Lupus anticoagulant is an immunoglobulin that binds to phospholipids and proteins associated with the cell membrane. Its name is a partial misnomer, as it is actually a prothrombotic antibody in vivo. Lupus anticoagulant in living systems causes a decrease in clotting time. The name derives from their properties in vitro, as these antibodies increase coagulation times in laboratory tests such as the activated partial thromboplastin time (aPTT). Investigators speculate that the antibodies interfere with phospholipids used to induce in vitro coagulation. In vivo, the antibodies are thought to interact with platelet membrane phospholipids, increasing adhesion and aggregation of platelets, which accounts for the in vivo prothrombotic characteristics. The condition was first described by hematologist C. Lockard Conley in 1952. Terminology Both words in the term "lupus anticoagulant" can be misleading: Most patients with a lupus anticoagulant do not actually have lupus erythematosus, and only a small proportion will proceed to develop this disease (which causes joint pains, skin problems and kidney failure, amongst other complications). People with lupus erythematosus are more likely to develop a lupus anticoagulant than the general population. The term "anticoagulant" accurately describes its function in vitro. However in vivo, it functions as a procoagulant. Indications for testing The main indication for testing for lupus anticoagulant is a suspected antiphospholipid syndrome, whose main manifestations are blood clots (thrombosis) in both arteries and veins as well as pregnancy-related complications such as miscarriage, stillbirth, preterm delivery, and severe preeclampsia. In a suspected antiphospholipid syndrome, lupus anticoagulant is generally tested in conjunction with anti-apolipoprotein antibodies and anti-cardiolipin antibodies, and diagnostic criteria require one clinical event (i.e. thrombosis or pregnancy complication) and two positive blood test results spaced at least three months apart that detect at least one of the three types of antibodies. Testing for lupus anticoagulant can also be indicated by a prolonged aPTT test that is unexplained. Workup An aPTT is generally included in a differential diagnosis in vague symptoms, and is a non-specific test of coagulation. In contrast, the prothrombin time (PT), another non-specific coagulation test, is normally unaffected by lupus anticoagulant. Nevertheless, falsely increased PT has been reported, likely by lupus anticoagulant interfering with the phospholipid component of the PT reagent, particularly when using recombinant tissue factor and purified phospholipids. A mixing test is generally in the initial workup of a prolonged aPTT. In a mixing test, patient plasma is mixed with normal pooled plasma and