What is CRP?
C-reactive protein (CRP) is a substance that is released into the blood in response to inflammation, the process by which the body responds to injury. Elevated levels of CRP in the blood mean that there is inflammation somewhere in the body, but other tests are needed to determine the cause and location of the inflammation.
Physicians now believe that atherosclerosis, or hardening of the arteries, is an inflammatory process. Atherosclerosis causes only a small amount of CRP to be released into the blood. Therefore, a very sensitive test called a high-sensitivity CRP test (hs-CRP) is used to measure CRP levels.
Who might have a CRP test?
Whether you would benefit from a CRP test depends on your overall risk of heart and blood vessel disease:1,2
- If your overall risk is low (less than a 10% chance of developing heart or blood vessel disease in the next 10 years) no test is needed.
- If your risk is intermediate (10% to 20% risk), you may benefit from a CRP test to better understand your risk. Intermediate-risk patients with high CRP may benefit from more aggressive treatment to control their risk factors.
- If your risk is high (more than 20% in 10 years), or if you have already been diagnosed with heart disease or stroke, you should be treated aggressively regardless of your CRP level, so no test is needed.
How is the CRP test done?
There is no special preparation for a CRP test. A blood sample will be taken from a vein in your arm. The test takes less than a minute and you can go home immediately after.
What do the results mean?
If your CRP is high, you have a higher risk of developing heart disease. A low CRP level does not mean that you can ignore other risk factors for heart disease. A very high reading (more than 10 mg/L) indicates that you have an infection of some sort. The test should be repeated in about 2 weeks after the infection has cleared.1
|CRP Levels & Heart Risk1|
|Risk Category||CRP Level (mg/L)|
|Low||Less than 1.0|
|Average||1.0 to 3.0|
|High||3.0 or more|
The risk categories above were established by the American Heart Association (AHA) and the Centers for Disease Control and Prevention (CDC) in 2003 using information drawn from mostly white populations. When these cutoffs were developed, it was assumed that CRP levels were similar in women and men. However, since then studies have shown that average CRP levels are almost twice as high in women as in men. Even after accounting for other factors that affect CRP levels, women are 60% to 70% more likely to have high CRP than men. Further studies are needed to determine if CRP cutoffs should be different for women.
What are the risks and limitations of this test?
The CRP test is a simple blood test that carries no risks. Your CRP level can be affected by medications and other factors. Hormone therapy, pregnancy, birth control pills, and intrauterine devices (IUDs) can raise CRP levels. Cholesterol lowering statin drugs, anti-inflammatories (such as aspirin, Advil, Motrin, and naproxen) may lower CRP levels. If you have chronic inflammation (such as arthritis) or have recently been ill, your CRP level will be high.
Why is CRP testing controversial?
There is an ongoing debate about how useful CRP really is. Some scientists argue that it doesn't provide any more information about your risk for heart disease than a thorough assessment of established heart disease risk factors.3 The studies showing that CRP help predict heart disease risk take some, but not all, of the established risk factors into account.
In the third National Health and Nutrition Examination Survey of more than 15,000 people 18 years and older (53% were women), only 4% of men and 10% of women had high CRP without having a borderline or abnormal heart disease risk factor.4 The risk factors that were measured included cholesterol levels, blood sugar, blood pressure, smoking, and overweight (using body mass index, BMI). Other studies have also shown no additional benefit for CRP testing.5, 6 It is also unclear whether knowing your CRP level will change how you are treated.
1. Pearson TA, Mensah GA, Alexander RW, et al. Markers of inflammation and cardiovascular disease: application to clinical and public health practice: A statement for healthcare professionals from the Centers for Disease Control and Prevention and the American Heart Association. Circulation. Jan 28 2003;107(3):499-511.
2. Ridker PM, Buring JE, Rifai N, Cook NR. Development and validation of improved algorithms for the assessment of global cardiovascular risk in women: the Reynolds Risk Score. JAMA. Feb 14 2007;297(6):611-619.
3. Lloyd-Jones DM, Liu K, Tian L, Greenland P. Narrative review: Assessment of C-reactive protein in risk prediction for cardiovascular disease. Ann Intern Med. Jul 4 2006;145(1):35-42.
4. Miller M, Zhan M, Havas S. High attributable risk of elevated C-reactive protein level to conventional coronary heart disease risk factors: the Third National Health and Nutrition Examination Survey. Arch Intern Med. Oct 10 2005;165(18):2063-2068.
5. Folsom AR, Chambless LE, Ballantyne CM, et al. An assessment of incremental coronary risk prediction using C-reactive protein and other novel risk markers: the atherosclerosis risk in communities study. Arch Intern Med. Jul 10 2006;166(13):1368-1373.
6. Wilson PW, Nam BH, Pencina M, D'Agostino RB, Sr., Benjamin EJ, O'Donnell CJ. C-reactive protein and risk of cardiovascular disease in men and women from the Framingham Heart Study. Arch Intern Med. Nov 28 2005;165(21):2473-2478.