Rheumatoid factors are a variety of antibodies that are present in 70% to 90% of people with rheumatoid arthritis (RA). Rheumatoid factor (RF), however, can be found in people without RA or with other autoimmune disorders. In general, when no rheumatoid factor is present in someone with RA, the course of the disease is less severe.
Another test for rheumatoid arthritis that measures levels of antibodies that bind citrulline modified proteins (anti-CCP) is more specific and tends to be elevated in patients with rheumatoid arthritis or in those about to develop rheumatoid arthritis. The presence of anti-CCP antibodies can be used to predict which patients will get more severe rheumatoid arthritis.