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 other types of arthritis, RA causes swollen and painful joints.

Unlike osteoarthritis, which is the result of the natural wear and tear of aging joints with a low level of inflammation, RA is the result of your immune system attacking your joints, causing a lot of inflammation. The exact reason for why this happens is still not fully understood.

Keep reading to learn more about inflammation as well as C-reactive protein (CRP) levels and testing.

If you have RA, your joints are 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. Eventually it resolves itself.

RA-induced inflammation occurs because your immune system mistakes your joints for an invader. Instead of resolving itself, it persists.

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.

The levels of CRP in your blood will also rise when you have an infection or major tissue injury. High CRP levels will fall when the underlying trigger is under control.

No single test can confirm that you have RA. However, measuring levels of CRP in your blood can be part of a comprehensive diagnosis. It can also be used to follow the degree of inflammation over time.

To confirm a diagnosis of RA, your doctor will:

  • Analyze other lab tests, such as the rheumatoid factor antibody and cyclic citrullinated peptide (CCP) antibody.
  • Assess the amount of swelling and pain in your joints and morning stiffness.
  • Document the duration of your symptoms.
  • Examine X-rays of the hands and feet to check for erosions or bone damage.

To get a CRP test, all you need to do is give a sample of blood. Once your blood is drawn, it’ll go to a lab for testing. Your doctor will tell you the results, or you may check them online.

There’s almost no risk associated with having blood drawn for the CRP test.

Your CRP levels should be normal if you don’t have any infections or chronic inflammatory conditions such as RA, Crohn’s disease, or lupus.

CRP is usually measured in milligrams of CRP per liter of blood (mg/L). Normal CRP levels are below 3.0 mg/L. Keep in mind the normal reference range often varies between labs.

A high-sensitivity CRP test can detect levels below 10.0 mg/L. This kind of test is performed primarily to determine risk for cardiovascular disease.

Levels of CRP over 3.0 mg/L are thought to put you at a higher than average risk for heart disease. Levels of CRP over 10.0 mg/L signify infection or an inflammatory condition like RA.

If you’re getting tested for RA, your doctor will likely order a standard CRP test rather than a high-sensitivity test. If your CRP levels are elevated, it may be a sign of RA or another inflammatory condition. However, this alone doesn’t confirm the diagnosis.

Once your doctor confirms an RA diagnosis, they 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 after starting it.

If your levels have dropped, the medication is probably helping. If your CRP levels rise, your doctor will know that you’re having a flare-up. You may need to adjust your medications or try a new treatment.

Measuring CRP levels isn’t a perfect method for diagnosing RA or determining the effectiveness of a treatment. This is because CRP isn’t specific to RA. Elevated levels of CRP can indicate any type of infection or inflammatory condition.

Some studies have shown that up to of people tested had normal CRP levels, yet were considered to have RA.