Rheumatoid factor testing measures the amount of the rheumatoid factor in the blood. Rheumatoid factor is an autoantibody produced by the immune system. While normal antibodies attack pathogens like bacteria and viruses, autoantibodies such as RF mistakenly attack the body’s healthy cells and tissues. A doctor may recommend a rheumatoid factor test if a patient is experiencing symptoms of rheumatoid arthritis. Fresh serum is the recommended specimen