A rheumatoid factor test measures the amount of rheumatoid factor in your blood. Rheumatoid factors are proteins produced by your immune system.High levels of rheumatoid factor in the blood are most often associated with autoimmune diseases, such as rheumatoid arthritis and Sjogren's syndrome. But rheumatoid factor may be detected in some healthy people. Also, 20-30% of people with rheumatoid arthritis have normal levels of rheumatoid factor.