CALCA and bacterial infectious disease: Therefore, more specific markers are required in the detection and follow-up of infections.5 Procalcitonin (PCT) is a 116 amino acid glycopeptide produced by C cells of the thyroid gland under normal conditions and is a precursor to calcitonin.6,7 It is known that serum PCT levels increase significantly in cases of sepsis and severe invasive bacterial infections and decrease rapidly with appropriate antibiotic treatment.