Clinical and Interpretive

Sepsis is a systemic inflammatory response to infection that can result in a number of life-threatening conditions including organ dysfunction. Septic shock is a subset of sepsis in which circulatory and cellular metabolism abnormalities are profound enough to substantially increase mortality. Despite advances in modern medicine like vaccines, antibiotics and advanced acute care, sepsis is the primary cause of death from infection, causing an estimated 5.3 million deaths worldwide each year. Early identification and appropriate management in the initial hours after sepsis develops has been found to improve outcomes for patients. Procalcitonin (PCT), a prohormone of calcitonin, is a protein consisting of 116 amino acids with a molecular weight of approximately 13 kDa. In healthy individuals, PCT is produced in the thyroid C-cells and is subsequently converted into calcitonin in the thyroid with almost no PCT entering the circulation. PCT levels in healthy individuals are typically less than 0.1 ng/mL. In individuals with systemic inflammation or bacterial infections, PCT levels rise in the circulation in response to bacterial endotoxins and inflammatory cytokines. PCT levels show an increase in levels within 2-6 hours and peak within 6-12 hours of infection. PCT levels have been found to correlate with the severity of bacterial infections and also with the probability of a positive blood culture, making it a clinically useful marker in the assessment of patients with possible sepsis or septic shock. Moreover, studies have found that PCT release is blocked by cytokines that are characteristically present in the immune response to viral infections. For this reason, PCT is more specific to bacterial infections, and can aid in the differential diagnosis between nonbacterial illnesses and bacterial illnesses and sepsis. In circulation, PCT has a half-life of 25-30 hours when controlled by the host immune system or appropriate treatment. PCT results may become elevated by causes other than systemic bacterial infection, including burns, trauma, autoimmune disorders or prolonged cardiogenic shock. Therefore, PCT should not be used alone but always in conjunction with patient history and clinical signs and symptoms.

Concentrations under 0.5 ng/mL do not exclude local infections or systemic infections in their initial stages (e.g. under six hours from onset of illness). PCT concentrations between 0.5 and 2.0 ng/mL should be interpreted with consideration of the patient’s history. In this range, it is recommended to retest PCT within 6 to 24 hours.