Blood tests to diagnose rheumatoid arthritis
Although blood tests cannot give a firm rheumatoid arthritis diagnosis, they can help doctors and specialist rheumatologists narrow down clues in the blood about arthritis and inflammation.
What blood markers are used to diagnose rheumatoid arthritis?
Rheumatoid factors are a variety of antibodies that are present in seventy to ninety percent of people with rheumatoid arthritis (RA). However a low level of rheumatoid factor can be found in people without rheumatoid arthritis or with other autoimmune disorders. In general, when no rheumatoid factor is present in someone with rheumatoid arthritis, the course of the disease is less severe. A new test for rheumatoid arthritis, which measures levels of antibodies that bind citrulline modified proteins (anti-CCP), is more specific and tends to be only elevated in patients with rheumatoid arthritis or in patients about to develop rheumatoid arthritis. The presence of anti-CCP antibodies can be used to predict which patients will get more severe rheumatoid arthritis.
Are there tests to determine inflammation?
Yes. The erythrocyte sedimentation rate (sed. rate or ESR) is an indication of the degree of inflammation in the body. It is actually a measurement of the speed with which red blood cells fall in a test tube of blood. When the inflammation in the blood goes up, these inflammatory substances attach to red blood cells and the cells fall faster. In healthy persons the ESR is low, but it climbs with inflammation. It doesn't point to any particular disease, but is a general indication of the amount of inflammation in the body. In lupus and polymyalgia rheumatica, the ESR often correlates with disease activity.
C-reactive protein (CRP) also indicates the amount of inflammation present. It is thought to be a better test than the ESR. In patients with rheumatoid arthritis, if the CRP is high, it suggests that there truly is significant inflammation or injury in the body.
Both CRP and ESR levels are used to monitor disease activity and how well someone is responding to treatment.
What tests indicate connective tissue disorders such as lupus?
The ANA is a blood test that is used in the evaluation of possible lupus or other connective tissue disorders. When the ANA is positive, it indicates that someone may have an autoimmune disorder, but alone it can't make the diagnosis.
The ANA profile is a series of tests consisting of an ANA and other related antibodies. This may be done if the ANA is found to be positive or possibly at the same time as the ANA. This profile helps look for diseases such as Sjögren's syndrome, lupus and drug-induced lupus.
What does the presence of HLA-B27 indicate?
HLA-B27 is a genetic test. In patients with inflammatory arthritis of the spine and joints (not osteoarthritis), a positive HLA-B27 test is associated with the presence of one of a group of diseases called seronegative spondyloarthropathies. This includes diseases such as ankylosing spondylitis (AS), psoriatic arthritis and Reiter's syndrome. HLA-B27 is present in about 90% of people with ankylosing spondylitis, but the gene can also be seen in people with no sign of arthritis or inflammation.