There is a clinical need for plasma tests to detect and quantify the in vivo destruction of pancreatic beta cells in type 1 diabetes. We previously developed a time-resolved fluorescence immunoassay to glutamate decarboxylase 65kDa (GAD65-TRFIA) that was able to detect the synchronous necrotic destruction of transplanted beta cells in the hours after their infusion in the liver. This GAD65-TRFIA, however, lacked sensitivity to detect continued beta cell rejection beyond this acute phase. The aim of present study was to gain at least an order of magnitude in analytical sensitivity by switching to Becton Dickinson Cytometric Bead Array (GAD65-CBA) enhanced sensitivity format, using the same couple of monoclonal antibodies. We compared the performances of GAD65-CBA and -TRFIA using CLSI protocols for linearity, imprecision, specificity, limit of detection (LoD) and functional sensitivity. We conducted a method comparison and assessed the biologic potential on samples from human recipients of islet grafts. The GAD65-CBA showed acceptable linearity and imprecision. Switching from TRFIA to CBA lowered functional sensitivity by a factor 35, and lowered LoD by a factor 11 with minimal need for method optimization. The enhanced sensitivity greatly expands the application domain of our biomarker and allowed for the first time to detect ongoing beta cell destruction up to at least 1 day post-islet transplantation. We conclude that the GAD65-CBA is suitable for biological and clinical assessment of the real-time destruction of beta cells in intraportal transplantation.