The RF (rheumatoid factor) test is a blood test that looks for antibodies called rheumatoid factors. These are auto-antibodies that are produced by the immune system that can attack a variety of healthy tissues in the body.
The RF test is usually ordered when a person is suspected of having Rheumatoid Arthritis (RA) or other rheumatic diseases. However, the RF test is not a very specific test. In the early stages of RA, about half of people will have a positive RF test and the other half will have a negative RF test. So having a negative RF test does not necessarily rule out a diagnosis of RA.
In established (meaning definitively diagnosed) RA, 75-80% of people will have a positive RF test. Once the RF test is positive, it rarely becomes negative unless the RF test was very low in the first place.
The RF test can be helpful in diagnosing RA. Since not everyone with RA will test positive, doctors interpret the results in the context of a person’s other signs and symptoms, as well as the results of other tests.
Science Behind this Test
Rheumatoid factors are a type of autoantibody: an antibody produced by the immune system that targets something that the body’s own tissues are made of.
Why the RF test is done
The RF test is usually ordered when a person has signs and symptoms of RA. It is one of many tests that might be ordered including other autoimmune tests (e.g. anti-nuclear antibody [ANA] test, anti-cyclic citrullinated peptide [anti-CCP] test), a complete blood count (CBC), and other makers of inflammation (e.g. C-reactive protein [CRP], erythrocyte sedimentation rate [ESR]).
An RF test can be positive years before the onset of symptoms of RA.
RF test results
This test’s results are important to consider because it provides a lot of insight into a person’s potential case of RA.
In a person with signs and symptoms of early arthritis and a positive RF test:
- The arthritis is likely to be persistent (it won’t go away)
- A high RF level (in the 100s or 1000s) can mean the arthritis is more severe
- It is very likely that the person has RA
In a person without signs and symptoms of early arthritis and a positive RF test:
- The risk of developing RF in the future is increased
- There may be another disease process causing the positive RF test such as a chronic infection (e.g. hepatitis B or C) or another autoimmune disease (e.g. Sjogren’s syndrome)
In a person with signs and symptoms of early arthritis and a negative RF test:
- The RF test can become positive over time and the arthritis can persist
- The RF test can remain negative over time and the arthritis can persist
- The arthritis can get better