THIS TEST MEASURES THE AMOUNT OF THE RF ANTIBODY IN THE BLOOD. RHEUMATOID FACTORS ARE PROTEINS PRODUCED BY YOUR IMMUNE SYSTEM THAT CAN ATTACK HEALTHY TISSUE IN YOUR BODY; HIGH LEVELS OF RF IN THE BLOOD ARE MOST OFTEN ASSOCIATED WITH AUTOIMMUNE DISEASES, SUCH AS RHEUMATOID ARTHRITIS AND SJOGREN'S SYNDROME. 80% OF ADULTS WHO TEST POSITIVE FOR RF IN THEIR BLOOD ARE DIAGNOSED WITH RHEUMATOID ARTHRITIS.