Rheumatoid Factor

Rheumatoid factor is an autoantibody test that looks for antibodies, most often IgM, that target part of your own IgG antibodies. These immune proteins can appear when the immune system is activated in certain ways, especially in rheumatoid arthritis and some other conditions.

The test is performed on a blood sample using immunoassay methods and is reported in international units per milliliter. It is one piece of information your clinician uses, together with your symptoms, exam findings, and other tests such as anti-CCP antibodies, to understand possible inflammatory joint disease.

Clinicians order RF when you have persistent joint pain, swelling, morning stiffness, or other signs of inflammatory arthritis. A positive result can support the diagnosis of rheumatoid arthritis and may relate to the likelihood of more extensive disease, but the test alone cannot confirm or rule out the condition.

RF can also be present with other autoimmune disorders, chronic infections, lung disease, and in some healthy older adults. Knowing your RF status can guide further testing, prompt earlier referral to a rheumatology specialist, and inform treatment planning along with imaging and inflammatory markers.

Results are interpreted in the context of your story and exam. A negative RF does not exclude rheumatoid arthritis, particularly early on. A positive RF makes rheumatoid arthritis more likely, especially when paired with typical symptoms or a positive anti-CCP antibody, but it does not by itself prove the diagnosis.

If your result is unexpected or borderline, your clinician may repeat the test, check anti-CCP antibodies, ESR or CRP, or look for other causes such as chronic infection. Treatment decisions focus on how you feel and function, not just a lab number, so discuss next steps and follow-up with your care team.