Procalcitonin (PCT) is a 116 amino acid peptide precursor to calcitonin, a hormone involved in calcium homeostasis. PCT increases significantly in the context of proinflammatory bacterial infections but remains low in viral and other inflammatory states. In serum, PCT rises in 2-4 hours and peaks at 4-30 hours, but interestingly enough, it does not have an appreciable effect on calcium levels despite high circulating concentrations during bacterial infections. PCT is helpful in determining when the inflammatory state has subsided and an appropriate time to wean antibiotic therapy.
A very low PCT is reassuring and may support avoiding antibiotics; however this should not trump a high clinical suspicion for sepsis and the early administration of antibiotic therapy. Additionally, false positives can occur with surgery, burns, pancreatitis, severe trauma, and shock. False negatives are can occur if a PCT is drawn too early in the infection’s course or if the infection is contained like mediastinitis or empyema.
Randomized controlled trials (RCTs) have demonstrated the utility of procalcitonin-centered algorithms in determining when to start/stop antibiotics in two situations thus far – respiratory infections in adults and critically ill patients with any source of infection (ie, our “septic patients”). If I trend procalcitonin on a patient, I check daily and usually stop antibiotics when the value is less than 0.5 µg/L or decreases by 80% or more from its peak value – especially if other markers of infection have subsided (symptomatology, white blood cell count, fevers, etc.)