Clinical Significance

Rheumatoid factors (RF) are antibodies directed against antigenic determinants on the Fc fragment of IgG. These are usually IgM antibodies, but may be IgG, IgA or IgE.

Rheumatoid factor sensitivity in rheumatoid arthritis varies from 30% in population-based studies to 70 – 80% in hospital-based studies, where the disease tends to be more severe. Higher titers of RF are more specific for the diagnosis of rheumatoid arthritis (RA) and are more common in patients with rapidly progressive joint destruction and in those with extraarticular manifestations such as subcutaneous rheumatic nodules. However, RF is a non specific test and a positive RF is observed in 1 – 5% of the healthy population at low titers and in 15 – 20% of elderly subjects with other chronic disease states. Similarly a negative result does not exclude RA. Approximately 25% of patients with a diagnosed case of RA may present with a negative result for RF.

A positive RF is also seen in autoimmune rheumatic diseases and in non-rheumatic conditions with variable frequency e.g. SLE, Sjögren’s syndrome, subacute bacterial endocarditis and other bacterial infections, infectious hepatitis, chronic liver diseases, chronic active pulmonary diseases, parasitic infections and viral infections.