Information about RA and Rheumatoid Factors
Rheumatoid factors are proteins found in the blood. These proteins, which also are called antibodies or immunoglobulins, are produced by white blood cells (lymphocytes) in the immune system to help the body neutralize harmful substances (antigens), such as viruses and bacteria, and fight disease.
There are three types of rheumatoid factors: IgM-rheumatoid factor (IgM RF), IgG-rheumatoid factor (IgG RF), and IgA-rheumatoid factor (IgA RF). Most, but not all, patients with rheumatoid arthritis (RA) have elevated blood levels of at least one type of rheumatoid factor. High levels of all three factors indicate RA in 99% of cases. Approximately 5% of healthy people also have increased rheumatoid factor levels.
In addition to rheumatoid arthritis, other autoimmune diseases (e.g., systemic lupus erythematosus, Sjögren's syndrome, chronic liver disease, scleroderma, sarcoidosis) and infections (e.g., syphilis, endocarditis, mononucleosis, tuberculosis) can result in high levels of rheumatoid factor. A high blood level of these antibodies sometimes occurs in family members of patients with RA who do not have symptoms of rheumatoid arthritis themselves.
Rheumatoid factor tests (e.g., latex agglutination test) can be used to help diagnose rheumatoid arthritis and other inflammatory autoimmune diseases; however, the blood tests are not used as a screening tool for RA because they do not provide a definitive diagnosis. Additional diagnostic tests that may be performed include blood tests to measure anti-CCP antibodies, C-reactive protein (CRP) level, and erythrocyte sedimentation rate (ESR), and imaging tests (e.g., MRI scan, ultrasound). In patients with joint pain, inflammation, and other symptoms of rheumatoid arthritis, high levels of rheumatoid factor often indicate severe, progressive disease.
Normal RF test results are indicated by rheumatoid factor levels that are less than 40–60 u/mL or by a ratio of less than 1:80 titer (i.e., the concentration of RF antibodies). Risks associated with blood tests are rare and include difficulty obtaining a blood sample, excessive bleeding, lightheadedness, fainting, and infection.