Why are EliA dsDNA and Varelisa dsDNA Antibodies only against IgG and not IgG and IgM?
IgM for dsDNA are, similar to IgG, a heterogeneous group of antibodies, some with high avidity, some with low avidity. Farr RIA is a highly specific assay for SLE which measures only IgG and IgM antibodies with high avidity. Low avidity antibodies are not found with this assay. ELISAs in general find antibodies of both, low and high avidity, which leads to a lower specificity but higher sensitivity of these assays.
Anti-dsDNA IgM antibodies have a lower specificity than IgG, particularly when they are of low avidity. Determined by ELISA, they have been demonstrated in a variety of diseases like Sjögren's syndrome, scleroderma, MCTD and chronic active hepatitis.
Specificity is a big aim in autoimmune diagnostics, particularly - to ensure the highest specificity possible, we do not measure IgM.
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