The C-reactive protein (CRP) test is a blood test that measures the level of CRP in the blood. CRP is an inflammatory marker, a substance released by the body in response to inflammation or infection.

High levels of CRP indicate an inflammation somewhere in the body. But this is not a specific test, so other tests are needed to determine the cause and location of the inflammation. 
 
Blood levels of CRP rise in response to inflammatory conditions such as rheumatoid arthritis, gout, lupus, polymyalgia rheumatica, sarcoidosis, Lyme disease, encephalitis and temporal arteritis. They can also rise in response to physical trauma such as burns, fractures or surgery.

Other illnesses that may cause a rise in CRP levels include bronchitis, cancer, diabetes and high blood pressure. Studies indicate that CRP levels may also rise in response to a heart attack, stroke or other cardiovascular conditions, but its role in coronary artery disease remains unclear.

Levels of CRP drop when inflammation passes, which makes this test valuable in monitoring the effectiveness of medical treatment.