Rheumatoid Factor (RF) is an antibody(of IgM class) which is directed against the Fc portion of IgG. RF helps to classify patients with polyarticular juvenile idiopathic arthritis(PJIA) into RF positive and RF negative PJIA.
ILAR (International League Against Rheumatism) criteria require 2 positive tests for RF 3 months apart as there can be transient appearance of RF with any intercurrent illness. RF is positive in over 60% of adult RA patients but is seen in only 5-10% of children with JIA.
RF can also be positive in patients with Sjogren’s syndrome, sarcoidosis, infective endocarditis, tuberculosis. RF can be measured by latex agglutination, sheep RBC agglutination, nephelometry and ELISA. Anti-CCP (Anti-cyclic citrullinated peptide) antibody has a comparable sensitivity to RF in adults, with a higher specificity.
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