The antiphospholipid syndrome (APS) is a systemic autoimmune disorder characterized by persistent autoantibodies directed against either phospholipids or plasma proteins bound to anionic phospholipids. The pathogenesis of the syndrome is complex, involving a multiplicity of factors. Patients with the APS may display a constellation of clinical features including venous, arterial, and microvascular thromboses, which can be associated with valvular heart disease, thrombocytopenia, livedo reticularis, neurologic findings, and recurrent fetal losses.

Antiphospholipid-related kidney disease, and so-called antiphospholipid nephropathy, may be found with any of these other clinical features. This disorder is referred to as the primary APS when it occurs alone without another associated autoimmune disease. Among patients with systemic lupus erythematosus (SLE), 20 to 30 percent will have clinically relevant elevated levels of antiphospholipid antibody titers. Elevated antibody titers can also be found in other rheumatic diseases, with certain infections, in patients taking specific medications, and among patients with pregnancy complications, venous thromboses, or stroke.