A Case of Type 2 Diabetes Mellitus Revealed Transient Positivity of Glutamic Acid Decarboxylase (GAD) Antibodies Following Immunoglobulin Administrations

Glutamic acid decarboxylase (GAD) antibodies are frequently measured in diabetes care as islet-associated autoantibodies that are useful in the diagnosis of type 1 diabetes. However, GAD antibodies derived from other persons may contaminate immunoglobulin preparations, and there have been cases of transiently positive GAD antibodies after intravenous immunoglobulin (IVIg) in patients who were originally negative for GAD antibodies. Clinicians may be unaware of such contamination and misdiagnose some cases as type 1 instead of type 2 diabetes mellitus based on positivity for GAD antibodies. Herein, we present a case of type 2 diabetes mellitus that revealed transiently positive GAD antibodies following immunoglobulin administrations. A 68-year-old woman with a medical history of diabetes mellitus was admitted to our hospital for the treatment of Guillain-Barré syndrome, and IVIg was started on the day of admission. Blood tests on admission revealed negative for GAD antibodies but showed weak positivity on day one after IVIg. Afterward, GAD antibodies turned negative on day 72. Immunoglobulin preparations were revealed to have a high concentration of GAD antibodies. Based on changes in GAD antibody titers and all negativity for anti-insulinoma-associated antigen-2 (IA-2), insulin, and zinc transporter 8 (ZnT8) antibodies, the patient was diagnosed with type 2 diabetes mellitus rather than slowly progressive type 1 diabetes mellitus (SPIDDM). This case demonstrates that it is important for the medical clinician to be aware of the possible presence of GAD antibodies in immunoglobulin preparations and to measure antibody titers before and after their use for diagnosing the type of diabetes mellitus.