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Rheumatoid factor is an antibody that can be detected in the blood through laboratory testing. This test is commonly ordered when doctors suspect a patient may have rheumatoid arthritis or other autoimmune conditions. Rheumatoid factor is an antibody that mistakenly targets the body's own tissues, specifically attacking other antibodies called immunoglobulin G. While the presence of rheumatoid factor is often associated with rheumatoid arthritis, it can also appear in other conditions and even in some healthy individuals.

The rheumatoid factor test is typically performed using a blood sample drawn from a vein in the arm. The sample is then analyzed in a laboratory to measure the level of rheumatoid factor antibodies present. Results are usually reported as a titer or concentration, with higher levels generally indicating a greater likelihood of autoimmune disease. However, the test results must be interpreted carefully alongside other clinical findings, as not all people with rheumatoid arthritis test positive for rheumatoid factor, and not everyone with a positive result has the disease.

Healthcare providers may order this test when patients present with symptoms such as joint pain, swelling, stiffness, or other signs suggesting rheumatoid arthritis. The test can help support a diagnosis, but it is rarely used alone. Doctors typically combine rheumatoid factor results with other laboratory tests, imaging studies, and physical examination findings to make an accurate diagnosis. Additional tests that may be ordered alongside rheumatoid factor include anti-CCP antibodies, C-reactive protein, erythrocyte sedimentation rate, and complete blood count.

A positive rheumatoid factor test does not automatically mean a person has rheumatoid arthritis. Elevated levels can occur in other autoimmune diseases such as lupus and Sjogren's syndrome, as well as in certain infections, liver diseases, and even in a small percentage of healthy older adults. Conversely, some people with rheumatoid arthritis may test negative for rheumatoid factor, especially in the early stages of the disease. These individuals are referred to as having seronegative rheumatoid arthritis. The interpretation of rheumatoid factor results should always be done by a qualified healthcare provider who can consider the complete clinical picture.