C-Reactive Protein (CRP)

Test Overview

A C-reactive protein (CRP) test is a blood test that measures the amount of a protein called C-reactive protein in your blood. C-reactive protein measures general levels of inflammation in your body.

High levels of CRP are caused by infections and many long-term diseases. But a CRP test cannot show where the inflammation is located or what is causing it. Other tests are needed to find the cause and location of the inflammation.

Why It Is Done

A C-reactive protein (CRP) test is done to:

• Check for infection after surgery. CRP levels normally rise within 2 to 6 hours of surgery and then go down by the third day after surgery. If CRP levels stay elevated 3 days after surgery, an infection may be present.
• Identify and keep track of infections and diseases that cause inflammation, such as:
  • Cancer of the lymph nodes (lymphoma).
  • Diseases of the immune system, such as lupus.
  • Painful swelling of the blood vessels in the head and neck (giant cell arteritis).
  • Painful swelling of the tissues that line the joints (rheumatoid arthritis).
  • Swelling and bleeding of the intestines (inflammatory bowel disease).
  • Infection of a bone (osteomyelitis).
• Check to see how well treatment is working, such as treatment for cancer or for an infection. CRP levels go up quickly and then become normal quickly if you are responding to treatment measures.

A special type of CRP test, the high-sensitivity CRP test (hs-CRP), may be done to find out if you have an increased chance of having a sudden heart problem, such as a heart attack. Inflammation can damage the inner lining of the arteries and make having a heart attack more likely. However, the connection between high CRP levels and heart attack risk is not very well-understood.

How To Prepare

There is no special preparation for a C-reactive protein (CRP) test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will indicate. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).

How It Is Done

The health professional taking a sample of your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure on the site and then put on a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (such as Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A C-reactive protein (CRP) test is a blood test that measures the amount of a protein called C-reactive protein in your blood.

Normal

Normal values may vary from lab to lab. Results are usually available within 24 hours.

Any condition that results in sudden or severe inflammation may increase your CRP levels.

Some medicines may decrease your CRP levels.

High-sensitivity C-reactive protein (hs-CRP) levels

The hs-CRP test measures your risk for heart problems. It may be done to find out if you have an increased chance of having a sudden heart problem, such as a heart attack. However, the connection between high CRP levels and heart attack risk is not very well-understood.

Many conditions can change CRP levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and medical history.

What Affects the Test

You may not be able to have the test or the results may not be helpful if:

• You have just exercised.
• You take certain medicines, such as hormone replacement therapy (HRT), birth control pills, nonsteroidal anti-inflammatory drugs (NSAIDs), aspirin, corticosteroids, or medicine to lower your cholesterol (for example, pravastatin).
• You have an intrauterine device (IUD) in place.
• You are pregnant.
• You are very overweight (obese).

What To Think About

• High-sensitivity C-reactive protein (hs-CRP) measures very low amounts of CRP in the blood. This test may help find your risk for heart problems, especially when it is considered along other risk factors such as cholesterol, age, blood pressure, and smoking. But the connection between high CRP levels and heart disease risk is not understood very well.
• High CRP levels before a major surgery may indicate that you are at risk for developing an infection after surgery.
• CRP testing can be used to see how well you respond to cancer treatment or treatment for an infection. Your CRP levels will rise quickly and then quickly return to normal if the treatment is working.
• High CRP levels may increase your chances of having other diseases, such as age-related macular degeneration and colon cancer.

References

Other Works Consulted

• Buckley DI, et al. (2009). C-reactive protein as a risk factor for coronary heart disease: A systematic review and meta-analysis for the U.S. Preventive Services Task Force. Annals of Internal Medicine, 151(7): 483–495.
• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
• Erlinger TP, et al. (2004). C-reactive protein and the risk of incident colorectal cancer. JAMA, 291(5): 585–590.
• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
• Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.
• Pearson TA, et al. (2003). Markers of inflammation and cardiovascular disease: American Heart Association and the Centers for Disease Control and Prevention scientific statement. Circulation, 107(3): 499–511.
• Seddon JM, et al. (2004). Association between C-reactive protein and age-related macular degeneration. JAMA, 291(6): 704–710.
• U.S. Preventive Services Task Force (2009). Using nontraditional risk factors in coronary heart disease risk assessment. Rockville, MD: Agency for Healthcare Research and Quality. Available online: http://www.ahrq.gov/clinic/uspstf09/coronaryhdrs.htm.

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