Procalcitonin (PCT), a precursor substance of calcitonin, has been widely used in the clinical diagnosis of infectious diseases and their differential diagnosis since it was first reported as an early inflammatory indicator of infectious diseases by Assicot et al. in the 1990s. PCT is now widely considered to be an endogenous non-steroidal anti-inflammatory substance produced in response to bacterial infections, which has a role in regulating cytokines and can be an important diagnostic marker for bacterial infections. PCT was initially used as an aid in the diagnosis of systemic systemic infections due to sepsis and in determining the severity of the disease. Currently, many foreign scholars have conducted a series of studies on the guiding effect of changes in serum PCT concentration on antibiotic therapy as a new type of infection marker, and have developed a set of therapeutic strategies for PCT-guided antibiotic application. When PCT is <0.1 ng/m, bacterial infection is unlikely and antibiotics should be avoided; 2. When 0.1 ng/ml ≤ PCT <0.25 ng/ml, bacterial infection is unlikely and antibiotics are not recommended; 3. When PCT concentration is ≥0.5ng/ml, systemic infection exists and antibiotics are strongly recommended; 5.When PCT concentration is >2ng/ml, it indicates sepsis; 6.When PCT concentration is >10ng/ml, it indicates severe sepsis. There are certain limitations in the detection of any index, i.e. false positives and false negatives of the test, and PCT is no exception. Causes of significant PCT elevation in non-bacterial infections include: 1. The body often shows moderate elevation of PCT levels in a state of intense stress (e.g., severe trauma and its surgical procedures); however, with the removal of stress, PCT concentrations can decrease quickly during dynamic reexamination; 2. Patients with hypothermia after cardiac arrest show elevated PCT levels that are not dependent on the underlying infection; 3; 3. Patients with Legionella infection pneumonia have significantly higher serum PCT levels, even exceeding those of patients with general bacterial infection. The above elevated PCT levels are non-specific and are not caused by bacterial infection, but by the inflammatory response of the organism. There are several cases of false negatives: 1. In the early stage of bacterial infection, the serum PCT level may appear as a false negative, but the PCT concentration will increase in the subsequent review; 2. In patients with subacute infective endocarditis, the sensitivity of the PCT test is very low and the PCT level is maintained at a low level. Therefore, a highly sensitive PCT assay is particularly important, and subtle monitoring of changes in patient serum PCT concentrations is likely to have a bearing on the effectiveness and safety of treatment.