What is rheumatoid factor?

Rheumatoid factor (RF) is an antibody the immune system produces against parts of other antibodies. About two-thirds of people with rheumatoid arthritis test positive, and higher levels tend to mean more aggressive disease and more chance of joint erosion or nodules. However RF can also appear in Sjögren's syndrome, chronic infections and in some healthy older adults, so it is never used alone. UK rheumatologists interpret it alongside symptoms, examination, anti-CCP and imaging.