The Role of Laboratory Testing in Differentiating Type 1 Diabetes from Type 2 Diabetes in Patients Undergoing Bariatric Surgery

Background: It may be difficult to distinguish between adults with type 1 diabetes and type 2 diabetes by clinical assessment. In patients undergoing bariatric surgery, it is critical to correctly classify diabetes subtype to prevent adverse perioperative outcomes including diabetic ketoacidosis. This study aimed to determine whether testing for C-peptide and islet cell antibodies during preoperative evaluation for bariatric surgery could improve the classification of type 1 versus type 2 diabetes compared to clinical assessment alone. Methods: This is a retrospective analysis of the Improving Diabetes through Lifestyle and Surgery trial, which randomized patients with clinically diagnosed type 2 diabetes and BMI 30–40 kg/m² to medical weight loss or bariatric surgery; one participant was discovered to have type 1 diabetes after experiencing postoperative diabetic ketoacidosis. Using blood samples collected prior to study interventions, we measured islet cell antibodies and fasting/meal-stimulated C-peptide in all participants. Results: The participant with type 1 diabetes was similar to the 11 participants with type 2 diabetes in age at diagnosis, adiposity, and glycemic control but had the lowest C-peptide levels. Among insulin-treated participants, fasting and stimulated C-peptide correlated strongly with the C-peptide area-under-the-curve on mixed meal tolerance testing (R = 0.86 and 0.88, respectively). Three participants, including the one with type 1 diabetes, were islet cell antibody positive. Conclusions: Clinical characteristics did not correctly identify type 1 diabetes in this study. Preoperative C-peptide testing may improve diabetes classification in patients undergoing bariatric surgery; further research is needed to define the optimal C-peptide thresholds.