Rheumatoid Factor is an antibody that is measurable in the blood. It is commonly used as a blood test for the diagnosis of rheumatoid arthritis. Rheumatoid factor is present in about 80% of adults with rheumatoid arthritis. It is also present in patients with other connective tissue diseases and some with infectious diseases. Rheumatoid factor blood test measures the amount of the RF antibody present in most people with rheumatoid arthritis.