When Serum C-Peptide Measurement Drives Adequate Diabetes Mellitus Diagnosis and Therapy: A Case Report

; Background: Therapeutic targets in type 2 diabetes mellitus (T2D) are oriented towards nephron- and cardio-protection and the prescription of antihyperglycemic agents with proven renal and cardiovascular benefits are increasing over time. Failure to promptly diagnose insulinopenic diabetes may adversely affect the adequacy of treatment and have harmful consequences, including severe hyperglycemia and diabetic ketoacidosis. Case Presentation: Herein we present the case of a 57-year-old woman referred to our clinic due to poor glycemic control (HbA1c 80 mmol/mol, therapeutic target <53 mmol/mol), class III obesity (BMI 41 kg/m2; normal value <25 kg/m²), and high cardiovascular risk misdiagnosed with T2D several years before. C-peptide measurement (0.3 ng/dL; reference range 1.1 – 4.4 ng/mL) confirmed the diagnosis of an insulinopenic form of diabetes, and the islet autoimmunity was consequently measured (GADA 2,000 UA/mL, reference range <5 UA/mL; IA2 17.1 UA/mL, reference range <7.5 UA/mL) and defined the diagnosis of an autoimmune form of diabetes. Discussion: Deprescribing insulin therapy in T2D patients in favor of other antihyperglycemic medications has become a growing therapeutic opportunity to provide adequate glucose control, promote weight loss, improve insulin sensitivity, and ameliorate cardiovascular and renal outcomes. However, a blunted insulin reserve poses significant restriction on the prescription of non-insulin agents (e.g., diabetic ketoacidosis due to gliflozins). According to our experience, the routine testing of insulin reserve provides detailed information on diabetes pathophysiology with positive implications for the appropriateness of pharmacological prescriptions. Conclusion: As part of our routine evaluation of diabetic patients, C-peptide measurement is a valuable and inexpensive tool to reclassify diabetes types and provide more appropriate disease management.