Supplementary Material for: Trajectory of glycemic control from late adolescence to young adulthood with type 1 diabetes: A longitudinal study

Introduction: Improving outcomes for transition-aged patients with type 1 diabetes (T1D) requires understanding glycemic trajectories and modifiable factors. We evaluated longitudinal glycated hemoglobin (HbA1c) trends, target attainment, and associated factors in Korean youth with T1D. Methods: This retrospective cohort included 354 patients diagnosed before age 14 years with HbA1c data at three or more distinct ages between 15 and 22 years, followed at Seoul National University Children’s Hospital 1999–2024. Linear mixed-effects models assessed factors associated with HbA1c trajectory. Results: Mean HbA1c declined from 9.0% (75 mmol/mol) at age 15 years to 8.2% (66 mmol/mol) at age 22 years (–0.103% [−1.1 mmol/mol] per year, p < 0.001). Older age, male sex, continuous glucose monitoring (CGM) use, and parental college education were independently associated with lower HbA1c over time (all p < 0.05). At age 22 years, there were no CGM users in the 2006–2015 cohort, whereas 25.2% used CGM in the 2016–2024 cohort. At this age, 19.6% achieved HbA1c < 7% (53 mmol/mol), whereas 24.8% remained at ≥ 9%. Among those with HbA1c ≥ 9% at age 15 years, nearly half remained ≥ 9% at age 22 years, while approximately one-tenth improved to < 7%. Conclusions: Although glycemic control improved with age, a substantial proportion of adolescents and young adults with T1D failed to meet HbA1c targets. Given that CGM use was a key factor associated with better control, increasing CGM uptake alongside tailored support may improve outcomes during the transition to adulthood.