The C-reactive protein (CRP) test is a blood test that measures the level an inflammatory marker – a substance that the body releases in response to inflammation. High levels of CRP in the blood mean there is inflammation somewhere in the body.  

The interest in CRP as an indicator of heart disease has emerged as researchers have learned more about atherosclerosis, or "hardening of the arteries." In recent years, scientists have learned that atherosclerosis is an inflammatory process in which immune system cells are activated in response to a local injury inside the arteries. These white blood cells release inflammatory chemicals that contribute to the atherosclerotic process and are one component of arterial plaque deposits. When a plaque deposit rupture, a heart attack or stroke can result. In response to this inflammation, CRP and other inflammatory markers in the blood rise.

Physicians may use a particularly sensitive type of CRP test called a high-sensitivity CRP (hs-CRP) assay to assess a patients risk for heart problems. By measuring CRP, physicians can measure the level of inflammation in the body and help identify people who may be at risk for a heart attack or other cardiovascular event.

CRP, however, has drawbacks as an indicator for heart disease. Because CRP levels rise in response to inflammation anywhere in the body, it is a non-specific marker, meaning that elevated CRP levels do not necessarily indicate atherosclerosis or increased risk for cardiovascular events. In fact,  CRP levels rise in response to many illnesses and chronic diseases, including bronchitis, gastritis, diabetes, high blood pressure and cancer. It can also rise in response to physical trauma (e.g., burns, fractures, surgery).

Because CRP levels are not specific to any one area of the body, some researchers have concluded that CRP testing is not as reliable as cholesterol, blood pressure and other risk factors in predicting cardiovascular disease in healthy people. Further studies in this area are necessary. Nevertheless, most experts agree that CRP levels can be a very useful measurement in people diagnosed with cardiovascular disease.

Patients who are scheduled for a CRP test will be asked not to eat or drink anything for 12 hours before the test.