Procalcitonin (PCT) is a 116 amino acid precursor of calcitonin. Calcitonin is exclusively produced by C-cells of the thyroid gland in response to hormonal stimuli, whereas PCT can be produced by several other cell types from a wide range of organs in response to inflammation or infection.

PCT is a good marker of bacterial infection because its level in the blood of normal subjects is very low and because viral infections cause only a minor increase in PCT concentration. Plasma PCT has a half-life of 25–30 hours. Levels typically increase within 3–6 hours of the stimulus, and higher levels are associated with poorer prognosis. Elevated values are highly suggestive of an infection, typically bacterial, with a systemic response (sepsis, or severe sepsis or septic shock). In addition, the diagnostic value of PCT is further supported by the close correlation between PCT concentration and the severity of inflammation. 

PCT has been shown to be a useful tool that can aid in the diagnosis of sepsis. PCT levels can be helpful in facilitating early diagnosis thus allowing for early therapeutic interventions that have been shown to improve patient outcomes.