Procalcitonin

Procalcitonin (PCT) is a peptide precursor of the hormone calcitonin, the latter being involved with calcium homeostasis. 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 (10 pg/mL) of clinical assays. The level of procalcitonin raises in a response to a proinflammatory stimulus, especially of bacterial origin. In this case, it is produced mainly by the cells of the lung and the intestine. It does not raise significantly with viral or non-infectious inflammations. With the derangements that a severe infection with an associated systemic response brings, the blood levels of procalcitonin may rise to 100 ng/ml. In serum, procalcitonin has a half-life of 25 to 30 hours. Remarkably the high procalcitonin levels produced during infections are not followed by a parallel increase in calcitonin or serum calcium levels.

Diagnosis and prognosis of sepsis
Measurement of procalcitonin can be used as a marker of severe sepsis and generally grades well with the degree of sepsis, although levels of procalcitonin in the blood are very low. PCT has the greatest sensitivity (85%) and specificity (91%) for differentiating patients with SIRS from those with sepsis, when compared with IL-2, IL-6, IL-8, CRP and TNF-alpha. Evidence is emerging that procalcitonin levels can reduce unnecessary antibiotic prescribing to people with lower respiratory tract infections. Currently, procalcitonin assays are widely used in the clinical environment.

Diagnosis of bacteremia
A meta-analysis reported a sensitivity of 76% and specificity of 70%.

Prognosis of pneumonia
A cluster randomized trial found that the procalcitonin level can help guide antibiotic therapy. In this trial, "on the basis of serum procalcitonin concentrations, use of antibiotics was more or less discouraged (<0.1 mcg/L or <0.25 mcg/L) or encouraged (> or =0.5 mcg/L or > or =0.25 mcg/L), respectively". However, an earlier nonrandomized, observational study reported "limited, prognostic value" of procalcitonin measurement.

Procalcitonin levels may be useful to distinguish bacterial infections from nonbacterial infections. Trials from 2008 and 2009 have shown that they may help guide therapy and reduce antibiotic use, which can help save on cost of antibiotic prescriptions and drug resistance.