PCT is a protein that is found at extremely low levels in the serum of healthy individuals and is virtually undetectable. Its levels are elevated in plasma when severe bacterial, fungal, and parasitic infections are present, as well as in sepsis and multiple organ failure. PCT levels increase significantly with increasing severity of infectious disease, decrease rapidly after effective antibiotic treatment, and correlate with patient prognosis. PCT reflects the active level of systemic inflammatory response. Factors that influence PCT levels include the size and type of the infected organ, the type of bacteria, the degree of inflammation and the status of the immune response. PCT is a specific indicator of severe bacterial inflammation and fungal infections, but also a reliable indicator of multiple organ failure associated with sepsis and inflammatory activity. PCT is not only an acute indicator used for differential diagnosis, but also a parameter for monitoring inflammatory activity: differential diagnosis of bacterial diseases: in unexplained inflammatory diseases, an elevated PCT concentration indicates the possibility of a bacterial cause. Elevated PCT concentrations in the further development of the disease may be bacterial repeat infections or sepsis. For example, spontaneous bacterial peritonitis in cirrhosis, acute pancreatitis combined with bacterial infection and other diseases. Differential diagnosis of autoimmune diseases and bacterial infections: It is now believed that PCT levels can distinguish autoimmune diseases from concomitant bacterial infections. PCT levels have been found to be more useful than C-reactive protein in the diagnostic evaluation of patients with co-infections. Monitoring of patients at risk of infection: PCT can be used to monitor critically ill patients at risk of infection. This is because PCT is only synthesized in systemic bacterial infections or sepsis, but not in local inflammation or minor infections. Disease course monitoring and prognosis: In severe bacterial infectious diseases such as sepsis and multiorgan dysfunction syndrome, the degree of PCT elevation is a reflection of inflammatory activity. the advantage of PCT over other inflammatory indicators lies in the fact that severe infections cause a significant increase in PCT concentration (>10ug/L).