Rheumatoid Factor (RF)

Rheumatoid factors are antibodies directed against the Fc fragment of IgG. These are usually IgM antibodies, but may be IgG or IgA. Rheumatoid factor is present in the serum of a majority of patients with rheumatoid arthritis, depending in part on what method is used. Latex beads coated with human IgG will be positive in 70% to 85% (and have significant numbers of false-positives). Sheep RBCs coated with rabbit IgG will be positive in 60% to 70% (and have fewer false-positives). Many rheumatic conditions and other chronic inflammatory processes also may produce rheumatoid factors. The presence of rheumatoid factor, especially in low titer, is far from diagnostic for rheumatoid arthritis. Furthermore, with increasing age, people with no clinical illness may have rheumatoid factor. The presence of IgM RFs has been reported in as much as 10% of the normal population. Statistically, patients with rheumatoid arthritis who have high titer rheumatoid factor are more likely to have severe disease and systemic involvement than other patients. Rheumatoid factor can be detected in synovial fluid, pleural fluid, and pericardial fluid, but contributes little more than a positive serum test. Some rheumatoid factors may behave as cryoglobulins.