Rheumatoid Arthritis: What CRP Levels Say About You
Rheumatoid arthritis (RA) is a type of arthritis that can affect anyone at any age. However, it’s more common in women and often first appears in middle age. Like any type of arthritis, RA causes swollen, painful joints.
Unlike osteoarthritis, which occurs because of the natural wear and tear on joints as you age, RA is a result of your immune system attacking your joints. Why this happens is not fully understood.
If you have RA, your joints are painful because they’re inflamed. Inflammation is a natural process that occurs when your immune system attacks a foreign invader.
When working correctly, immune cells rush to an area of infection, like a cut, and go to work. This causes the area to become inflamed, red, and painful.
RA-induced inflammation occurs because your immune system mistakes your joints for an invader.
C-reactive protein (CRP) is a protein that’s produced by your liver and can be found in your blood. CRP levels in your blood rise in response to inflammation. For example, the levels of CRP in your blood will rise when you have an infection. High CRP levels will fall when the infection is under control.
CRP and Diagnosis of RA
No single test can confirm that you have RA, but measuring levels of CRP in your blood can be part of a comprehensive diagnosis. The criteria for diagnosing RA include:
- other lab tests, such as scanning blood for an antibody called rheumatoid factor
- the swelling and amount of pain in your joints
- the duration of your symptoms
The CRP Test
To get a CRP test, all you need to do is give a small sample of blood. Your doctor will tell you ahead of time if you need to fast or stop taking certain medications. Once your blood is drawn, it will go to a lab for testing, and your doctor will inform you of the results.
There is almost no risk associated with having blood drawn for the CRP test.
Normal CRP Levels
Your CRP levels should be normal if you don’t have any infections or chronic inflammatory illnesses such as RA, Crohn’s disease, or lupus. Normal CRP levels are below 10.0 milligrams per deciliter (mg/dL). A standard CRP test often can’t even detect normal levels because they’re so low.
A high sensitivity CRP test can detect levels below 10.0 mg/dL. This kind of test is performed to determine risk for cardiovascular disease. Levels of CRP over 3.0 mg/dL are thought to put you at a higher-than-average risk for heart disease.
Elevated CRP Levels
If you’re being tested for RA, your doctor will likely order a standard CRP test rather than a high-sensitivity test. Levels of CRP that can be detected with a standard test are considered elevated. Elevated levels of CRP are indicative of an inflammatory disease, but don’t confirm a definitive RA diagnosis.
CRP Levels and Response to Treatment
Once you have been diagnosed with RA, your doctor may order occasional CRP tests. Your CRP levels are useful in indicating how well your treatments are working. For instance, if you try a new medication, your doctor may test your CRP levels a few weeks later.
If your levels have dropped, the medication is helping. If your CRP levels rise, your doctor will know that you’re having a flare up and may choose to try new treatments.
Problems With CRP Tests
Measuring CRP levels isn’t a perfect method for diagnosing RA or the effectiveness of treatments. This is because CRP isn’t specific to RA. Elevated levels of CRP can indicate any type of infection or inflammatory disease.
Though it’s not understood why, some studies have shown that up to 45 percent of patients had normal levels, yet were considered to have RA.
2010 Rheumatoid Arthritis Classification. (2010). American College of Rheumatology. Retrieved
July 31, 2013, from http://www.rheumatology.org/practice/clinical/classification/ra/ra_2010.asp
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● Handout on Health: Rheumatoid Arthritis. (2009, April). National Institutes of Health. Retrieved July 31, 2013, from http://www.niams.nih.gov/Health_Info/Rheumatic_Disease/#ra_9
● Rheumatoid Arthritis. (2013, March 22). National Institutes of Health. Retrieved July 31, 2013, from http://www.nlm.nih.gov/medlineplus/ency/article/000431.htm
● Sokka, T., and Pincus, T. (2009, January 23). Erythrocyte Sedimentation Rate, C-Reactive Protein, or Rheumatoid Factor are Normal at Presentation in 35% - 45% of Patients with Rheumatoid Arthritis Seen Between 1980 and 2004: Analyses from Finland and the United States. J. Rheum., 2009; 36: 7, 1387-1390. Retrieved July 31, 2013, from http://www.jrheum.org/content/36/7/1387.abstract