Procalcitonin (PCT) is a protein that consists of 116 amino acids and is the peptide precursor of calcitonin. Calcitonin is initially biosynthesized as PCT which under normal conditions, is found in low levels in the circulation (≤ 0.1 ng/mL). 12 PCT was first described as a marker of bacterial infection in 1993 when high concentrations of calcitonin-like immunoreactivity were detected in the blood of patients with extra-thyroid diseases. 3 Using a monoclonal immunoradiometric assay for calcitonin precursors, investigators measured serum concentrations of procalcitonin in 79 children with bacterial and viral infections. They found serum PCT levels to be very high (6-53 ng/mL) in patients with severe invasive bacterial infections compared to patients with mild local bacterial infections or viral infections (0.1-1.5 ng/mL); additionally, they noticed PCT levels decreased rapidly during antibiotic therapy and that calcitonin levels were normal in all subjects irrespective of PCT levels. They concluded PCT levels are raised during bacterial septic conditions, and serum concentrations are correlated with severity of microbial invasion.A 1994 study also found PCT levels rise in response to bacterial infection, by injecting healthy volunteers with endotoxin and measuring serial PCT levels. 4 Levels were detectable