What is CRP?
CRP is a pentameric protein composed of five identical, noncovalently-linked, 23-kD subunits. It is present in trace concentrations in the plasma of all humans and it has been highly conserved over hundreds of millions of years of evolution. Although its exact function is unknown, it shows important recognition and activation properties. Ligands recognized by CRP include phosphatidylcholine as well as other phospholipids and some histone proteins. CRP can activate the classical complement pathway; it can bind to and modulate the behavior of phagocytic cells in both proinflammatory and antiinflammatory ways. CRP is produced as an acute-phase reactant by the liver in response to IL-6 and other cytokines. Elevation occurs within 4 hours of tissue injury and peaks within 24–72 hours. In the absence of inflammatory stimuli, it falls rapidly, with a half-life of about 18 hours. A normal value is typically < 0.5–1.0 mg/dL (<5.0–10.0 mg/L) depending on the laboratory. Age, race, and obesity may affect the normal values. Table 6.0 below outlines the age and obesity-adjusted upper limit of normal for CRP. CRP is measured by immunoassay or nephelometry. The 2018 Medicare test cost is $6.39 ($15.98 for high-sensitivity CRP).
Pearl : Levels >8–10 mg/dL (> 80-100 mg/L) should suggest bacterial infection, systemic vasculitis, acute polyarticular crystal disease, or widely metastatic cancer.