Rheumatoid Factor
September 12th, 2020
Overview
Rheumatoid factor is a heterogeneous group of immunoglobulins against antigens classically found to be present in rheumatoid arthritis.
Normal Range
- <20 U/mL
Causes of Elevated Rheumatoid Factor
- Physiologic - elevated in 15% of normal people
- Inflammatory disease - rheumatoid arthritis, Sjögren's, SLE, sarcoidosis, pulmonary fibrosis, cryoglobulinaemia
- Bacterial infection - subacute bacterial endocarditis, TB, syphilis
- Viral infection - hepatitis C, EBV, CMV, HIV, rubella
- Parasitic infection - malaria, toxoplasmosis, Chagas disease
- Cirrhosis
- Malignancy
Pearls
- Rheumatoid factor is classically used as a marker of rheumatoid arthritis, with 70% sensitivity and 70% specificity - therefore a positive test does not confirm RA and a negative test does not rule out RA
- The test is generally interpreted with anti-CCP antibodies as the two tests together are more predictive
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