Elevated blood levels of C-reactive protein (CRP) have been shown to indicate an increased risk for cardiovascular events, such as heart attack and stroke. But because nobody knows for sure whether the CRP itself increases risk, or whether instead it is merely a marker for increased risk, doctors can't agree on when to measure it - or what to do about it when it is elevated.
CRP is a protein that is released into the bloodstream during periods of inflammation. Since atherosclerosis (the process that forms plaques in the arteries) is now thought to be at least partly an inflammatory process, it makes sense that CRP levels might be elevated during periods of active plaque formation, and that high levels of CRP would correlate with cardiovascular events.
CRP is measured using a high sensitivity test (called the hs-CRP blood test). In general, the higher the hs-CRP level, the higher the risk. Levels of hs-CRP below 1 are considered low; levels of 1 - 3 are considered moderately elevated; levels greater than 3 are considered high. Levels greater than 10 are usually only seen with active, obvious inflammatory processes, such as severe infection, major trauma, or chronic inflammatory diseases - these ultra-high levels cannot be used to interpret cardiac risk. Because CRP levels can fluctuate over time, most experts now recommend measuring 2 CRP levels a few weeks apart, and averaging the two values.
Deciding whether measuring CRP should be part of the routine screening process has been difficult for two reasons. First, interpreting the significance of an elevated CRP level is often not straightforward, and can lead to more confusion rather than less. Second, it is not clear that treatment should be changed based on the results of CRP values. Specifically, there is no direct evidence that reducing CRP levels will reduce risk.
It is particularly important to note that, in most patients, elevated levels of CRP are associated with the presence of several other risk factors for cardiovascular disease, such as smoking, obesity, sedentary lifestyle, increased cholesterol, hypertension and metabolic syndrome. The highest levels of CRP are usually seen in patients with the greatest number of additional risk factors. So often, finding a high CRP level merely confirms what already is quite apparent - the patient has a high risk of cardiac disease.
On the other hand, an elevated CRP level in patients whose other risk factors place them in a moderate risk category does add information. For these patients, a high CRP level probably indicates that their risk is higher than it otherwise appeared. At the very least, this information should make the patient (and his/her doctor) much more serious about risk reduction.
It is particularly difficult to interpret elevated CRP levels, however, in patients whose cardiac risk is otherwise low. In general, whether the elevated CRP in these patients means their cardiac risk is increased is unknown. However, this generalization may not always be true in women, in whom elevated CRP levels have been reported to increase risk even if cholesterol levels are normal.
The bottom line
CRP levels do not need to be measured in everybody. The first thing that ought to be done is for your doctor (or you yourself) to assess your baseline risk of cardiac disease, based on the more obvious risk factors. ( You can do your own cardiac risk assessment here.) This assessment will tell you whether you are in a high, intermediate, or low risk category.
If you are in a high risk category, don't bother measuring CRP. You're probably going to have a heart attack or stroke unless you aggressively reduce your risk, no matter what the CRP level.
It makes sense to measure a CRP level if you are in the moderate risk category. An elevated CRP level here should send up the red flags.
With today's state of knowledge, most people in the low risk category don't stand to gain much by measuring a CRP level. If the CRP is elevated, nobody will really know what it means or what to do about it. An exception can be argued for individuals with apparently low risk, but a strong family history of cardiovascular disease - especially if you are a woman.
If you do have your CRP measured and it comes back high, you may want to read this article: What To Do When Your CRP Is Elevated.
Sources:
1. Cook NR, Buring JE, and Ridker PM. The effect of including C-reactive protein in cardiovascular risk prediction models for women. Ann Intern Med 2006; 145:21-29.
2. Lloyd-Jones DM, Liu K, Tian L, and Greenland P. Narrative review: Assessment of C-reactive protein in risk prediction for cardiovascular disease. Ann Intern Med 2006; 145:35-42.
3. Davey Smith G, Timpson N and Lawlor D. C-reactive protein and cardiovascular disease risk: Still an unknown quantity? Ann Intern Med 2006; 145:70-72.
