What does procalcitonin (PCT) indicate?

Procalcitonin as a Biomarker for Bacterial Infection

Procalcitonin (PCT) is primarily a biomarker that helps differentiate bacterial infections from viral infections and non-infectious inflammatory conditions, with higher levels strongly correlating with increased probability of bacterial infection and infection severity. 1

What PCT Indicates

• PCT is a precursor hormone of calcitonin produced by parafollicular cells of the thyroid gland and neuroendocrine cells of the lung and intestine that rises in response to bacterial infection 1
• PCT begins to rise approximately 4 hours after exposure to bacteria, reaching maximum levels after 6-8 hours, and decreases rapidly after appropriate antibiotic treatment 1
• PCT values in healthy individuals are typically less than 0.05 ng/mL 1
• Higher PCT levels correlate with infection severity: 0.6-2.0 ng/mL for systemic inflammatory response syndrome (SIRS), 2-10 ng/mL for severe sepsis, and >10 ng/mL for septic shock 2

Clinical Utility of PCT

• PCT can help distinguish bacterial from non-bacterial causes of inflammation, particularly in respiratory infections and sepsis 3
• PCT has demonstrated value in guiding antibiotic therapy decisions, especially for discontinuation of antibiotics in critically ill patients and patients with acute peritonitis 1
• PCT-guided antibiotic stewardship has been shown to reduce antibiotic exposure and associated side effects in patients with respiratory infections and sepsis 3
• PCT levels <0.5 ng/mL have a high negative predictive value (96-98.6%) for bacterial infections, particularly for gram-negative infections 2

Limitations of PCT

• PCT cannot be used alone to justify withholding antibiotics from patients with community-acquired pneumonia due to variable sensitivity (38-91%) 1
• Recent studies have shown that PCT may be elevated during severe viral illnesses including influenza and COVID-19, potentially reducing its discriminatory power 1
• No single PCT threshold has been identified that perfectly discriminates between viral and bacterial pathogens, though higher values strongly correlate with bacterial infection 1
• PCT should not be routinely used in the setting of sepsis and septic shock due to uncertain benefit, cost, and availability issues 1

Clinical Applications

• In critically ill patients with new fever and low to intermediate probability of bacterial infection, measuring PCT can help rule out bacterial infection 1
• PCT should not be measured to rule out bacterial infection when the probability of infection is already deemed high 1
• Serial PCT measurements may be more valuable than a single reading, particularly in post-surgical patients 2
• PCT ratio (day 1 to day 2) following surgical procedures can help indicate successful surgical intervention, with a ratio >1.14 suggesting successful source control 2

Comparison with Other Biomarkers

• C-reactive protein (CRP) is another commonly used biomarker that rises in response to inflammation or infection 1
• Unlike PCT, CRP rises more slowly (12-24 hours after inflammatory stimulus) and peaks after 48 hours 1
• CRP is less specific than PCT for bacterial infections but may be more accessible and less expensive 1
• In some studies, CRP >30 mg/L has demonstrated superior diagnostic value compared to PCT for identifying bacterial pneumonia 4

Best Practice for PCT Use

• Combine PCT results with careful patient assessment and clinical judgment rather than using PCT values in isolation 3
• Consider the pretest probability of bacterial infection and the patient's clinical condition when interpreting PCT results 3
• In high-risk individuals or those with high pretest probability for infection, empiric antibiotic treatment should not be delayed while awaiting PCT results 2, 4
• Low PCT levels help rule out bacterial infection primarily in patients with both low pretest probability for bacterial infection and low-risk general condition 3

PCT is a valuable biomarker that, when used appropriately in conjunction with clinical assessment, can help differentiate bacterial from non-bacterial causes of inflammation and guide antibiotic therapy decisions.