Procalcitonin (PCT) is a small protein, of approximately 13kD, which belongs to the calcitonin family. PCT is encoded by the CALC-1 gene and is the peptide precursor of the hormone calcitonin. In healthy individuals, PCT is primarily synthesized by thyroid C cells and is present in the bloodstream at very low levels. In contrast, the level of PCT is rapidly elevated in response to bacterial infection and is produced ubiquitously by a wide range of tissues and cells. Elevated levels of PCT have also been reported to correspond with an increase in the severity of bacterial infection and PCT is therefore considered to be a valuable biomarker for diagnosing sepsis and assessing disease severity. Furthermore, elevated PCT is not associated with chronic inflammatory conditions and has, therefore, been used to differentiate inflammatory from infectious conditions.