Casual C peptide index: Predicting the subsequent need for insulin therapy in outpatients with type 2 diabetes under primary care

Background Evaluation of residual beta cell function is indispensable in patients with type 2 diabetes as it informs not only diagnoses but also appropriate treatment modalities. However, there is a lack of convenient biomarkers for residual beta cell function. Therefore, we evaluated endogenous insulin level as a biomarker in outpatients who were being treated with insulin therapy and in patients who were introduced to insulin therapy after 4 years. Methods Data of 174 outpatients with type 2 diabetes (50% male) whose glycemia was moderately controlled (glycated A1c 7.3% [5.2%-14.8%]) were reviewed. Twenty patients whose estimated glomerular filtration rate was lower than 30 ml/min/1.73 m(2) were excluded from the evaluation of endogenous insulin level with both casual C-peptide index (C-CPI) and urinary C-peptide/creatinine ratio (determined at any time, generally 1-2 h after breakfast). Patients were stratified based on the provision of insulin therapy. Results C-CPI and UCPCR were significantly lower in the insulin-treated patients than in the insulin-untreated patients (0.9 vs. 2.2, p < 0.0001; 24.7 vs. 75.5, p = 0.0003, respectively). Moreover, C-CPI were significantly lower in the insulin-requiring patients for 4 years than in the insulin-unrequiring patients (1.0 vs. 1.7, p = 0.0184). The multivariate logistic regression analysis revealed that both indicators of insulin secretion influenced the requirement for insulin therapy, but C-CPI could serve as the most convenient and useful biomarker for not only current insulin therapy requirements (p = 0.0002) but also the subsequent requirement for insulin therapy (p = 0.0008). Conclusions C-CPI could be determined easily, and it was found to be a more practical marker for outpatients; therefore, our findings would have critical implications for primary care.