Rheumatoid arthritis

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 and 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.

RA inflammation

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, but eventually resolves.

RA-induced inflammation occurs because your immune system mistakes your joints for an invader and persists.

C-reactive protein

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 rise when you have an infection. High CRP levels will fall when the infection is under control.

CRP and the 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. It can also be used to follow the degree of inflammation over time.

The criteria for diagnosing RA includes:

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

The CRP test

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

There’s 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.

C-reactive protein is measured in milligrams of CRP per liter of blood (mg/L). Normal CRP levels are below 3.0 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. If your CRP levels are elevated, it may be a sign of RA or another inflammatory disease. However, it’s not a confirmed diagnosis.

CRP levels and response to treatment

Once you’ve 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 after starting the medication. 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 and you may need to try a new treatment.

Problems with CRP tests

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 disease.

Some studies have shown that up to 45 percent of people tested had normal CRP levels, yet were considered to have RA.