What does procalcitonin (PCT) measure?

Procalcitonin: Measurement and Clinical Significance

Procalcitonin (PCT) is a precursor protein of calcitonin that is produced throughout the body in response to bacterial infections, serving as a valuable biomarker for diagnosing bacterial sepsis and guiding antibiotic therapy decisions. 1, 2

What PCT Measures

• PCT is released 3-4 hours after an inflammatory stimulus (particularly bacterial infection), reaches peak plasma levels within 6-24 hours, and has a half-life of 22-35 hours 1
• In healthy individuals, PCT levels are typically less than 0.05 ng/mL 3
• PCT is produced ubiquitously throughout the body in response to bacterial toxins and inflammatory cytokines, unlike calcitonin which is primarily produced in the thyroid 1, 3
• PCT levels correlate with infection severity, with values of 0.6-2.0 ng/mL suggesting systemic inflammatory response syndrome (SIRS), 2-10 ng/mL indicating severe sepsis, and >10 ng/mL correlating with septic shock 3

Clinical Significance and Interpretation

• PCT has demonstrated superior diagnostic accuracy for bacterial infections with a sensitivity of 77% and specificity of 79% for early diagnosis of sepsis in critically ill patients 1
• PCT levels begin rising within 2-3 hours of bacterial infection onset, making it a valuable early indicator of serious bacterial infection 2, 3
• A PCT level of 8 ng/mL is approximately 160 times higher than normal levels and strongly indicates bacterial sepsis requiring immediate antibiotic therapy 2
• PCT values are markedly influenced by renal function, different techniques of renal replacement therapy, and neutropenia 1

Clinical Applications

• PCT is most widely used for antibiotic stewardship, helping guide both initiation and discontinuation of antibiotics across various clinical scenarios 1
• The Society of Critical Care Medicine suggests measuring PCT in critically ill patients with new fever and no clear focus of infection when the probability of bacterial infection is low to intermediate 1, 3
• Serial measurements showing decreasing PCT levels (≥80% from peak or to <0.5 ng/mL) correlate with improved outcomes and can guide antibiotic discontinuation decisions 2, 3
• PCT should not be used to rule out bacterial infection when the probability of infection is deemed high 1, 3

Limitations and Considerations

• PCT should always be interpreted in conjunction with clinical judgment and not used as the sole decision-making tool 3
• PCT levels can be elevated in severe viral illnesses and non-infectious conditions 3
• PCT has limited generalizability in severely immunocompromised patients 3
• When choosing between PCT and C-reactive protein (CRP) for evaluating fever in critically ill patients, current guidelines do not strongly favor one over the other, though PCT rises more quickly than CRP 1, 3

Clinical Decision Algorithm

1. Measure PCT in patients with suspected bacterial infection, particularly when infection probability is low to intermediate 1, 3
2. Interpret PCT values based on clinical context:
   • <0.5 ng/mL: Bacterial infection unlikely in low-risk patients 3
   • 0.5-2.0 ng/mL: Possible bacterial infection 3

   • 2.0 ng/mL: High likelihood of bacterial infection 3

   • 10 ng/mL: Severe sepsis/septic shock highly likely 3

3. Use serial measurements to monitor treatment response and guide antibiotic duration 2, 3
4. Consider discontinuing antibiotics when PCT decreases by ≥80% from peak or to <0.5 ng/mL in clinically improved patients 3