Infectious diseases are common and frequently-occurring diseases in the clinic. If not diagnosed early and treated in time, the progression of the disease can lead to sepsis, multiple organ dysfunction syndrome (MODS), and even life-threatening. The early clinical signs of infection are often atypical. How to select high-sensitivity and high-specificity laboratory diagnostic markers to assist clinicians in early diagnosis is of great significance to make a rapid and accurate diagnosis of the patient's condition.
 
In recent years, with the rapid development of the in vitro diagnostic industry, more rapid, simple and accurate serological testing has played an increasingly important role in the diagnosis of early infectious diseases. Routine blood, C-reactive protein (CRP) and procalcitonin (PCT) are traditional detection indicators, which are widely used in the diagnosis and prognosis evaluation of clinical infectious diseases.

 
C-reactive protein (CRP)

——Auxiliary diagnostic indicators of early infection

CRP is an acute phase response protein of the body, which is often used to reflect the severity of infection and is widely used in the clinical diagnosis and anti-infective treatment of acute infectious diseases. Studies have confirmed that the serum CRP level in normal healthy people is extremely low. When the body is damaged by trauma, infection and tumor, CRP rises sharply within a few hours, and even reaches hundreds of times of the normal value, which is helpful for the diagnosis of early infection. good value. As a non-specific diagnostic index, CRP is elevated in many inflammatory processes, but in infectious diseases caused by bacteria, the elevated level of CRP is significantly higher than in non-infectious diseases such as immune, blood, and tumor-related diseases and physical and chemical damage. Some studies have concluded that with CRP of 8 mg/L as the cut-off value for distinguishing infectious fever from non-infectious fever, the sensitivity and specificity are 90.3% and 68.6%, respectively. Although CRP can indicate the presence of severe infection, it is not a good predictor of the risk of death in patients with septic shock and sepsis. There are many factors affecting CRP, and common leukocyte changes can have a significant impact on CRP. Therefore, it should be considered whether the increase of CRP is combined with other influencing factors in the application. If combined with other infection markers, the utilization value of CRP may be greatly improved.
 
Procalcitonin (procalcitionin, PCT)

——Reliable indicators for sepsis diagnosis and guidance of medication

PCT is the precursor of calcitonin. Generally, when the body is infected with bacteria, the level of serum PCT increases significantly. It is an important biological indicator reflecting the degree of infection and has important value in the early diagnosis and prognosis evaluation of sepsis. Studies have shown that dynamic monitoring of serum PCT levels is helpful to reflect the severity of systemic inflammatory response in patients with sepsis, to screen for the recovery period of the inflammatory process, to guide treatment, and to judge efficacy. PCT has a certain degree of diagnostic value for infections that occur in different systems and tissues of the body, such as the nervous system, urinary system, abdominal organs, especially the respiratory system. As an infection-related biological marker, it has higher specificity and sensitivity than CRP. Some studies have proposed: when the serum PCT value is less than 0.25ng/L, it is generally considered that infection will not occur, and antibiotics are not recommended; when it is between 0.25 and 0.5ng/mL, it indicates that there may be infection, and antibiotics can be considered; /mL, it indicates that there is a high possibility of infection, and the application of antibiotics is strongly recommended. Using the above PCT threshold to guide the antibiotic treatment of patients with suspected bacterial infection in the ICU can not only reduce the antibiotic selection pressure, but also will not significantly increase the poor prognosis; at the same time, as the basis for drug withdrawal, it can also significantly shorten the period of antibiotic application. Likewise, PCT has also been shown to be significantly superior to CRP in the prognostic assessment of the diagnosis of sepsis and blood-borne infections. However, PCT has high application value in severe diseases. It does not highlight its diagnostic advantages in local infection. Compared with blood routine and CRP, PCT is expensive in terms of price. It is not suitable for the detection of common infectious diseases such as respiratory tract infection and pharyngitis. . Therefore, although PCT is superior to CRP in the diagnosis and evaluation of infection, PCT cannot replace CRP in terms of application cost and extensiveness. The combined application of the two can improve the accuracy of diagnosis of various infectious diseases.
 
Overview

1. Routine blood and CRP are generally used to differentiate bacterial and viral infections, but they are not good predictors of mortality risk in patients with septic shock and sepsis.
 
2. PCT is currently one of the most effective tools for the diagnosis, evaluation of efficacy and prognosis of sepsis. At present, the academic community has reached a consensus that the level of PCT is highly positively correlated with the severity of sepsis, and the reduction of its concentration can be regarded as infection control and treatment effect. Some scholars have successfully predicted the 90-day mortality of critically ill patients through continuous monitoring of PCT levels. However, using PCT alone to evaluate the prognosis of patients with sepsis will inevitably lead to distortion. The current mainstream academic view recommends using PCT in combination with other indicators [such as C-reactive protein (CRP), acute physiology and chronic health status scoring system II (APACHE II) ) and the Sequential Organ Failure Assessment (SOFA) score, etc.)] to improve the predictive efficacy threshold for sepsis prognosis.

 
Test items
applicability
Blood routine + CRP
Identify bacterial infections
Blood routine+CRP+PCT
Differentiate systemic infection from sepsis