II. Mechanism
- Polyclonal B Cell activation synthesizes RF autoantibody
- RF IgM (rarely IgG) Antibody
- Binds IgG constant region (crystallizable fragment)
- Serum Detection
- Latex fixation
- Agglutination of latex particles
- Latex coated with human IgG
- Sheep Cell Agglutination Assay indications
- Negative latex fixation despite high suspicion
- Latex fixation
III. Causes: Negative
- Normal
- Conditions in which Rheumatoid Factor is expected to be negative
IV. Causes: Positive
- Rheumatoid Factor present in normal persons
-
Rheumatoid Arthritis (33-90%)
- RF Titer alone does not diagnose Rheumatoid Arthritis
- Test Sensitivity: 69%
- Test Specificity: 85%
- Positive Likelihood Ratio (LR+): 4.86
- Negative Likelihood Ratio (LR-): 0.38
- Prognosis worse with high titer at onset
- Higher titers are associated with erosive Arthritis, extraarticular involvement
- RF Incidence increases with duration of disease
- Three months: 33%
- Six months: 45%
- One year: 75%
- Eighteen months: 90%
- Up to 20% of RA patients remain seronegative
- RF Titer alone does not diagnose Rheumatoid Arthritis
- Other Autoimmune Disease
- Sjogren's Syndrome (75 to 95%)
- Systemic Lupus Erythematosus (15 to 35%)
- Scleroderma (20 to 30%)
- Polymyositis or Dermatomyositis (10%)
- Cryoglobulinemia (40 to 100%)
- Mixed Connective Tissue Disease (50 to 60%)
- Infections
- Bacterial Endocarditis
- Osteomyelitis
- Tuberculosis
- Syphilis
- Hepatitis (Acute and Chronic)
- Mononucleosis
- Parasitic Infection
- Viral Infection (esp. Mumps, Rubella, Influenza)
-
Lung Disease
- Asbestosis
- Diffuse interstitial pulmonary fibrosis
- Sarcoidosis
- Silicosis
- Gastrointestinal Conditions
- Cancer