Assessment of the extent or activity of inflammation: The serum CRP concentration provides a rough guide to the amount of tissue involved in inflammation and to the intensity of the inflammatory response.
Monitoring therapy in inflammation: Many chronic inflammatory disease are difficult to monitor clinically and serial CRP measurements can be very valuable in assessing the response to therapy and changes in disease activity. This is particularly true of rheumatoid arthritis, polymyalgia, giant cell arteritis, systemic vasculitis and inflammatory bowel disease.
Detection of bacterial infection: Bacterial infection is the most potent activator of the acute phase response. A sudden rise in CRP concentration may be a useful pointer to intercurrent sepsis, particularly in diseases where the kinetics of the acute phase response are known, such as following major surgery, or in those with little intrinsic response such as leukaemia, systemic lupus erythematosus and during peritoneal dialysis. In all these cases there is a risk of silent but serious sepsis. Viral infections cause little acute phase response and the CRP measurement may be useful in indicating bacterial aetiology in meningitis, neonatal illness and pneumonia. Falling CRP concentrations are a useful indication of response to antibacterial therapy.
Infection in neonates: The neonate is a poor inducer of CRP synthesis to an extent that concentrations seldom rise above 10 mg/L even in severe bacterial infection. Whilst this in no way limits the value of CRP in the assessment of neonatal infection, it does put constraints on the assay used by a laboratory serving a neonatal unit.
Sample requirement: 2 mL serum.
Median normal concentrations are probably less than 1 mg/L, but an operational upper normal limit is usually taken as 4 mg/L.
There are no age or gender related differences in the reference interval.
Significant bacterial infections are unusual below 10 mg/L except in the neonate.