A C-reactive protein (CRP) blood test measures the level of CRP in a sample of your blood. CRP is a protein made by the liver and released into the bloodstream in response to inflammation. The test can show whether you have inflammation in your body and how much, but it cannot show whats causing the inflammation or which part of your body is inflamed. The CRP test is especially useful for tracking infections because CRP levels often go up before you have symptoms of pain or fever and drop down as you recover. The test can also help diagnose or monitor chronic inflammatory diseases, such as rheumatoid arthritis or lupus, and autoimmune disorders, such as vasculitis. Additionally, a high-sensitivity CRP test (hs-CRP) may be used to measure your risk of heart disease even if you seem healthy. The hs-CRP test is more sensitive than a standard CRP test and can find smaller increases in CRP than a standard test can. A healthcare provider may order the CRP test to check for infection, determine if anti-inflammatory medicine is working to treat a disease or condition, or to see how well you are responding to treatment.

In general, healthy people have very low amounts of CRP in their blood. Any increases above normal mean you have inflammation in your body. However, labs measure CRP levels in different ways, and they define "normal" CRP ranges differently, so its best to ask your healthcare provider what your results mean. The normal CRP values vary from lab to lab, but the majority of healthy adults have levels less than 0.3 mg/dL. A positive test means you have inflammation in the body, and the cause of a high CRP cannot always be determined by the test alone.