Procalcitonin

# Overview
Procalcitonin (PCT) is a precursor of the hormone calcitonin, which is involved with calcium homeostasis, and is produced by the C-cells of the thyroid gland.  It is there that procalcitonin is cleaved into calcitonin, katacalcin and a protein residue.  It is not released into the blood stream of healthy individuals.  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 blood serum, procalcitonin has a half-life of 25 to 30 hours.
The test is commercially available and produced by Thermo Fisher Scientific.
Triggering receptor expressed on myeloid cells-1 (TREM1) may be a more accurate serum biomarker for diagnosing infection.
Comparisons of the procalcitonin and c-reactive protein give conflicting results.

# Uses

## 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. In a cross-sectional study 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.  However, the test is not routinely used and has yet to gain widespread acceptance.
A review for diagnosing sepsis in 2013 of 30 studies:
- Sensitivity 77% (95% CI 72% - 81%)
- Specificity 79% (95% CI 74%  84%)
A review for diagnosing sepsis in 2007 of 18 studies:
- Sensitivity 71% (95% CI 6776)
- Specificity 71% (95% CI 6776)
Subsequent meta-analyses have summarized the relevant studies for diagnosing sepsis among immunocompromised patients.

## Diagnosis of bacteremia
Meta-analyses are available. A meta-analysis reported a sensitivity of 76% and  specificity of 70%.
Diagnosis of bacteremia in the elderly has been studied.
- Sensitivity 96%
- Specificity 68%
Diagnosis of bacteremia in the neutropenic patients with Systemic inflammatory response syndrome (SIRS) suggests lower sensitivity and higher specificity due to lower PCT levels in neutropenic patients.

## Prognosis of pneumonia
Various algorithms for interpreting and responding to the procalcitonin level are available.
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 ( or =0.5 microg/L or > or =0.25 microg/L), respectively".. However, a nonrandomized, observational study reported "limited, prognostic value" of the procalcitonin.
Procalcitonin has been used in prediction of mortality in community-acquired pneumonia:
- Sensitivity 35%
- Specificity 92%

## Guiding antibiotic therapy

### Subjects with sepsis
Low quality evidence suggests that PCT-guided therapy may aid antimicrobial stewardship without change in mortality. However, the results of trials are heterogeneous and in most trials, the quality of care in the control group .
An earlier systematic review by the Cochrane Collaboration concluded that among subjects whose care was guided by procalcitonin "antibiotic consumption was significantly reduced". The first author of this analysis receives financial support from the manufacturer of the procalctonin test.

### Subjects with lower respiratory tract infection
PCT-guided therapy does not reduce mortality. An earlier meta-analysis by the Cochrane Collaboration concluded that mortality is reduced.