C-reactive protein (CRP) is found in the bloodstream of people who have an inflammatory condition such as RA. This protein is made in the liver and its production increases when inflammation is present in the body (though it is not the only protein that is produced when inflammation occurs). CRP plays an important role in the immune system, helping other proteins to destroy foreign material such as bacteria, viruses, and fungi. Physicians test for it because CRP is reasonably specific for inflammation and the test is easy to perform.
A higher level of CRP in the blood indicates a higher level of disease activity. In this sense, the CRP is similar to the erythrocyte sedimentation rate (ESR, also called the “sed rate”).
Like the sed rate, the CRP level is an indicator of inflammation, but it is not a specific test for RA. A high level of CRP does not mean a person has RA, and some people with active RA have a low level of CRP—or even no CRP at all. The reason for this aberration is not known. Thus a low CRP level does not always mean that inflammation is not present.
A normal, healthy person should not have a CRP level. Physicians consider a CRP level higher than 1 mg/dL to be elevated. For most infections and significant inflammatory diseases, CRP levels in excess of 10 mg/dL are the norm.
A positive CRP test may be an indicator of several conditions, including RA, rheumatic fever, cancer, tuberculosis, pneumonia, heart attack, bone, joint and skin infections, and systemic lupus erythematosus. An elevated CRP level also can be detected during the last half of a normal pregnancy or in women who are taking oral contraceptives.
Although the sed rate and the CRP level both tend to increase with increased inflammation, sometimes one measure will be raised while the other is normal. This discrepancy can occur because CRP levels rise more rapidly and disappear more quickly than changes in the sed rate. As a result, your CRP level may fall to normal if you have been treated successfully, such as for a flare of arthritis, but your ESR may still be abnormal for a while longer. In light of these differences, doctors often order both the sed rate and CRP tests to make sure they can detect changes in your disease process. The CRP is repeated regularly to monitor the level of your inflammation and your response to medication, because CRP levels drop as inflammation subsides.
The “normal” range for the CRP level may change depending on which laboratory is analyzing your blood sample. If your CRP level is tested repeatedly at the same lab, increases and decreases in this level may be meaningful. Conversely, if your blood is tested in different laboratories that use different “normal ranges,” then changes in the CRP level must be judged with caution.
A CRP level is measured from a small sample of blood that is drawn from a vein, usually from the inside of your elbow or the back of your hand. No special preparation or fasting is required before the blood is drawn.