Clinical and Interpretive

Rheumatoid factor is used in the diagnosis and prognosis of rheumatoid arthritis.  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 that have been sensitized with rabbit IgG; and agglutination of polystyrene latex particles that 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.

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.