Use of Biomarkers in Clinical Practice

Characteristics and Kinetics

PCT is a precursor hormone of calcitonin produced by the thyroid gland's parafollicular cells and neuroendocrine cells in the lung and intestine, with a half-life of 22-35 hours 1, 2
PCT rises rapidly within 4 hours after bacterial exposure, peaks at 6-8 hours 2, 3
Normal values in healthy individuals are <0.05 ng/mL 2
CRP is an acute-phase protein synthesized by the liver in response to inflammation or infection, with normal levels typically <5 mg/L 2, 3
CRP rises more slowly, starting 12-24 hours after inflammatory stimulus, with peak values at 48 hours 2

The American College of Critical Care Medicine recommends ordering PCT or CRP for low to intermediate probability of bacterial infection in critically ill patients with new fever and no clear focus of infection 1, 2
PCT can be used to guide decisions about antibiotic discontinuation, especially in ICU settings 2
CRP can be used as an adjunct to clinical assessment in community-acquired pneumonia, with CRP >100 mg/L making pneumonia likely 5
The Infectious Diseases Society of America recommends using CRP for monitoring the inflammatory response over time in various conditions 2, 3

PCT has higher specificity (77%) than CRP (61%) for bacterial infections 3
PCT-guided algorithms have been shown to safely reduce antibiotic use in respiratory infections and critical care 2
Serial measurements of PCT or CRP are often more valuable than single determinations 2, 4

Neither biomarker should replace clinical judgment or delay antibiotic administration in critically ill patients 1, 2
PCT may be elevated during severe viral illnesses, including influenza and COVID-19, potentially reducing its discriminatory power 2
The Society of Critical Care Medicine recommends using serial measurements to monitor treatment response and guide antibiotic duration 2, 4