Among the many causes of fever, bacterial infections have always played an important role. The identification of bacterial infections from viral infections, autoimmune diseases, mycoplasma infections, drug fever and other febrile diseases has always been a problem for clinicians. In recent years, a biomarker, procalcitonin (PCT), has gradually come to the attention of many clinicians. PCT is a glycoprotein, a precursor of calcitonin (CT), which is synthesized and secreted in the parafollicular cells of the thyroid gland under physiological conditions, and may be synthesized and secreted by tissue cells of the body, including liver, lung, kidney, fat and muscle, when bacterial infection occurs. PCT may be degraded by specific proteases. It is rarely excreted via the kidney, and plasma PCT is not significantly elevated in patients with renal failure. Plasma PCT concentrations do not change significantly with continuous intravenous hemofiltration or hemodialysis. Therefore plasma PCT assay is also indicated for patients with renal failure or artificial kidney therapy. Clinical significance of PCT (ng/ml): 1. Judgment of the presence or absence of bacterial infection: <0.1 very unlikely to be bacterial infection, strongly against the application of antibiotics; 0.1-0.25 unlikely to be bacterial infection, against the application of antibiotics; 0.25-0.5 possible bacterial infection, recommended the application of antibiotics; >0.5 very likely to be bacterial infection, strongly recommended the use of antibiotics. 2. Judgment of the degree of bacterial infection: <0.05 no signs of bacterial infection; <0.5 presence of local bacterial infection; <2 manifestation of systemic infection (sepsis); <10 severe concentration; >10 possible septic shock. 3, other pathogens that can cause PCT elevation: invasive fungal infections: 0.05-2, Aspergillus infection can be further elevated in the late stages. Legionella: average 13.5, <0.5 is less likely. Tuberculosis: more > 2. Mycoplasma: average 0.2-0.96.