What is procalcitonin (PCT) and why is it tested?

Procalcitonin (PCT): Definition and Clinical Applications

Procalcitonin is a precursor hormone of calcitonin that serves as a biomarker primarily used to distinguish bacterial infections from viral or non-infectious causes, and to guide antibiotic therapy decisions in critically ill patients. 1

What is Procalcitonin?

• PCT is produced by parafollicular cells of the thyroid gland and neuroendocrine cells of the lung and intestine 1
• Normal values in healthy individuals are less than 0.05 ng/mL 1
• Rapid biomarker that begins to rise within 4 hours after bacterial exposure, reaching peak levels after 6-8 hours 1
• Serum levels correlate with infection severity and decrease rapidly after effective antibiotic treatment 1
• Test results are typically available within one hour using point-of-care testing or routine laboratory methods 1

Why is PCT Tested?

1. Diagnosis of Bacterial Infections and Sepsis

• PCT helps discriminate bacterial infections from viral and non-infectious causes of inflammation 1
• Higher diagnostic accuracy for sepsis compared to C-reactive protein (CRP):
  • PCT: SROC curve 0.85, sensitivity 80%, specificity 77% 2
  • CRP: SROC curve 0.73, sensitivity 80%, specificity 61% 2
• Clinical interpretation of PCT levels:
  • <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 2

2. Guiding Antibiotic Therapy

• PCT-guided therapy can significantly reduce antibiotic exposure in hospitalized patients, including ICU patients 1
• Serial PCT measurements showing trends are more valuable than single measurements:
  • Decreasing levels indicate good response to treatment
  • Safe antibiotic discontinuation supported when levels decrease by ≥80% from peak or to <0.25 ng/mL 2

3. Specific Clinical Scenarios for PCT Testing

• Critically ill patients with new fever and no clear infection focus:

  • Suggested when probability of bacterial infection is deemed low to intermediate 1
  • Not recommended when probability of bacterial infection is already high 1

• Intra-abdominal infections:

  • May help determine treatment response and guide antibiotic duration 1
  • However, some studies show limitations in predicting treatment response in perioperative septic shock from intra-abdominal infections 1

Limitations and Caveats

• PCT can be elevated during severe viral illnesses including influenza and COVID-19, reducing its discriminatory power 1
• Not recommended for routine use in sepsis and septic shock due to uncertain benefit, cost, and availability issues 1
• Should always be interpreted in conjunction with clinical assessment, not as a standalone test 1
• CRP may be affected by neutropenia, immunodeficiency, and use of nonsteroidal anti-inflammatory drugs, while PCT is less affected by these factors 1
• PCT is not a perfect biomarker and can be elevated in non-infectious conditions 3

Advantages Over Other Biomarkers

• Rises earlier (4 hours) than CRP (12-24 hours) after infection onset 1, 4
• Returns to normal concentration more rapidly than CRP, allowing for earlier diagnosis and better monitoring 4
• More specific for bacterial infections than CRP 4
• Better for monitoring response to antibiotic therapy 2

PCT testing represents an important tool in antimicrobial stewardship programs, helping clinicians make more informed decisions about initiating, continuing, or discontinuing antibiotic therapy, ultimately improving patient outcomes and reducing unnecessary antibiotic use.