161-OR: Type A Insulin Resistance Can Be Misdiagnosed as Type 1 or Type 2 Diabetes—A Cautionary Tale

We present the case of a 19-year-old male presumed to have type 1 diabetes diagnosed at age 14. He was treated as such until requiring escalating doses of insulin and metformin. He was then lost to follow-up and after self-discontinuing insulin for approximately 1 year, he re-presented with hyperglycemia, the absence of ketonuria or acidosis, and elevated insulin level without use of exogenous insulin. Subsequent genetic testing revealed a mutation in the insulin receptor gene (INSR) consistent with type A insulin resistance. We then contrast the case of his 15-year-old maternal half-brother who has an identical gene mutation. However, this second patient was independently evaluated and diagnosed at age 9 with type 2 diabetes. He presented with significantly elevated BMI, acanthosis nigricans, as well as suboptimal diet and exercise habits. He required escalation of type 2 diabetes treatment to achieve effective glucose control and management of obesity. The case of a half-brother pair with essentially opposite presentations of the same genetic mutation consistent with type A insulin resistance has never been described in the literature.