C-reactive protein is a test that measures the amount of a protein in the blood that signals acute inflammation.

Blood will be taken from a vein, usually from the inside of your elbow or the back of the hand. The health care provider will clean the area with a germ-killing product (antiseptic). An elastic band may be placed around your arm. The band helps apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

The health care provider will insert the needle into your vein. Blood collects into an air-tight container or syringe connected to the needle. While the blood is flowing into the container, the band is removed to restore circulation.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

At the laboratory, your blood sample is mixed with a liquid called an antiserum, which contains substances that looks for the specific protein.

No preparation is necessary for this test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

C-reactive protein is produced by the liver. The level of CRP rises when there is body-wide (systemic) inflammation. The CRP test is considered a general test, not a specific one. In other words, it can reveal that there is inflammation present in the body, but it cannot tell you where it is.

Your health care provider might use the CRP test to check for flare-ups of inflammatory diseases like rheumatoid arthritis, lupus, or vasculitis. The test might also be used to tell if anti-inflammatory medicine is working. However, a low CRP level does not always mean that there is no inflammation present. Levels of CRP may not be increased in persons with rheumatoid arthritis and lupus. The reason for this is not known.

A more sensitive CRP test, called a high-sensitivity C-reactive protein (hs-CRP) assay, is available to determine a person's risk for heart disease. Many consider a high CRP level to be a positive risk factor for heart disease. However, is not known whether CRP is merely a sign of cardiovascular disease or whether it actually plays a role in causing cardiovascular problems.

Normal CRP values vary from lab to lab. Generally, there is no CRP detectable in the blood.

Your doctor may also use a highly sensitive test called hs-CRP to determine your risk of heart disease. According to the American Heart Association:

• You are at low risk of developing cardiovascular disease if your hs-CRP level is lower than 1.0mg/L.
• You are at average risk of developing cardiovascular disease if your levels are between 1.0 and 3.0 mg/L.
• You are at high risk for cardiovascular disease if your hs-CRP level is higher than 3.0 mg/L.

A positive test means you have inflammation in the body. This may be due to a variety of different conditions, including:

• Cancer
• Connective tissue disease
• Heart attack
• Infection
• Inflammatory bowel disease (IBS)
• Lupus
• Pneumococcal pneumonia
• Rheumatoid arthritis
• Rheumatic fever
• Tuberculosis

This list is not all inclusive.

Note: Positive CRP results also occur during the last half of pregnancy or with the use of birth control pills (oral contraceptives).

The risks associated with puncturing a vein are slight:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.