A Predictive Level of C-peptide for Glutamic Acid Decarboxylase Antibody Positivity in Autoimmune Diabetes

Objective: Diabetes mellitus (DM) is one of the most prevalent chronic diseases and is considered a universal health issue. This study aimed to establish a C-peptide cut-off level to predict glutamic acid decarboxylase antibody 65 (GADA) positivity in autoimmune diabetes. Methods: Designed as a retrospective methodological study, the study reviewed diabetic patients aged over 16 years who presented to adult internal medicine outpatient clinics between 2008 and 2022. The data were collected retrospectively from the hospital records. Patients were categorized into two groups according to GADA positivity, and their sensitivity and specificity were calculated for fasting C-peptide levels. Results: GADA positivity was observed in 96 of a total of 343 patients. The group with antibody positivity was younger (p < .001), had lower C-peptide levels (p < .001) and higher HbA1c levels (p = .03). Thyroid antibody positivity was more common in this group (p = .009). Sensitivity was 80.2% and specificity was 72.5% for the optimal C-peptide level cut-off point of ≤ 1.35 ng/mL, which predicted GADA positivity. Conclusion: GADA positivity was highly prevalent at a fasting C-peptide level cut-off point of ≤1.35 ng/mL. Hence, in clinical practice, GADA could be routinely examined in patients with a value below this level for accurate diagnosis and early initiation of treatment in autoimmune diabetes.