Beta-2 glycoprotein 1 (B2GP1) is a member of the complement superfamily and may be involved in the coagulation cascade. It is the principal target of autoantibodies which most frequently develop in people with an autoimmune disorder called antiphospholipid syndrome (APS). B2GP1 autoantibodies are one of three primary antiphospholipid antibodies and less common than the other two, cardiolipin antibody and lupus anticoagulant. B2GP1 IgG and IgM antibody tests are primarily used along with cardiolipin antibody and lupus anticoagulant testing to help diagnose APS, the cause of an unexplained blood clot, or pregnancy complications such as pre-eclampsia and recurrent miscarriages. A positive APS diagnosis requires two tests to be carried out on two or more specimens drawn at least 12 weeks apart to determine if the presence of antiphospholipid antibodies is persistent.