[10] During inflammation, LPS, microbial toxin, and inflammatory mediators, such as IL-6 or TNF-α, induce the CALC-1 gene in adipocytes, but PCT never gets cleaved to produce CT.[10] In a healthy individual, PCT in endocrine cells is produced by CALC-1 by elevated calcium levels, glucocorticoids, CGRP, glucagon, or gastrin, and is cleaved to form CT, which is released to the blood.

A microbial infection induces a substantial increase in the expression of CALC-1, leading to the production of PCT in all differentiated cell types.

[13] The function of PCT synthesized in nonneuroendocrine tissue due to a microbial infection is currently unknown, but, its detection aids in the differentiation of inflammatory processes.

[14] The table below is a summary from Schuetz, Albrich, and Mueller,[14] summarizing the current data of selected, relevant studies investigating PCT in different types of infections.

PCT has the greatest sensitivity (90%) and specificity (91%) for differentiating patients with systemic inflammatory response syndrome (SIRS) from those with sepsis, when compared with IL-2, IL-6, IL-8, CRP and TNF-alpha.

[52] Evidence is emerging that procalcitonin levels can reduce unnecessary antibiotic prescribing to people with lower respiratory tract infections.

[57] Immune responses to both organ rejection and severe bacterial infection can lead to similar symptoms such as swelling and fever that can make initial diagnosis difficult.

To differentiate between acute rejection of an organ transplant and bacterial infections, plasma procalcitonin levels have been proposed as a potential diagnostic tool.

[60] In adults with acute respiratory infections, a 2017 systematic review found that PCT-guided therapy reduced mortality, reduced antibiotic use (2.4 fewer days of antibiotics) and led to decreased adverse drug effects across a variety of clinical settings (ED, ICU, primary care clinic).

[59] Procalcitonin-guided treatment limits antibiotic exposure with no increased mortality in patients with acute exacerbation of chronic obstructive pulmonary disease.

[61] Using procalcitonin to guide protocol in acute asthma exacerbation led to reduction in prescriptions of antibiotics in primary care clinics, emergency departments and during hospital admission.

Be that acute asthma exacerbation is one condition that leads to overuse of antibiotics worldwide, researchers concluded that PCT could help curb over-prescribing.

[63] The European Society of Cardiology recently released a PCT-guided algorithm for administering antibiotics in patients with dyspnea and suspected acute heart failure.

[64] This coincides with a 2017 review of literature which concluded that PCT can help reduce antibiotic overuse in patients presenting with acute heart failure.

[69] Evidence shows that an elevated PCT above .5 ng/mL could help diagnose infectious complications of inflammatory bowel disease such as abdominal abscesses, bacterial enterocolitis etc.

PCT can help the clinical decision making while identifying invasive bacterial infection in children with unexplained fever.

[80] In adult emergency department patients with respiratory tract illnesses, PCT-guided treatment groups had reduced antibiotic use.