SUN-644 - Diagnostic Dilemma in A Patient with Early Onset Diabetes Mellitus

Introduction:
Differentiating Type 1 diabetes (T1D), Type 2 diabetes (T2D), and Maturity-Onset Diabetes of the Young (MODY) can present a diagnostic dilemma. Approximately 17% of adults managed as T1D are misclassified, possessing negative Islet autoantibodies and preserved C-peptide. No single feature – including age of onset, Body Mass Index (BMI), diabetic ketoacidosis (DKA), or family history – confirms T1D. Integration of clinical context, biochemical markers, and potentially genetic testing is necessary to avoid misclassification and ensure adequate pathophysiology-based management.

Clinical

Case:
A 23-year-old female presented to endocrine clinic in September 2025 for evaluation of insulin-treated diabetes complicated with proteinuria. Outside records listed her diagnosis as T1D. She presented at age 11 during a routine school checkup involving blood and urine tests, leading to a hospital admission requiring continuous insulin drip treatment. It was unclear if she had DKA. She has been on multiple daily insulin injections for diabetes management since then, most recently on Glargine 14 units daily, Lispro 9-12 units 2-3 times daily before meals. Her only other hospital admission was a year ago after running out of insulin for a few weeks following relocation, when she ended up having pneumonia but not DKA.

Family history is notable for mother diagnosed with T1D at age 17 but later re-classified as T2D, maternal grandmother with T1D, and two sisters (ages 31 and 25) with T2D. No family history of other autoimmune diseases.

Her BMI was 30.92 kg/m2. Labs revealed elevated hemoglobin A1C of 10.1%, urine microalbumin > 570 mg/L, and normal serum creatinine and lipid panel.

Given the clinical ambiguity, additional tests were ordered. Results showed a preserved C-peptide of 2.3 ng/mL, and negative Glutamic Acid Decarboxylase, Islet Antigen-2 and Islet Cell antibodies, favoring T2D or MODY over T1D. MODY genetic testing is pending insurance approval. She continues intensive insulin, due to poorly controlled glucose levels, with plans to introduce non-insulin agents later pending clinical course.

Conclusion:
This case highlights the peril of diagnostic anchoring and the complex implications of reclassification of a diabetes diagnosis. An accurate diagnosis guides tailored pathophysiology-based therapies. Unlike T1D, T2D can be treated with Glucagon-like peptide-1 agonists and Sodium-glucose cotransporter-2 inhibitors, which can confer cardio-renal protection and weight loss benefits and reduce insulin-related hypoglycemia and weight gain risks. Some forms of MODY can be treated with sulfonylureas. Misdiagnosing T1D may place patients on unnecessary lifelong insulin therapy. Rigorous diagnostic verification is essential to ensure patients receive the most appropriate management.

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