Blood Tests for Rheumatoid Arthritis
The Diagnostic Puzzle for RA
A chronic inflammation of the joints, rheumatoid arthritis (RA) affects around 1.3 million U.S. adults. RA resembles a number of other disorders and conditions, so it often takes time to diagnose.
If you have morning stiffness and pains in symmetrical joints, you and your doctor have some detective work to do. You could have RA, osteoarthritis, or something as serious as cancer. Or you may simply have a bad mattress.
The Role of Blood Tests
Blood tests don’t give you a simple yes-or-no answer to whether you have RA. But they help your doctor and rheumatologist steer toward a diagnosis. Blood tests narrow down options and suggest how your disease might progress.
After you receive a diagnosis of RA, continued blood tests monitor the side effects of drugs used in treatment. They can also help track the progression of the disorder.
The Diagnostic Tests
Once you leave a sample of your blood at the lab, it may be checked for these factors:
- rheumatoid factor. The lab will check for rheumatoid factor (RF) antibodies that collect in the synovium of the joint. These antibodies are present in about 80 percent of RA patients.
- erythrocyte sedimentation rate (ESR) and C-reactive protein. A high erythrocyte sedimentation rate may indicate inflammation in the joints—as do high levels of C-reactive protein.
- complete blood count. This includes hemoglobin and hematocrit tests, which may reveal anemia, an indicator of RA.
The RF Test
Rheumatoid factor antibodies gather in the synovium, the tissue lining the joints. But the antibodies can also show up in patients with other disorders. The most common form of the antibody is called immunoglobulin IgM. When this antibody attaches to healthy body tissue, it can create damage.
The RF test is not perfect for determining if you have rheumatoid arthritis. While 80 percent of people with RA display this factor, 20 percent don’t (Weinblatt, 2000). And 10 percent of people without rheumatoid arthritis test positive for RF due to hepatitis, leukemia, lupus, or other disorders (Weinblatt, 2000).
Interpreting RF Results
The results of your RF test will be reported as units per milliliter (u/mL) or the concentration of the antibodies in your blood. Normal levels of RF antibodies in your blood are less than 40 to 60 u/mL, or a concentration under 1:80. Higher numbers indicate a positive RF result.
The higher the figure, the more likely you have rheumatoid arthritis or possibly Sjogren syndrome, an autoimmune disorder that causes dry eyes and a dry mouth.
The ESR Test
The erythrocyte sedimentation rate (ESR) test, also called a sed rate, measures inflammation by seeing how red blood cells clump together. In this test, your blood is put into a test tube to observe how quickly erythrocytes—red blood cells—float to the bottom.
The proteins that indicate inflammation cause the red blood cells to stick together and tumble to the bottom of the tube. A faster sedimentation rate indicates greater inflammation.
C-reactive protein (CRP) is produced by the liver as a result of inflammation. CRP doesn’t confirm rheumatoid arthritis; it indicates severity of a condition rather than what specifically is wrong. The CRP test may be more sensitive than the ESR test.
If you have a false negative on the RF test, the ESR or C-reactive protein test may provide an indication of rheumatoid arthritis.
However, the fact that either an autoimmune response or a bacterial infection can result in C-reactive protein can complicate CRP test results.
Complete Blood Count
Complete blood count is a routine test. The test looks at your counts of white and red blood cells. The hemocrit test measures the volume of red blood cells. The hemoglobin test determines your body’s oxygen carrying ability.
Low hemoglobin and hemocrit results can indicate rheumatoid arthritis. Complete blood count tests can also indicate the presence of anemia, often found in tandem with RA.
Putting the Puzzle Together
Your health care providers will study your blood test results. If you have a combination of RF antibodies and symmetric joint pain, this may indicate RA. Joint pain is most typically found in the wrists, hands, elbows, shoulders, ankles, or knees.
X-rays showing joint damage can help confirm a diagnosis. Your doctor may also periodically request X-rays to monitor the progress of your disorder.
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