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The presence of abnormal levels of all three rheumatoid factor (RF) isotypes has a specificity of 99% for Rheumatoid Arthritis. RF in any isotype combination may be found in hepatitis C, Sjogren Syndrome, and other chronic infections. Immunoglobulin M (IgM) autoantibodies against the Fc fragment of immunoglobulin G (IgG) are called rheumatoid factors (RFs). These proteins are produced by B cells and can be found circulating in the blood. Their role is unknown in both healthy individuals and in those with rheumatoid arthritis. Borderline/Equivocal RF results warrant redraw & retesting to confirm. Rheumatoid Factor (RF) levels IgM: Negative: <10 IU/mL Borderline/Equivocal: 10-12.5 IU/mL Positive: >12.5 IU/mL
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