The Glomerular Basement Membrane (GBM) Antibody Test is a blood test used to detect antibodies directed against the glomerular basement membrane, a specialized tissue in the kidneys that helps filter waste and excess fluid from the blood. The presence of these antibodies may indicate an autoimmune disorder known as Anti-GBM disease, which can cause rapid kidney damage and, in some cases, lung bleeding (known as Goodpasture syndrome).