Rheumatoid factor

Background

• IgM antibody to the Fc region of IgG
• Most asymptomatic people with positive RF do not progress to rheumatoid arthritis
• Neither specific nor sensitive for RA

Differential Diagnosis

• Rheumatic disorders
  • Rheumatoid arthritis (26-90%)
  • Systemic lupus erythematosus (15-35%), Sjögren syndrome (75-95%), and mixed connective tissue disorder (50-60%)
  • Type II and III cryoglobulinemia (40-100%)
  • Polymyositis and dermatomyositis (5-10%)
• Chronic infections
  • Bacterial endocarditis (25-50%)
  • Hepatitis B and hepatitis C (20-75%)
  • Other viruses, such as HIV, Epstein-Barr virus, and CMV
  • Some parasites, including malaria, schistosomiasis, and toxoplasmosis
• Inflammatory disorders
  • Cirrhosis (25%)
  • Primary biliary cirrhosis (45-70%)
  • Idiopathic pulmonary fibrosis (10-50%)
  • Sarcoidosis (3-33%)
  • Silicosis and asbestosis (30-50%)
• Malignancies
  • B-cell disorders (5-25%)
• Normal aging
  • ≥65 years of age (5-25%)