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The C-reactive protein (CRP) test is used to find swelling and infection in your body. It does this by measuring the amount of CRP in your blood. CRP is a protein made by the liver and sent into the bloodstream. Blood levels may be higher when you have swelling (inflammation) or an infection. Because CRP levels often go up before you have symptoms of pain or fever and drop down as you recover, the CRP test is especially useful for tracking infections.
Because C-reactive protein is part of the immune system, your levels of it rise whenever you have swelling in your body. In fact swelling can trigger a large jump in CRP. But the test doesn't show where the swelling is or what is causing it.
A high-sensitivity CRP test (hs-CRP) may be used to measure your heart disease risk even if you seem healthy. It can find much smaller changes in CRP levels than the regular CRP test.
You may need this test if your healthcare provider thinks you have an infection. This may be osteomyelitis, which involves bone. Or it may be a potentially life-threatening condition called sepsis or blood poisoning.
Symptoms of sepsis may include fever and chills, headache, pain, nausea, vomiting, confusion, rash, and shortness of breath. The level of CRP in your blood goes up within a few hours of a serious infection. CRP levels can also rise when you have a viral infection. But they don’t go as high as during a bacterial infection.
Your provider may also order the test if he or she thinks you have an inflammatory condition or an autoimmune disease, such as rheumatoid arthritis. CRP levels are higher in people with these conditions. This test may also be used to watch flare-ups and recovery. Symptoms of rheumatoid arthritis include joint swelling and pain, morning stiffness, tiredness, weight loss, and low-grade fevers.
If you are being treated for a long-term inflammatory condition or an infection, this test may be used to see if the treatment is working.
The more specialized CRP test for heart disease risk is not covered in this article.
Because the CRP test tells you only if you have swelling and doesn't pinpoint what is causing it, your healthcare provider will likely order more tests. These may include:
Erythrocyte sedimentation rate, or ESR, test. Like the CRP, this test measures inflammation. It's not as sensitive as the CRP, but it's easy to do and gives additional information, so many doctors will order it at the same time.
Antinuclear antibody (ANA) test. The ANA test measures autoantibodies that attack your cells. This is important in the diagnosis of some autoimmune diseases.
Rheumatoid factor, or RF. This test is done to help diagnose and monitor rheumatoid arthritis.
Anticyclic citrullinated peptide, also called anti-CCP, antibodies. This test is done to help diagnose and monitor rheumatoid arthritis.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.
The normal range for this test depends on the lab and the procedure used in the lab. Generally, a CRP level under 10 milligrams per liter (mg/L) is considered normal. If the level of CRP in your blood is higher than that, it may mean your body is having an inflammatory reaction to something. More tests will be needed to figure out what's causing the inflammation.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
A number of things may cause your CRP levels to be slightly higher than normal. These include obesity, lack of exercise, cigarette smoking, and diabetes.
Medicines such as nonsteroidal anti-inflammatory drugs, aspirin, and steroids can cause your CRP levels to be lower than normal.
You don’t need to prepare for this test. Be sure your healthcare provider knows about all medicines, herbs, and supplements you are taking. This includes medicines that don’t need a prescription and any illicit drugs you may use.
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