Autoimmune Disease

Rheumatoid Factor (RF) testing is a quantitative or semi-quantitative serological assay (nephelometry, turbidimetry, latex agglutination, or ELISA) for detecting autoantibodies against the Fc portion of immunoglobulin G, primarily of the IgM isotype. RF is a key marker in the ACR/EULAR classification criteria for rheumatoid arthritis, with sensitivity 60-80% and specificity 70-90%. Test requires serum samples; results reported in IU/mL (reference <15-20 IU/mL) or titer. Primary clinical applications include diagnosis and classification of rheumatoid arthritis, prognostic assessment (high-titer RF correlates with severe, erosive disease), differential diagnosis of inflammatory arthritis, monitoring disease activity, diagnosis of Sjögren's syndrome, evaluation of mixed cryoglobulinemia (hepatitis C-associated), and assessment of other autoimmune and chronic inflammatory conditions. Critical safety precautions include awareness of false positives (other autoimmune diseases, chronic infections, healthy elderly), false negatives (seronegative RA 20-30%), need for clinical correlation, and anti-CCP antibody testing to improve diagnostic accuracy. Essential test for rheumatology practice, autoimmune disease evaluation, and differential diagnosis of inflammatory arthritis.