The RA test measures rheumatoid factor (RF) in the blood. RF is an auto-antibody that causes inflammation and joint damage. RF can be detected in nearly 80% of individuals with rheumatoid arthritis. However, its presence is not entirely specific, and RF may be present in other autoimmune disorders (30% to 80% of cases of Sjögren’s syndrome, certain persistent bacterial, viral or parasitic infections, and certain cancers). More rarely, positive RA tests can occur in individuals with lung, liver or kidney conditions. Positive results can also be seen in 5% of healthy subjects under age 50 and in 15% of healthy individuals over age 70.
RA results must be interpreted in light of symptoms and clinical history. In individuals with signs and symptoms of rheumatoid arthritis, a positive RA titre suggests the presence of the disease. (Other criteria including radiological criteria, CRP levels, number of joints affected, etc. are also taken into account.)
A negative RA test does not rule out this diagnosis. About 20% of patients with rheumatoid arthritis will have a negative RA test.