The anti-beta-2-glycoprotein I (β2GPI) antibody test is a quantitative immunoassay that detects and measures the level of IgG autoantibodies directed against β2GPI. This protein binds to cell surfaces, and it is believed that the binding of antibodies to it (particularly IgG) leads to the activation of endothelial cells and platelets, promoting hypercoagulability. The presence of these antibodies is one of the key laboratory criteria (along with anticardiolipin and lupus anticoagulant) for the diagnosis of antiphospholipid syndrome (APS).