Rheumatoid factor (RF) is an autoantibody against degenerative IgG, usually of the IgM class, and is found mostly in the serum or synovial fluid of patients with rheumatoid arthritis. About 90% of patients with rheumatoid arthritis (RA) have positive RF. IgA-RF is associated with bone destruction, and early elevation of IgA-RF often indicates severe disease and poor prognosis; when IgE-RF is elevated, the disease is already advanced. Certain autoimmune diseases, such as cold globulinemia, progressive systemic sclerosis, dry syndrome, SLE and other patients have a high positive rate; some other diseases such as vasculitis, liver disease, chronic infection can also appear RF. RF in rheumatoid arthritis patients have a high detection rate, RF positive support the tendency to diagnose early RA, such as young women should be distinguished between RA and rheumatic fever; and For the diagnosis of inactive RA, reference to medical history is required. However, RF, like ANA, is not a specific antibody unique to RA. RF is positive in 50% of patients with SLE, and can be positive to varying degrees in other connective tissue diseases such as SS, scleroderma, chronic active hepatitis, and the elderly.