The clinical relevance of the IgM isotype of antiphospholipid antibodies in the vascular antiphospholipid syndrome

•Currently, clinical value of IgM antiphospholipid antibodies is a matter of debate.•Isolated positivity for IgM isotype allowed us to classify as APS in 12.3% of patients.•IgM isotype seems significantly associated with retinal thrombosis as onset of APS.

ObjectivesBecause it has been suggested that the IgM isotype of antiphospholipid (aPL) antibodies should no longer be included in the laboratory criteria for antiphospholipid syndrome (APS) classification, we assess the clinical relevance of IgM isotype of aPL in a cohort of patients with vascular APS.Patients/methodsMean age, sex, the presence of autoimmune diseases other than systemic lupus erythematosus, risk factors for thrombosis, the type/s and site/s of thromboembolic events, the levels of C3 and C4, and autoantibody profile were evaluated in a large cohort of persistently aPL positive patients fulfilling the Sydney criteria for APS. Patients with isolated IgM isotype were compared for each variable with those with any other aPL antibody combination.ResultsOne hundred six patients were assessed; of these 55 (51.9%) had venous thromboembolism, 48 (45.3%) arterial thrombosis, and 3 (2.8%) small vessel thrombosis. Positivity to only IgM aPL made possible to classify 13 patients (12.3%) as vascular APS. In all cases the presence of IgM aPL was at medium-high titer, confirmed, and found to be stable in the time. There were four patients with retinal thrombosis (3.8%) and the prevalence of this event was significant in the isolated IgM isotype positive patients (p = 0.005).ConclusionsData from this investigation give clinical value to the IgM isotype of aPL and suggest to consider aCL and anti-β2GPI of IgM class as valid laboratory criteria for APS classification, especially when they are associated and stable overtime.