The C-reactive protein (CRP) test is used to find inflammation in your body. Inflammation could be caused by different types of conditions, such as an infection or autoimmune disorders like rheumatoid arthritis or inflammatory bowel disease. This test measures 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 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 CRP is part of the immune system, your levels of it rise whenever you have inflammation in your body. But the test doesn't show where the inflammation 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. For example, it 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 inflammation and doesn't pinpoint what is causing it, your healthcare provider will likely order more tests. These may include:
Erythrocyte sedimentation rate (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 more information. So many healthcare providers will order it at the same time.
Antinuclear antibody (ANA) test. The ANA test measures autoantibodies that attack your cells. This is important in diagnosing some autoimmune diseases.
Rheumatoid factor (RF). This test is done to help diagnose and monitor rheumatoid arthritis.
Anticyclic citrullinated peptide (anti-CCP) antibodies. This test is done to help diagnose and monitor rheumatoid arthritis.
Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you.
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 is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand.
Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be 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.
Certain medicines can cause your CRP levels to be lower than normal. These include nonsteroidal anti-inflammatory drugs (NSAIDs), aspirin, and steroids.
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 illegal drugs you may use.