Rheumatoid factors in patients with rheumatoid arthritis.

180 Canadian Family Physician • Le Médecin de famille canadien d VOL 52: FEBRUARY • FÉVRIER 2006 I is important for family physicians to understand the role of rheumatoid factor (RF) in management of patients with rheumatoid arthritis (RA). Rheumatoid factors are antibodies that recognize the Fc portion of IgG molecules as their antigens; RFs can be of any immunoglobulin isotype (IgM, IgG, IgE). Most of the RFs measured clinically are IgM RFs. Rheumatoid factors probably developed in humans as a mechanism for helping remove immune complexes from the circulatory system. Many conditions associated with chronic infl ammation are also associated with RF (Table 1). Incidence and titre increase with age. By age 70, up to 14% of the population will test positive for RF.1 Testing for RF is not a screening test for RA in patients with musculoskeletal pain without joint swelling. The predictive value of a test for RA depends on the sensitivity and specificity of the test and the pretest probability of RA. Because RF is present in numerous conditions other than RA, its specifi city is limited; because RF is not present in all patients with RA, sensitivity is also reduced. In patients with diff use musculoskeletal pain without joint swelling, the predictive value of the test is about 16%. In patients with a high pretest probability of RA (ie, those with symmetric polyarticular joint swelling), the predictive value of the RF factor increases to 80%. For patients with RA, RF is an important predictor of poor prognosis.2 In high titres it portends increased joint erosion, extra-articular manifestations, and greater disability. Rheumatoid factor deposited in tissue is responsible for the formation of rheumatoid nodules. About 70% of RA Rheumatoid factors in patients with rheumatoid arthritis