What does a procalcitonin (PCT) level measure?

Procalcitonin (PCT) Level Measurement and Clinical Significance

Procalcitonin is a precursor hormone of calcitonin produced by the parafollicular cells of the thyroid gland and neuroendocrine cells of the lung and intestine that primarily helps discriminate systemic response due to bacterial infection from viral and non-infectious etiologies. 1

Biological Characteristics

• PCT begins rising approximately 4 hours after exposure to bacterial pathogens, reaching peak levels after 6-8 hours 1
• Normal PCT values in healthy individuals are less than 0.05 ng/mL 1
• PCT has a half-life ranging from 22 to 35 hours, allowing for effective monitoring of infection progression 1
• PCT production is ubiquitous throughout the body in response to bacterial infection, unlike calcitonin which is primarily produced in the thyroid 1

Clinical Interpretation of PCT Levels

• PCT levels correlate with infection severity and decrease rapidly after effective antibiotic treatment 1
• PCT values along the infection continuum:
  • <0.05 ng/mL: Normal range 1
  • 0.5-2.0 ng/mL: Systemic inflammatory response syndrome 1
  • 2.0-10 ng/mL: Severe sepsis 1

  • 10 ng/mL: Septic shock 1

• A PCT level of 8 ng/mL strongly indicates bacterial sepsis, representing approximately 160 times higher than normal levels 2

Diagnostic Utility

• PCT has a sensitivity of 77% and specificity of 79% for early diagnosis of sepsis in critically ill patients 1
• PCT is superior to other inflammatory markers in distinguishing bacterial from non-bacterial causes of inflammation 1
• PCT levels are not typically elevated in chronic inflammatory states, making it more specific for acute bacterial infections 1
• Recent studies have shown PCT may be elevated during severe viral illnesses including influenza and COVID-19, potentially reducing its discriminating power for predicting causative microorganisms 1

Clinical Applications

• PCT is most useful when the probability of bacterial infection is deemed low to intermediate in critically ill patients with new fever and no clear focus of infection 1
• PCT testing is not recommended when the probability of bacterial infection is already high, as it would not change management 1
• PCT can guide decisions about antibiotic initiation and discontinuation, potentially reducing unnecessary antibiotic exposure 1, 3
• Serial PCT measurements showing decreasing levels correlate with improved outcomes and can guide treatment decisions 2

Practical Considerations

• PCT test results are usually available within one hour with point-of-care testing devices or in routine laboratories 1
• PCT levels are markedly influenced by renal function, different techniques of renal replacement therapy, and neutropenia 1
• When using PCT for antibiotic stewardship, consider the clinical context, timing of measurement, and trending of values 3

Comparison with Other Biomarkers

• Unlike C-reactive protein (CRP), PCT is less affected by neutropenia, immunodeficiency, and use of nonsteroidal anti-inflammatory drugs 1
• PCT rises earlier (4 hours) than CRP (12-24 hours) after an inflammatory stimulus 1
• Either PCT or CRP can be used to rule out bacterial infection when the probability is low to intermediate 1

PCT measurement provides valuable information for diagnosing bacterial infections, assessing severity, and guiding antibiotic therapy decisions, particularly when used in conjunction with clinical assessment and other diagnostic tools.