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Antinuclear Antibody, Rheumatoid Factor, and Cyclic-Citrullinated Peptide Testing for the Evaluation of Musculoskeletal Complaints in Pediatric Populations

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Background: Rheumatoid Factor Test

Rheumatoid factors (RFs) specifically react with the Fc fragment of the immunoglobulin (Ig)G molecule. RFs are found in all Ig isotypes (i.e., IgA, IgG, IgD, IgM, and IgE), but 19S IgM-RF is the isotope most frequently used to test for rheumatoid arthritis. The presence of RFs is typically determined by agglutination assays, nephelometry, or EIA. The agglutination assay method mainly employs latex beads as a substrate to which human or rabbit IgG is bound. Nephelometry is a photometric test in which complexes formed between IgG and RF are detected by light scattering, which is dependent on the concentration of those immune complexes. Latex agglutination and nephelometry only measure 19S IgM-RF, whereas enzyme immunoassays have been designed to measure the various RF isotypes. RFs are prevalent among adults with rheumatoid arthritis but are uncommon among children with juvenile idiopathic arthritis (<10% of children with juvenile idiopathic arthritis have a positive RF test result).