A rheumatoid factor (RF) test looks for rheumatoid factor (RF) in a sample of your blood. Rheumatoid factors are proteins made by the immune system.

Normally, your immune system makes proteins called antibodies to attack germs that could make you sick. But rheumatoid factors are antibodies that sometimes attack healthy cells and tissues in your body by mistake. When this happens, you have an autoimmune disorder.

Not everyone has rheumatoid factors in their blood. And some people who have them are healthy. But if you have certain symptoms and higher levels or rheumatoid factors, you may have an autoimmune disorder or another health problem related to high RF levels.

RF testing is mostly used with other tests to help diagnose rheumatoid arthritis. Rheumatoid arthritis (RA) is a type of autoimmune disorder that damages your joints and causes pain, swelling, and stiffness. It is a chronic (long-lasting) condition that can also affect your organs and cause other symptoms.