Procalcitonin

Figure 1: Immature Calcitonin
A 3D cartoon of procalcitonin's daughter compound, calcitonin

Procalcitonin (PCT) is a peptide precursor of the hormone calcitonin, the latter being involved with calcium homeostasis. It arises once preprocalcitonin is cleaved by endopeptidase.^[1] It was first identified by Leonard J. Deftos and Bernard A. Roos in the 1970s.^[2] It is composed of 116 amino acids and is produced by parafollicular cells (C cells) of the thyroid and by the neuroendocrine cells of the lung and the intestine.

The level of procalcitonin in the blood stream of healthy individuals is below the limit of detection (0.01 μg/L) of clinical assays.^[3] The level of procalcitonin rises in a response to a pro-inflammatory stimulus, especially of bacterial origin. It is therefore often classed as an acute phase reactant.^[4] The induction period for procalcitonin ranges from 4–12 hours with a half-life spanning anywhere from 22–35 hours.^[5] It does not rise significantly with viral or non-infectious inflammations. In the case of viral infections this is due to the fact that one of the cellular responses to a viral infection is to produce interferon gamma, which also inhibits the initial formation of procalcitonin.^[6] With the inflammatory cascade and systemic response that a severe infection brings, the blood levels of procalcitonin may rise multiple orders of magnitude with higher values correlating with more severe disease.^[7] However, the high procalcitonin levels produced during infections are not followed by a parallel increase in calcitonin or a decrease in serum calcium levels.^[8]