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C-Reactive Protein (CRP) is an acute-phase protein that sharply rises in plasma when the body is infected or when tissue is damaged. It plays a role in activating complement and enhancing the phagocytosis of macrophages.As an inflammatory marker, CRP has been widely used in clinical practice. Common methods for detecting CRP levels in patients’ serum and other body fluids include rate nephelometry, single radial immunodiffusion, ELISA, and latex agglutination.
High-sensitivity C-Reactive Protein (hs-CRP) serves the same purpose as traditional CRP, but its measurement methods are more precise and sensitive. The rate nephelometry method has been established as a routine testing method for hs-CRP in clinical settings. It is a dynamic assay that measures the antigen-antibody binding reaction, allowing for quick and accurate measurement of antigen levels in samples, with results produced in 15-30 minutes and can be measured on various automated testing devices. The normal reference range is <3mg/L.
Clinical Applications of CRP
Many factors can lead to an increase or decrease in CRP levels in clinical practice. Understanding the release patterns and influencing factors is crucial for accurate diagnosis.
Related to Inflammation
CRP is synthesized in the liver and increases within 4-6 hours after the onset of inflammation or injury, doubling every 8 hours and peaking at 36-50 hours.Due to CRP’s short half-life (4-7 hours), plasma concentration depends solely on the rate of synthesis; after inflammation subsides, CRP levels drop rapidly.
Elevated CRP can accompany various acute and chronic inflammatory conditions caused by both infectious and non-infectious diseases.
Indicates Infection
In most inflammatory cases, CRP significantly rises as part of the acute-phase reactants.Significant increases in CRP levels are closely associated with infections.
Bacterial infections account for about 80% of patients with CRP >100mg/L and 88%-94% of patients with CRP >500mg/L. Viral infections may also elevate CRP levels, but the extent is often lower than that seen in bacterial infections. Notably, infections caused by adenovirus, influenza virus, and cytomegalovirus without complications can also lead to CRP levels as high as 100mg/L.
Guides Antibiotic Therapy
In clinical and laboratory indicators for newborns, typical signs of sepsis (such as fever and leukocytosis) may not be present, making CRP useful in determining whether to initiate antibiotic therapy.
Monitoring trends in CRP levels can also determine treatment responses, especially when clinical assessments of treatment effectiveness are challenging. A study on newborns suggested measuring CRP levels at least twice with a 24-hour interval; if the 8-48 hour measure is <10mg/L, it is considered unlikely to be a bacterial infection.
Monitors Treatment of Inflammatory Diseases
CRP is widely used to assess and monitor inflammatory and autoimmune diseases.For instance, in the treatment of Kawasaki disease, CRP levels typically return to normal within a few days.Persistently elevated CRP levels indicate ongoing inflammation and may require additional treatment.
In patients with inflammatory bowel disease, CRP levels sustained above 45mg/L indicate uncontrolled inflammation and a risk of colorectal cancer, potentially necessitating colectomy.
Assesses Cardiovascular Risk
The level of hs-CRP is closely related to the occurrence, severity, and prognosis of coronary heart disease and acute cerebral infarction, serving as a prognostic indicator for acute coronary syndrome and a predictive marker for future coronary heart disease events.
The predictive model combining hs-CRP with lipid levels [such as the total cholesterol (TC)/high-density lipoprotein cholesterol (HDL-C) ratio] is currently the best model for assessing coronary heart disease risk. The cardiovascular risk assessment proposed by the CDC and AHA states that low, moderate, and high-risk serum hs-CRP values are <1mg/L, 1-3mg/L, and >3mg/L, respectively; if hs-CRP exceeds 10mg/L, attention should be paid to identifying sources of infection or inflammation. Follow-up hs-CRP tests should be conducted after two weeks to assess whether levels remain elevated.
Mild Elevation Indicates Low-Grade Inflammation
Plasma CRP concentrations in the population exhibit a skewed distribution rather than a normal distribution.In samples from reference populations, 70%-90% of CRP concentrations are below 3mg/L, but some individuals may have mildly elevated CRP concentrations, with a maximum of 10mg/L.The so-called normal range (or reference range) for CRP varies significantly across laboratories, to the extent that biological or technical factors cannot explain the differences.Therefore, values classified as “elevated” may not truly be high; it is best to consider CRP concentrations >10mg/L as indicative of clinically significant inflammation, while concentrations between 3-10mg/L are taken as indicative of low-grade inflammation.
Low-grade inflammation does not present with typical signs of inflammation and may be triggered by various metabolic stresses, such as atherosclerosis, obesity, obstructive sleep apnea, insulin resistance, hypertension, and type 2 diabetes. Low-grade inflammation is also associated with many unhealthy states and lifestyles, including low physical activity, prehypertension, and unhealthy diets.
Summary
CRP affects multiple stages of inflammation, and it can initiate the clearance of pathogens and target cells through interactions with the fluid and cellular effect systems of inflammation;in some cases, CRP responses triggered by tissue damage can exacerbate tissue injury.
In summary, CRP can be used to diagnose infectious or inflammatory diseases, and clinicians can refer to CRP levels in diagnosing diseases and throughout antibiotic treatment, making it a valuable tool for monitoring treatment responses in infectious diseases. Proper use of this indicator can compensate for the limitations of clinical history and physical examination, playing an important role in clinical practice.
(Original article published in “Doctor Online” magazine, September 10, 2020, Issue 25, Total Issue 419)
Author of this article:Tian Xinlei (Attending Physician, Beijing Western Medical Area, PLA General Hospital) Meng Qingyi (Chief Physician, Emergency Department, First Medical Center, PLA General Hospital)Image source:pixabay Free Image WebsiteDuty Editor: Liu XiaomeiContact Email: [email protected]
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