Abstract-The antiphospholipid syndrome is a relatively common acquired cause of venous thrombosis. Up to 20% of cases of deep vein thrombosis, with and without pulmonary embolism, may be associated with antiphospholipid antibodies. These antibodies are typically detected in lupus anticoagulant assays and tests for anticardiolipin antibodies. Most antiphospholipid antibodies are directed against several phospholipid-binding plasma proteins. The most common antigens are ␤ 2 -glycoprotein I and prothrombin. Immunoassays using these purified antigens are now available. In addition to being markers for thrombotic risk, antiphospholipid antibodies have been shown to directly contribute to hypercoagulability in animal models and in various in vitro studies. Prevention of recurrent venous thrombosis in patients with the antiphospholipid syndrome requires long-term anticoagulation. The optimal intensity of warfarin therapy is an ongoing issue, but most clinicians currently favor a target INR in the 2.0 to 3.0 range. In certain patients, antiphospholipid antibodies may interfere with determination of the INR, requiring other approaches to monitor and adjust the warfarin dose. Low-dose aspirin is typically recommended for primary prevention of thrombosis in asymptomatic patients with moderate to high levels of antiphospholipid antibodies, although strong supporting data are lacking. Key Words: antiphospholipid Ⅲ anticardiolipin Ⅲ lupus anticoagulant Ⅲ ␤2-glycoprotein I Ⅲ thrombosis T he antiphospholipid syndrome (APS) is the association of thrombosis or recurrent pregnancy loss with persistent antiphospholipid antibodies (aPLs). The spectrum of thrombosis in APS includes both venous and arterial events and thrombosis at nearly every site in the vasculature has been reported. This review will focus on venous thrombosis (VT) in the setting of APS. The most common type of venous thrombosis associated with APS is lower extremity deep vein thrombosis (DVT) with or without pulmonary embolism (PE). APS is one of the more common acquired causes of venous thrombophilia. In one prospective cohort study, approximately 20% of patients with DVT or PE had moderate to high levels of aPLs before the thrombotic event. 1 About one-third of patients present with VT at the time of diagnosis of APS. 2 APS occurring in the absence of other autoimmune diseases is termed primary APS. Secondary APS refers to APS in the setting of other autoimmune diseases, most commonly systemic lupus erythematosus (SLE). International consensus criteria for the classification of definite APS were initially published in 1999 3 and updated in 2006. 4 The classification criteria, summarized in the Table, are useful in research studies but are not intended as diagnostic criteria for clinical purposes. The criteria do not include a number of clinical manifestations associated with aPLs including thrombocytopenia, livedo reticularis, valvular heart disease (LibmanSacks endocarditis), a form of nephropathy, and certain neurological abnormalities. 4 Antiphospholipid Anti...

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