The Beta 2 Glycoprotein 1 Antibody (B2GP1) IgM test is used to detect the presence of IgM antibodies against Beta 2 Glycoprotein 1 (B2GP1), a protein involved in regulating the blood clotting process. This test is primarily used in the diagnosis and monitoring of certain autoimmune diseases, particularly antiphospholipid syndrome (APS).