Lupus anticoagulant is an autoimmune disorder caused by antibodies that bind to phospholipids and proteins associated with the cell membrane. This antibody has the peculiar nature of causing a delay in coagulation in laboratory tests (the partial thromboplastin time) while clinically predisposing for thrombosis. It is also known as lupus antibody, LA and lupus inhibitor. Together with anticardiolipin antibodies, LA causes the bulk of antiphospholipid syndrome.

Although it is named "lupus" anticoagulant, most patients with the antibody do not have lupus erythematosus, and only a small proportion will proceed to develop this disease (which causes joint pains, skin problems and renal failure, amongst other complications).

Symptoms include thrombosis and habitual abortion (repeated miscarriage). Various other symptoms (e.g. migraine and rash) may be reported, but these are very non-specific, and it is uncertain whether these are caused by the antibodies at all.

Diagnosis is initially with a laboratory test termed the "dillute Russell viper venom test". Mixing studies then confirm the presence of lupus anticoagulant.

There is no specific treatment, but miscarriages can be prevented with the administration of low molecular weight heparins (LMWHs), and thrombosis is treated with anticoagulants (LMWHs and warfarin).