A case of slowly progressive type 1 diabetes accompanied by positivity for a single islet cell antibody, initially suspected to be childhood-onset type 2 diabetes or maturity-onset diabetes of the young.

We encountered a 19-year-old woman who was found to have glucosuria (3+) on urinalysis at school at the age of 11 years and 10 months. She had a three-generation family history of diabetes. She was 150.6 cm tall (+ 0.28 standard deviations (SD)), weighed 50.8 kg (+ 1.0 SD), and did not have obesity or deafness. She had a glycated hemoglobin (HbA1c) of 10.9%, blood glucose of 334 mg/dL, negativity for urine ketones, serum C-peptide concentration of 1.63 ng/mL, and negativity for anti-glutamic acid decarboxylase (GAD) and tyrosine phosphatase-like protein IA-2 (IA-2) autoantibodies. The patient was initially diagnosed with childhood-onset type 2 diabetes mellitus without obesity because she could be weaned off insulin treatment within 1 month. Her HbA1c level was maintained at 6% using diet and metformin for 3 years after, but deteriorated to 9.9% in the 4th year. Subsequently, the oral hypoglycemic regimen was strengthened, but her HbA1c rose even higher. She stopped attending hospital for 19 months, but at the age of 19 years and 11 months, she was urgently admitted to our hospital with diabetic ketoacidosis, when her fasting serum C-peptide concentration had decreased to 0.49 ng/mL. No mutations in maturity-onset diabetes of the young-related genes were identified. She was negative for anti-GAD, IA-2, and zinc transporter 8 antibodies, but positive for islet cell antibody, confirming a diagnosis of slowly progressive type 1 diabetes. Therefore, the type of diabetes present in children and adolescents should be diagnosed with great care and reconsidered according to the clinical course.