Procalcitonin (PCT) is a protein produced by the body in response to bacterial infections. The PCT test is a blood test that measures the level of PCT in the blood. This test is used to help diagnose and monitor bacterial infections, especially in critically ill patients. PCT levels rise rapidly in response to bacterial infections, and decrease as the infection is treated and resolved. The PCT test is often used in conjunction with other diagnostic tests and clinical evaluations to help guide appropriate antibiotic therapy. It is a useful tool for healthcare providers to help differentiate between bacterial and viral infections, and to determine the appropriate course of treatment.