Factor, rheumatoid: An antibody that is measurable in the blood. Rheumatoid factor is commonly used as a blood test for the diagnosis of rheumatoid arthritis. Rheumatoid factor is present in about 80 percent of adults (and a much lower proportion of children) with rheumatoid arthritis. It is also present in patients with other connective tissue diseases, such as systemic lupus erythematosus and Sj'gren's syndrome, and in some with infectious diseases, including infectious hepatitis.
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