C Reactive Protein

C-reactive protein (CRP) is an acute phase protein synthesized in hepatocytes and alveolar macrophages in response to cytokines, particularly IL-6. CRP activates the classical complement pathway. It is a general marker of inflammation that begins to rise four to six hours after tissue injury. This is much earlier than other acute phase reactants, which do not begin to increase until 24 hours or more. CRP also increases to much higher levels than other acute phase proteins, making it the most sensitive indicator of small inflammatory stimuli. Plasma levels of CRP can be detected as early as 4 hours after injury and peak (1000-fold increase) between 24 and 72 hours after injury.

CRP is a nonspecific biochemical marker of inflammation that may be elevated in patients with infection, autoimmune disorders, cancer and trauma. CRP has been used clinically to monitor inflammatory disease activity, detect postoperative and neonatal infections and assess transplant rejection. When using CRP to evaluate infection, inflammation or tissue injury, a CRP level less than 5 mg/L is considered negative and a value greater than 10 mg/L is considered positive for inflammation. Severe infections and trauma can produce CRP levels greater than 100 mg/L. CRP levels may be muted in patients with liver disease. The upper limit of quantification is approximately 500 mg/L. Sequential measurement of CRP is important for effective monitoring of inflammation.Procalcitonin is a better biomarker than CRP for monitoring antibiotic therapy. 

CRP is usually measured with a particle-enhanced immunoturbidimetric assay or nephelometry.

Reference value is <5.0 mg/L. 

Preferred specimen is a red top tube (SST) of blood. Gross lipemia or hemolysis interferes with this test.

References

Sproston NR, Ashworth JJ. Role of C-Reactive Protein at Sites of Inflammation and Infection. Front Immunol. 2018 Apr 13;9:754. doi: 10.3389/fimmu.2018.00754.

Dias, R.F., de Paula, A.C.R.B., Hasparyk, U.G. et al. Use of C-reactive protein to guide the antibiotic therapy in hospitalized patients: a systematic review and meta-analysis. BMC Infect Dis 23, 276 (2023). https://doi.org/10.1186/s12879-023-08255-3