Rheumatoid factor is an antibody, a protein made by the immune cells to specifically recognize and help to destroy bacteria and viruses. Each person makes thousands of different antibodies, each of which can specifically recognize thousands of different foreign substances that may attack the body.

Rheumatoid factor (RF) can be detected with a blood test. Your doctor may order this blood test if there is a suspicion that you might have rheumatoid arthritis or if you are being tested for certain other autoimmune diseases. People with rheumatoid arthritis often have high levels of RF. So a finding of a high RF can support a diagnosis of rheumatoid arthritis in a person with other symptoms of rheumatoid arthritis, such as joint pain with joint swelling and morning stiffness. However, elevation of RF in the absence of symptoms has no bearing on the likelihood that you will develop rheumatoid arthritis.

All people make RF antibodies as part of the normal process of responding to infection. If sensitive enough measures are used, it can be shown that everyone has rheumatoid factors, and these may be elevated normally following infection, blood transfusion, or other events that stimulate the activity of the immune system. As the name implies, though, rheumatoid factor was first identified in patients with rheumatoid arthritis because these patients tend to make a great excess of these antibodies. Higher elevation of RF has been associated with a more aggressive disease progression in rheumatoid arthritis.