Should I have a C-reactive protein test?

DEAR DOCTOR K:

The last time I had blood work, my doctor didn’t check my CRP level. Wouldn’t my CRP level have given him a better idea of my risk of heart disease?

DEAR READER:

You ask a good — and controversial — question. Let me say up front that this test has been developed and studied by a colleague of mine at Harvard Medical School, and revenue from the test comes to my colleague and to the hospital where I practice.

Also, I’m talking only about the use of the CRP test to screen for future heart disease in people who are not known to have heart disease. I’m not talking about using the test in people who already have heart disease.

The C-reactive protein (CRP) blood test measures inflammation in the body. What does that have to do with heart disease? We now know that many heart attacks and strokes occur because cholesterol-rich plaques of atherosclerosis rupture. When that happens, blood clots form that can cut off the blood supply to part of the heart or brain. Plaques rupture because of inflammation inside them.

Many studies have shown that the CRP test does, indeed, help estimate a person’s risk for heart disease. The question is whether the test should be used in everyone and how much additional information it gives.

A recent article in the New England Journal of Medicine found that checking CRP levels may not tell us much about cardiovascular disease risk after all, at least in the average person. The review article looked at 52 studies that included 240,000 people. The researchers added the CRP results to standard risk factors, such as diabetes or high cholesterol. Using all of this information, they classified people by their risk of cardiovascular disease and whether they needed preventive treatment.

It turned out that the CRP tests would have prevented only one heart attack or stroke for every 400 to 500 people screened. In other words, the CRP test added little information about cardiovascular disease risk, beyond what the standard risk factors already showed, in the average person.

Several authoritative groups recommend that the test be performed in people who are at intermediate risk for heart disease based on other risk factors: high blood pressure (hypertension), smoking, high levels of LDL cholesterol, diabetes, or close relatives who have developed heart disease at a young age.

The logic in this recommendation is straightforward. If you are at high risk, according to these other risk factors, knowing your CRP level won’t change your doctor’s advice; you really need to eliminate these risk factors to protect your health.

And if you are at low risk, according to these other risk factors, finding that you have a high CRP level won’t lead to any additional testing or treatment that is likely to help protect you. But if you’re at intermediate risk, knowing your CRP level can be valuable.