Rheumatoid factor

An autoantibody directed against the Fc portion of IgG. Elevated RF levels may support the diagnosis of rheumatoid arthritis but it may also occur in other autoimmune, infectious, and inflammatory conditions.

• Low positive: 20–40 IU/mL

• High positive: >40 IU/mL

Causes

• Autoimmune and connective tissue diseases

  • Rheumatoid arthritis (sensitivity ~75%)

  • Sjögren's syndrome

  • Systemic lupus erythematosus (SLE)

  • Scleroderma (systemic sclerosis)

  • Sarcoidosis

  • Chronic inflammatory states

• Infectious causes

  • Hepatitis C virus (HCV)

  • HIV infection

  • Tuberculosis

  • Infective endocarditis

  • Malaria and other parasitic infections

• Leukemia

• Advanced age (low-level RF positivity without disease)