Clinical Characteristics and Development of Complications Differ Between Adult‐Onset and Child–Adolescent‐Onset Type 1 Diabetes: A Report From a Tertiary Medical Center in Türkiye

<b>Background and Aims:</b> The age-at-onset is of great importance in the heterogeneity of Type 1 diabetes mellitus (T1DM). This study was designed to define clinical and laboratory differences between child-adolescent-onset and adult-onset T1DM at presentation and during follow-up and determine the predicting factors for developing microvascular and macrovascular complications. <b>Material and Methods:</b> This retrospective observational study evaluated T1DM patients who were followed in the diabetes outpatient clinic between January 1, 2000, and December 31, 2019. <b>Results:</b> The study cohort included 490 individuals with T1DM (54.3% female, 58.8% adult-onset, and median follow-up: 5 years). In the adult-onset group, baseline C-peptide and GADA prevalence were higher, whereas presentation with ketoacidosis was 2.3-fold lower compared to the child-adolescent-onset group (<i>p</i> < 0.001). During follow-up, the adult-onset group had a 2.4-fold higher overweight/obesity (<i>p</i> < 0.001) and 1.7-fold higher dyslipidemia/hyperlipidemia (<i>p</i> = 0.002) than the child-adolescent-onset group. In multivariate analysis, fasting glucose (<i>p</i> = 0.024) in adult-onset, dyslipidemia/hyperlipidemia (<i>p</i> = 0.037) in child-adolescent-onset, and diabetes duration (<i>p</i> = 0.008 and <i>p</i> = 0.007) and hypertension (<i>p</i> = 0.001 and <i>p</i> = 0.01) in both groups were associated with increased risk of microvascular complications, whereas age-at-onset (<i>p</i> = 0.024), dyslipidemia/hyperlipidemia (<i>p</i> = 0.03), nephropathy (<i>p</i> = 0.003), and neuropathy (<i>p</i> = 0.001) in adult-onset and age (<i>p</i> = 0.002) and triglycerides (<i>p</i> = 0.013) in child-adolescent-onset groups were associated with increased risk of macrovascular complications. The cutoff C-peptide levels at baseline predicted microvascular complications in the whole cohort and adult-onset group were defined as 0.383 ng/mL (<i>p</i> < 0.001) and 0.41 ng/mL (<i>p</i> = 0.001), respectively. In the Kaplan-Meier analysis, C-peptide (< 0.383 ng/mL) but not age-at-onset predicted future development of microvascular and macrovascular complications (<i>p</i> = 0.003 and <i>p</i> = 0.032). <b>Conclusion:</b> Clinical presentation and prognosis differ in adult-onset and child-adolescent-onset T1DM. Low initial C-peptide may predict the development of microvascular and macrovascular complications.