How are Rheumatoid factor measured
What are Rheumatoid factors (RFs)?
Rheumatoid factor (RF) is the general term used to describe an autoantibody directed against antigenic determinants on the Fc fragment of immunoglobulin G. RF may be of any isotype: IgM, IgG, IgA, or IgE. IgM RF is the primary isotype routinely measured by clinical laboratories, using nephelometry, ELISA, and latex agglutination techniques. In RA, RF has a sensitivity of 60% to 80% and specificity of 80% to 90%. RA patients who are RF-positive tend to have more aggressive joint disease and are at an increased risk to develop extraarticular manifestations. Disease activity of rheumatoid arthritis is best determined by clinical assessment and not by RF titer. Teleologically, RFs probably developed in humans as a mechanism to help remove immune complexes from the circulation. Therefore, many conditions associated with chronic inflammation are also associated with RF positivity. Medicare cost for an RF test is $7.00.
RF can be positive in normal individuals but usually at a low titer (<50 IU, < 1:160), with males and females affected equally. Age affects the frequency of a positive RF in normal individuals with 2%–4% (20–60 years), 5% (60–70 years) and 10%–25% (>70 years) positive at low titers.