Biochemistry and Synthesis

• Procalcitonin (PCT) is a 116-residue peptide, the pro-hormone of calcitonin, with a molecular mass of about 13 kDa.
• Normally synthesized in the thyroid gland's C-cells, but production is complex and upregulated during systemic infections, particularly bacterial.
• Also synthesized by liver, kidney, adipocytes, and muscle cells when stimulated by inflammatory mediators like endotoxins and cytokines (TNF-α, IL-1β, IL-6).
• Escapes normal post-translational processing during infection, released into circulation as the intact prohormone.

Diagnostic Value in Bacterial Infections

• Useful biomarker for diagnosing bacterial infections and sepsis, more specific than CRP or white blood cell count.
• PCT levels are suppressed in viral infections and non-infectious inflammatory diseases, aiding in differentiation.
• Dynamics of PCT:
• Levels increase within a few hours of infection onset.
• Peak at 24-48 hours.
• Decrease rapidly with effective treatment.
• Normal PCT values: below 0.05 ng/ml.
• Suggestive of bacterial infection: 0.5 ng/ml or above.
• Suggestive of sepsis: 2 ng/ml or above.
• Valuable marker in emergency and intensive care units for timely antibiotic therapy initiation.

Clinical Applications and Decision Making

• Evolved clinical decision-making:
• Managing antibiotic therapy initiation and discontinuation.
• Assessing patient response to treatment.
• Determining patient prognosis in intensive care.
• PCT-based algorithms in antibiotic stewardship programs reduce antibiotic use with similar or better patient outcomes.
• Important amid growing antibiotic resistance issues.

Limitations and Confounding Factors

• Certain conditions can cause elevated PCT levels without bacterial infection: major trauma, surgery, severe burns, some autoimmune diseases.
• Some bacterial infections, including localized or intracellular bacteria, may not result in high PCT levels.
• Renal impairment can alter PCT clearance, raising levels.
• PCT levels should be assessed alongside clinical and laboratory data for accurate diagnosis.