections, acute COPD exacerbations, and acute asthma exacerbations likely requiring admission. 1

Surgical Infections:

• PCT can help diagnose surgical infections and guide early antibiotic discontinuation in critically ill patients and those with acute peritonitis. 4
• PCT has limited utility in complicated intra-abdominal infections, where an 80% decrease from peak level failed to accurately predict treatment response. 1

Critical Limitations and Pitfalls

When NOT to Use PCT

• Do NOT use PCT to rule out bacterial infection when clinical probability is high—empiric antibiotics must be started immediately regardless of PCT results. 1, 2
• The Surviving Sepsis Campaign explicitly states no recommendation can be given for using PCT to distinguish severe infection from other acute inflammatory states, as it cannot reliably discriminate sepsis from other causes of generalized inflammation. 1
• Do NOT use PCT based on fever alone to guide antibiotic decisions. 1
• Do NOT use PCT for patients with dyspnea and suspected/known heart disease. 1

False Positives

• PCT may be falsely elevated in severe viral illnesses (including influenza and COVID-19), acute respiratory distress syndrome, and other non-infectious conditions, potentially reducing its discriminatory power. 2, 3
• In COVID-19 patients, PCT may be elevated without bacterial pneumonia in approximately 21% of cases. 3
• PCT levels are markedly influenced by renal function and renal replacement therapy. 3

Practical Clinical Algorithm

For suspected bacterial infection: 1, 2

1. Perform thorough clinical evaluation and obtain cultures before starting antibiotics
2. Measure PCT as part of initial workup
3. If high clinical suspicion: Start empiric antibiotics immediately regardless of PCT level
4. If low-to-intermediate clinical suspicion with PCT <0.5 ng/mL: Consider withholding antibiotics with close monitoring
5. Once patient stabilizes on antibiotics: Use PCT <0.5 ng/mL or ≥80% decrease from peak to support discontinuation
6. Obtain serial PCT measurements (daily) to monitor treatment response—successful therapy shows rapid decline

Comparison with CRP

• PCT rises earlier (4 hours) than CRP (12-24 hours) after inflammatory stimulus and returns to normal more rapidly, allowing earlier diagnosis and better monitoring. 2, 5
• CRP is less specific than PCT for bacterial infections but may be more accessible and less expensive. 2
• Both CRP and PCT can help diagnose surgical infections, but PCT is superior for guiding antibiotic discontinuation. 4