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Background: Critically ill patients with coronavirus disease 2019 (COVID-19) have a profound hypercoagulable state and often develop coagulopathy which leads to organ failure and death. Because of a prolonged activated partial-thromboplastin time (aPTT), a relationship with anti-phospholipid antibodies (aPLs) has been proposed, but results are controversial. Functional assays for aPL (i.e., lupus anticoagulant) can be influenced by concomitant anticoagulation and/or high levels of C reactive protein. The presence of anticardiolipin (aCL), anti-beta2-glycoprotein I (anti-b 2 GPI), and anti-phosphatidylserine/ prothrombin (aPS/PT) antibodies was not investigated systematically. Epitope specificity of anti-b 2 GPI antibodies was not reported. Objective: To evaluate the prevalence and the clinical association of aPL in a large cohort of COVID-19 patients, and to characterize the epitope specificity of anti-b 2 GPI antibodies. Methods: ELISA and chemiluminescence assays were used to test 122 sera of patients suffering from severe COVID-19. Of them, 16 displayed major thrombotic events.

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