Procalcitonin

The Procalcitonin (PCT) test aids the diagnostic evaluation of sepsis, septicemia, and septic shock; supports assessment of systemic inflammatory response syndrome and multiorgan dysfunction; and helps identify severe bacterial infections at diverse anatomic sites. It is additionally applied to assess infectious complications in postoperative patients and those managed in intensive care units. Procalcitonin results are incorporated into antimicrobial stewardship to guide initiation and on‑treatment monitoring of antibacterial therapy and to assist decisions about discontinuing antibiotics when appropriate. Clinicians may refer to this as a procalcitonin assay or PCT level.

Procalcitonin is a 116–amino acid prohormone of calcitonin that is normally synthesized by thyroid C cells, with circulating concentrations at very low levels. During systemic bacterial infection, endotoxin and proinflammatory cytokines (eg, interleukin-1, interleukin-6, tumor necrosis factor-α) trigger extra‑thyroidal PCT production by neuroendocrine and parenchymal tissues, including liver, kidney, lung, muscle, and adipose, producing marked increases in blood PCT. Concentrations begin to rise within 2–4 hours, reach a peak at approximately 12 hours, and then fall with a half-life of about 22–26 hours. Viral infections generally suppress PCT generation, which underpins its utility for distinguishing bacterial from viral etiologies. Marked elevations may also accompany severe infections caused by fungi or protozoa, and higher levels correlate with disease severity and mortality among critically ill patients. In contrast, localized infections such as tonsillitis, pharyngitis, sinusitis, or gastritis rarely produce diagnostically meaningful increases. Noninfectious increases can occur after major surgery, cardiopulmonary bypass, multiple trauma, severe burns, acute graft rejection, subarachnoid hemorrhage, and in chronic heart failure. In neonates, a physiologic surge occurs within the first 12–36 hours of life with return to baseline by days 4–5; when possible neonatal sepsis is being assessed, repeating the measurement at 24 hours is recommended. Serial PCT measurements, interpreted with clinical findings and other laboratory data, may be used to evaluate response to antibacterial therapy and to support decisions on antibiotic discontinuation.