The potential of anti-glutamic acid decarboxylase antibodies to support a diagnosis of autoimmune diabetes mellitus.

Objective: Anti-glutamic acid decarboxylase (anti-GAD) antibodies are a frequently used diagnostic marker for autoimmune forms of diabetes mellitus (DM), namely, type 1 diabetes mellitus (T1DM) and latent autoimmune diabetes in adults (LADA). We sought to provide insight into a unique diagnostic application of anti-GAD antibodies in patients potentially misdiagnosed with type 2 diabetes mellitus (T2DM).

Methods: We present a case series of patients who had a change in diagnosis from T2DM to autoimmune DM that was supported by positive anti-GAD antibodies. Patients were identified via a retrospective chart review of all anti-GAD antibodies tests ordered between 1 January 2020 and 31 December 2021 at a tertiary care academic hospital.

Results: Of the 23 patients with previous diagnosis of T2DM, positive anti-GAD antibodies supported the clinician's decision to change the diagnosis to autoimmune DM. The prominent clinical reasons for ordering anti-GAD antibodies in patients previously diagnosed as T2DM were patient presentation with diabetic ketoacidosis, features of insulin insufficiency, inadequate effect of oral diabetes mellitus medications, young age at diagnosis, and a family history of autoimmune conditions.

Conclusion: Anti-GAD antibodies' positivity can support a change in diagnosis from T2DM to autoimmune DM, which has substantial impact on patient care. Timely and reliable clinical laboratory reporting of anti-GAD antibodies is highly recommended.