A "rheumatoid factor" or "RA factor" quantitative test is a blood test that measures the exact level of rheumatoid factor (RF) present in a person's blood, which is a protein produced by the immune system that can sometimes attack healthy tissues, and is often used to help diagnose conditions like rheumatoid arthritis (RA) and other autoimmune disorders; a high level of RF indicates a potential presence of these conditions, although further testing is usually required for confirmation.