Certain membrane phospholipids are normally prevented from activating the coagulation cascade by being bound to circulating phospholipid-binding proteins; antibodies to the phospholipids block attachment of the binding protein and thus predispose to thrombosis.

The antiphospholipid antibody syndrome consists of thrombosis and (in pregnancy) fetal demise associated with various autoimmune antibodies directed against one or more phospholipid-binding proteins (eg, β₂-glycoprotein I, prothrombin, annexin). These proteins normally bind to phospholipid membrane constituents and protect them from excessive coagulation activation. The autoantibodies displace the protective proteins and, thus, produce procoagulant endothelial cell surfaces and cause arterial or venous thromboses. In vitro clotting tests may paradoxically be prolonged because the antiprotein/phospholipid antibodies interfere with coagulation factor assembly and activation on the phospholipid components added to plasma to initiate the tests. The lupus anticoagulant is an antiphospholipid autoantibody that binds to protein/phospholipid complexes. It was initially recognized in patients with SLE, but these patients now account for a minority of people with the autoantibody.

The lupus anticoagulant is suspected if the PTT is prolonged and does not correct immediately upon 1:1 mixing with normal plasma but does return to normal upon the addition of an excessive quantity of phospholipids (done by the hematology laboratory). Antiphospholipid antibodies in patient plasma are measured by immunoassays of IgG and IgM antibodies that bind to phospholipid/β₂-glycoprotein I complexes on microtiter plates.

Heparin, warfarin, and aspirin have been used for prophylaxis and treatment.

Last full review/revision January 2013 by Joel L. Moake, MD