Infection markers are clinical tests that have been developed in recent years to be useful in the determination of infection, and can help emergency physicians quickly determine the presence of infection as well as infer the type of pathogen that may be infecting the patient. Currently, commonly used infection markers include serum C-reactive protein (CRP), serum procalcitonin (PCT), interleukin 6 (IL-6), and serum amyloid A (SAA), as well as heparin-binding protein (HBP).

HBP

HBP is a protein molecule released by the body’s activation of neutrophil eosinophil granules. HBP is a protein molecule released by the activated neutrophil eosinophil granules of the body, and it has been shown that HBP is significantly elevated in sepsis patients when IL-6 levels are normal or mildly elevated, and its diagnostic accuracy in sepsis is greater than that of other cytokines, especially in the early and rapid diagnosis of severe bacterial infections.HBP, as an acute temporal protein, is an effective biomarker for assessing the severity of disease in sepsis patients, and is more important for early diagnosis and efficacy monitoring of patients with septic shock. diagnosis and efficacy monitoring.

PCT

PCT is a non-hormonally active glycoprotein, and the level of PCT is positively correlated with the severity of bacterial infection. When the PCT is 0.10-0.25ng/mL, it suggests that bacterial infection is unlikely, when the PCT is 0.25-0.50ng/mL, there may be a bacterial infection that requires treatment, and when the PCT is >0.5ng/mL, there is likely to be a bacterial infection that requires treatment. Most experts recommend a PCT of 0.5ng/mL as the diagnostic threshold for sepsis.PCT rises rapidly after 2~4h of infection and peaks at 12~48h.PCT has a high degree of specificity and is recommended by many domestic and international guideline consensus recommendations. In the “2020 New Coronavirus Pneumonia Diagnosis and Treatment Program”, the PCT of most patients is normal, which indicates that PCT is generally not elevated in viral infections and can be used as a specific marker for bacterial infections.

IL-6

IL-6 is a multifunctional cytokine induced by IL-1 and tumor necrosis factor-alpha (TNF-α), and is a multifunctional glycoprotein composed of 212 amino acids, which plays a central regulatory function in the process of inflammation, and is a key inflammatory factor transporter.The test value of IL-6 is significantly elevated in bacterial infections, and is positively correlated with the levels of HBP and SAA, which can be used as a common indicator for evaluation and detection of infections. The concentration of IL-6 is consistent with the degree of damage of the patient’s disease. In addition, IL-6 is of great significance in the determination of sepsis and severity of disease, and some research results show that serum IL-6 levels in patients with sepsis are significantly higher than those in patients without sepsis, and serum IL-6 levels in patients with septic shock are significantly higher than those in patients without septic shock.

CRP

CRP is an acute time-phase reactive protein, and most infections trigger a rapid increase in its concentration, which usually begins to rise 2 h after infection and peaks at 24-48 h. CRP levels are significantly higher in bacterial infections than in non-septic shock patients. CRP is significantly elevated in bacterial infections, whereas it is mostly normal or slightly elevated in viral infections. Therefore, CRP is usually used as one of the reference indicators to identify bacterial or viral infections. However, the cut-off value of bacterial infection is not clear, some clinical studies suggest that CRP of 40 mg/L can be used as the cut-off value of bacterial infection, but some people think that CRP>20 mg/L can be considered as a bacterial infection.The half-life of CRP is 18h, and it can be decreased rapidly in 1~2d after the infection is controlled.

SAA

SAA is a sensitive acute temporal response protein. When the body is stimulated by bacteria, viruses and other stimuli to produce a series of cytokines, thereby stimulating the liver cells to synthesize and secrete a large number of SAA into the bloodstream, and within 5~6h, the amplitude of elevation reaches 10~1000 times the normal value. Therefore, SAA can be used as a sensitive indicator to reflect the control of infection and inflammation in the body.The level of SAA is not affected by gender and age, and is usually combined with CRP to identify bacterial and viral infections. When SAA and CRP are both elevated, it suggests the presence of bacterial infection; when SAA is elevated but CRP is not, it often suggests viral infection.SAA has a half-life of about 50 min, and when the body is cleared of antigens, SAA can be rapidly reduced to normal levels. Therefore, SAA can be used as a sensitive indicator of infection and inflammation control.

Determination of acute infection and suspected infection
According to the results of the above studies and clinical experience, the presence of acute infection can be considered according to the following manifestations: ① acute (within 72h) fever or hypothermia ② increase or decrease in the total number of leukocytes ③ CRP, IL-6 increased ④ PCT, SAA and HBP increased ⑤ there is a clear or suspected site of infection.

Determination of infection: 2 of the above items (①~3) plus ④ can help to determine the type of pathogen, or ⑤ can help to determine the site of infection if there are clear signs.

Suspected infection: 1 of the above items (①~3) plus (4) has no definite result, or (5) has a suspected site of infection.