GAD1 and diabetes mellitus: This report highlights that medical clinicians are crucial to be aware of the possible presence of GAD antibodies in immunoglobulin preparations and to measure patients’ antibody titers before and after their use for diagnosing the type of diabetes mellitus. If GAD antibodies become positive due to the administration of the immunoglobulin preparations, the patient could be misdiagnosed with SPIDDM and be disadvantaged physically, mentally, and financially in the future.