You Asked

What is C-reactive protein? My doctor tells me that mine is elevated, and he is concerned about it.

Dr Ockene answers:

C-Reactive protein (CRP) is a substance that is largely produced by the liver and is present in the blood in increased amounts when there is inflammation in the body. It was first discovered in the 1930s when it was used as a test for pneumonia, but it quickly became clear that it was a marker for many types of inflammation. It has been used for decades, for example, to judge how active a patient's rheumatoid arthritis is - the more active the arthritis, the higher the C-Reactive protein level.

In the 1990s, however, Paul Ridker and colleagues at Brigham and Women's Hospital in Boston developed a  high sensitivity C-reactive protein test that could look at very low levels of this substance. They demonstrated that when there were no other causes of inflammation, elevation of C-reactive protein using this high sensitivity test was picking up inflammation in arteries, a sign of vascular disease. In large research studies, they and others have shown that elevation of high sensitivity C-reactive protein predicts increased risk of heart disease and stroke, and also adds to the value of more traditional risk factors such as cholesterol. For an example of one of their articles, click here.

While high sensitivity C-reactive protein is very useful in larger studies, and while it can also be useful in judging the risk of individual patients, it always needs to be remembered that C-reactive protein is not specific for arterial inflammation but will go up in many other conditions. If you have a viral illness such as the flu, your C-reactive protein level can quickly rise a hundred fold or more, and will take several weeks to return to normal. If you have conditions such as rheumatoid arthritis or any other condition that causes inflammation, that too will cause increased C-reactive protein levels. So when a physician uses this test, an elevated level needs to be put into the context of what else is wrong with the patient and whether or not he or she has been ill in the recent past. If C-reactive protein is low, that is good, but if it is high, it is generally worth repeating at least once to see if the second reading is lower.