IgM,but not IgA rheumatoid factor interferes with anti-cardiolipin and anti-β2 glycoprotein I measurements: a quantitative analysis

IgM rheumatoid factor (RF) is sometimes referred to as capable of causing interference in the IgM anti-cardiolipin (aCL) testing. Published guidelines are, however, inconsistent, and evidence regarding the interference is limited. Our goal was investigate IgM and IgA RF cross-reactivity and/or interference in IgM and IgA aCL and anti-β2 glycoprotein I (aβ2GPI) testing. Serum specimens with high IgM and IgA RF levels were tested for IgG, IgA and IgM aCL and aβ2GPI antibodies to examine cross-reactivity. Samples containing IgG aCL and aβ2GPI antibodies were spiked with IgM (and IgA) RF, and samples with high RF levels were spiked with IgG aCL antibodies. The mixtures were tested for IgM and IgA aCL and aβ2GPI antibodies. Specimens with high IgM and IgA RF concentrations did not test positive for IgM or IgA aCL and aβ2GPI antibodies (except one weak positive IgA aβ2GPI result), indicating the lack of cross-reactivity. In the spiked specimens, addition of IgM RF caused significant positive bias in the measurement of both aCL and aβ2GPI antibodies of IgM isotype in the presence of IgG aCL and aβ2GPI antibodies. The threshold for triggering significant interference was 318IU/ml for IgM RF, and 77 GPLU/ml for IgG aCL. Neither IgM, nor IgA RF, however, affected the IgA antiphospholipid (aPL) antibody testing. IgM RF can cause a false-positive IgM aCL result in the presence of IgG aCL antibodies. In studies on the prevalence and clinical significance of IgM aPL antibodies, RF interference should be considered and RF testing should be performed.