The (non‐)sense of detecting anti‐cardiolipin and anti‐β2glycoprotein I IgM antibodies in the antiphospholipid syndrome

Background The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with the persistent presence of lupus anticoagulant (LAC), anti-cardiolipin (aCL) and/or anti-beta 2glycoprotein I (a beta 2GPI) antibodies of the immunoglobulin G/immunoglobulin M (IgG/IgM) isotype. However, the role of aCL and a beta 2GPI IgM as a serologic marker in APS is debated. Objectives We aimed to assess the diagnostic and clinical value of IgM antiphospholipid antibodies (aPL) in APS within the classification criteria. Patients/Methods Our multicenter study comprised 1008 patients, including APS patients and controls. Anti-CL and a beta 2GPI IgG and IgM antibodies were detected with four commercially available solid phase assays. Results Positivity for aCL and/or a beta 2GPI antibodies was significantly correlated with thrombosis and pregnancy morbidity, independent of the isotype and solid phase assay. Higher odds ratios were obtained for IgG compared to IgM positivity. Isolated IgM was rare in thrombotic APS, but more frequent in obstetric APS, ranging from 3.5% to 5.4% and 5.7% to 12.3%, respectively, dependent on the solid phase assay. In a multivariate logistic regression analysis of aPL, IgM positivity was found to be associated with pregnancy morbidity. However, detection of IgM was not independently associated with thrombosis. Combined positivity for LAC, IgG, and IgM was highly associated with thrombosis and pregnancy morbidity. Conclusions Our data support testing for aCL and a beta 2GPI IgM in women suspected of obstetric APS. However, no added value was found for testing IgM in patients suspected of thrombotic APS. Still, IgM aPL might be useful as a second-line test to improve thrombotic risk stratification.