PCT is a commonly used indicator for the differential diagnosis of bacterial infections^[1].PCT is a very stable protein secreted by thyroid C-cells and its level is very low in healthy individuals during normal metabolism of the body. When bloodstream infection occurs, bacteria and endotoxins stimulate macrophages, neutrophils, monocytes, lymphocytes and platelets to produce colony-stimulating factor, tumor necrosis factor, interleukins, vasoactive substances and other substances, which induces an increase in the level of PCT, resulting in clinical manifestations such as fever, endotoxemia, and shock ^[2-3]. In recent years, PCT has received more and more attention and has become an important indicator for the diagnosis of early infection^[4-5].

PCT of 0-0.5 ng/mL is normal; >0.5 ng/mL suggests that the patient is at risk of severe bacterial infection and sepsis.

PCT levels in plasma are elevated in severe bacterial, fungal, and parasitic infections, as well as in sepsis and multiple organ failure. In contrast, PCT is not elevated in autoimmune, allergic, and viral infections, limited localized bacterial infections, minor infections, and chronic inflammation.PCT reflects the degree to which the systemic inflammatory response is active. Factors affecting PCT levels include the size and type of infected organ, the type of bacteria, the degree of inflammation, and the status of the immune response.