Blood tests are never fun. And if you’re not even sure what is being tested for, they can be frustrating, too. Knowing what you’re being tested for — and why — can help you feel more in charge of your medical history. One blood test your doctor may order if you have rheumatoid arthritis, or if RA is suspected, is a C-reactive protein (CRP) test. Here’s what this test is for and what your results might mean.
Most people are familiar with the term arthritis, but are not always as familiar with its different categories. Arthritis is simply a term for joint inflammation. Unlike osteoarthritis, where the joint inflammation is caused by joint tissue breaking down over time, rheumatoid arthritis is an autoimmune condition where otherwise healthy joint tissue is attacked by the immune system.
In an autoimmune or inflammatory condition, the body’s ordinary tissues are seen as foreign and are attacked by the immune system. In RA, the specific type of tissue under attack is the synovial membrane. This is tissue that lines the joints and keeps them lubricated and nourished with a substance called synovial fluid. When the tissue is being attacked, just like any suspected foreign invader, inflammation results. This inflammation is responsible for the joint pain and stiffness that are early symptoms of RA.
A CRP test measures levels of C-reactive proteins in the blood. These proteins are made by the liver. If you have inflammation in any area of the body, CRP is sent through the blood to the affected area. A high level of these proteins indicates inflammation in the body.
RA is one of many conditions that can cause elevated C-reactive protein levels. If your doctor suspects you may have RA, this is typically one of the tests you will undergo. CRP levels of 10 milligrams per liter (mg/L) or lower are considered normal. Higher CRP levels may mean infection or chronic illness, or may be the result of physical trauma.
The body produces C-reactive protein in response to inflammation. Since inflammation is the main cause of RA symptoms, CRP is one of the markers that can be tested to see if an RA diagnosis is likely.
CRP may be produced in response to another protein agent called interleukin-6 (IL-6). Interleukin-6 is one of the proteins produced when the immune response from RA starts to attack the joints. These proteins, which belong to a category called cytokines, are typically produced in response to acute injury or infection. But if normal tissue is believed to be an invader, the immune system will produce cytokines to attack it.
After an RA diagnosis, your doctor may monitor your CRP levels periodically. One reason for monitoring is to check the response to a current treatment. Since CRP is produced in response to inflammation, a drop in CRP levels after treatment begins means there is less inflammation in the body. A drop in inflammation may have other causes, but a lower CRP level combined with other test results and an improvement of symptoms can mean a particular treatment is effective. Conversely, if CRP levels are the same or elevated after treatment, and other test results continue to show inflammation, it may show that treatment is not working.
Some treatments are monitored for effectiveness using CRP level tests. Actemra (Tocilizumab) is one of these. Actemra is believed to work by blocking interleukin-6, the protein responsible for elevated CRP levels in rheumatoid arthritis. If Actemra is working properly, CRP levels should decrease during use.
CRP levels are also used to determine if a person is having a flare-up of RA. While CRP levels alone aren’t diagnostic, a rise can indicate worsening inflammation. Because of this, your doctor may take periodic CRP levels to determine your baseline score. Once baseline results are established, it’s easier to see when your condition is beginning to flare again. Additional treatments can then be started before the flare-up gets to an unmanageable point.
CRP levels are only one tool used to help diagnose RA. A low or normal CRP level doesn’t necessarily mean that you don’t have RA, and an elevated one doesn’t necessarily mean you do. As one member of myRAteam wrote, “All of my CRP tests have been normal, even though I have visible inflammation and pain.”
A study of people with RA in Finland and the U.S. found that, while many participants had elevated CRP test results, between 44 percent and 58 percent had normal levels. This tells us that a normal CRP level does not rule out RA.
Another study periodically tested CRP levels in a large group of women to see which ones would later develop RA. The study did not find increased CRP levels directly before diagnosis. People who are obese have also been found to have higher CRP levels.
Since CRP measurement is not the only indicator of RA, and normal CRP levels can be found in those with RA, other blood tests may be ordered at the same time if RA is suspected. Two other biomarkers — erythrocyte sedimentation rate (ESR) and rheumatoid factor (RF) — are usually checked along with CRP.
RF tests measure a different kind of protein, and high levels of this protein are also a sign a person may have RA. ESR tests are another measure of inflammation. ESR is a less sensitive test than CRP and is not as useful a diagnostic marker, but since it’s easy to test for, it’s usually run at the same time as CRP. If all three levels are elevated, this is a stronger indication of RA — although it is not definitive.
One myRAteam member told another about their test results: “If [your CRP levels] plus your RF was positive, that’s pretty strong evidence of definite RA.” Even if all three levels are at normal rates, a diagnosis of RA can still be made if other symptoms are present. Conversely, elevated levels of all three do not mean a person has RA. Another member wrote, “Just for the record, blood tests for RA are only part of the diagnosis puzzle. … They are never enough alone to diagnose RA. Some folks actually have high RF levels, high anti-CCP, high CRP, and high ESR level, and definitely do NOT have RA.”
Lab tests, therefore, are only one part of how RA is diagnosed.
Do you have questions about CRP levels? What was your path to RA diagnosis? Comment below or start a conversation on myRAteam.