Antiphospholipid syndrome (APS) is diagnosed in the presence of vascular thrombosis or pregnancy morbidity occurring in patients with circulating antiphospholipid antibodies (lupus anticoagulant [LA] and/or IgG/IgM anticardiolipin [aCL] and/or IgG/IgM anti-beta2glycoprotein I [abeta2GPI] antibodies). Each test may identify different autoantibodies; a single test makes the diagnosis possible when positive on two or more occasions at least 12 weeks apart. However, single test positivity may be unrelated to pathogenic antibodies, which are now considered to be a subclass of abeta2GPI antibodies directed against the domain I of this protein. Conversely, all three positive tests identify a single class of abeta2GPI antibodies, thus identifying high-risk patients with APS.