Case Report: Euglycemic diabetic ketoacidosis as the initial presentation of adult-onset autoimmune diabetes in a treatment-naïve man

Background Autoimmune diabetes can begin in adulthood and is often initially mistaken for type 2 diabetes. Its first presentation as euglycemic diabetic ketoacidosis (EDKA), that is, ketoacidosis with a plasma glucose below 200 mg/dL (11.1 mmol/L), is uncommon and easily overlooked, particularly in a patient without known diabetes or sodium-glucose cotransporter-2 (SGLT2) inhibitor exposure. Case presentation A 34-year-old previously healthy, lean man (body mass index 21.4 kg/m²) presented with a three-day history of fatigue, nausea, reduced appetite, increased thirst, and mild dyspnea, with oral intake falling to approximately 700 kcal on the day before admission. He had a high anion gap metabolic acidosis (pH 7.23, bicarbonate 13 mmol/L, anion gap 22 mmol/L) with marked ketonemia (β-hydroxybutyrate 5.2 mmol/L), but a random blood glucose of only 198 mg/dL (10.99 mmol/L). He had no known diabetes, took no medications, and did not drink alcohol. Diabetes was established during the admission by a hemoglobin A1c of 7.1%, and positivity for GAD65, IA-2, and ZnT8 antibodies with a low fasting C-peptide (0.42 ng/mL) indicated adult-onset autoimmune diabetes. Infectious, alcoholic, toxicologic, lactic, renal, and adrenal causes were excluded, and the brief reduction in intake was insufficient to explain the severity of the ketoacidosis. The episode resolved within 24 hours with intravenous fluids, insulin infusion with concurrent dextrose, and potassium replacement. He was discharged on basal–bolus insulin and remained free of ketoacidosis at three months, with a persistently low C-peptide. Conclusion Clinically significant ketoacidosis can precede the diagnosis of diabetes and occur with a glucose below the conventional threshold. An unexplained high anion gap acidosis should prompt ketone measurement regardless of the glucose level or a history of diabetes. Whether this patient has latent autoimmune diabetes in adults (LADA) or adult-onset type 1 diabetes will require longer follow-up to determine.