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Rheumatoid factors (RF) are antibodies that are directed against the Fc fragment of IgG altered in its tertiary structure. Although the IgM class is the most common RF, IgG and IgA RFs have been reported.
A variety of tests for the determination of RF have been described including agglutination of sheep erythrocytes which have been sensitized with rabbit IgG, agglutination of polystyrene latex particles which have been coated with human IgG as well as radioimmunoassay and enzyme immunoassays.
Over 75% of patients with rheumatoid arthritis (RA) have an IgM antibody to IgG immunoglobulin. This autoantibody (RF) is diagnostically useful although it may not be etiologically related to RA.
The titer of RF correlates poorly with disease activity, but those patients with high titers tend to have more severe disease and thus a poorer prognosis than do sero-negative patients. Tests are usually positive for patients with nodules or clinical evidence of vasculitis.
Diagnosis and prognosis of rheumatoid arthritis
Positive results are consistent with rheumatoid arthritis.
Nonrheumatoid and rheumatoid arthritis (RA) populations are not clearly separate with regard to the presence of rheumatoid factor (RF) (15% of RA patients have a nonreactive titer and 8% of nonrheumatoid patients have a positive titer). Patients with various nonrheumatoid diseases, characterized by chronic inflammation may have positive tests for RF. These diseases include systemic lupus erythematosus, polymyositis, tuberculosis, syphilis, viral hepatitis, infectious mononucleosis, and influenza.
1. Winchester RJ: Characterization of IgG complexes in patients with rheumatoid arthritis. Acad Sci 1975;256:73-81
2. Roberts-Thomson PJ, McEvoy R, Langhans T, Bradley J: Routine quantification of rheumatoid factor by rate nephelometry. Ann Rheum Dis 1985;44:379-383