Rheumatoid factor (RF) is a protein (antibody) made by the immune system that can attack the body's own tissues. An elevated rheumatoid factor may mean the presence of an inflammatory disease.

Rheumatoid factor can be measured with a blood test. This is most helpful for diagnosing rheumatoid arthritis, lupus, and Sjögren's syndrome. Certain heart and liver diseases, infectious diseases (such as tuberculosis), and normal aging may also increase RF levels.