Aims/Introduction:  Type 2 diabetes is progressive in that therapy must be altered over time, which is partly as a result of the progressive loss of pancreatic β‐cell function. To elucidate the relationship between residual endogenous insulin secretion and the necessity of insulin therapy to achieve good glycemic control, indices using serum C‐peptide immunoreactivity (CPR) were analyzed in patients with type 2 diabetes.

Materials and Methods:  The data of 201 Japanese patients with type 2 diabetes who achieved the target of glycemic control during admission were analyzed retrospectively. Indices using CPR including fasting CPR (FCPR), CPR 6 min after intravenous injection of glucagon (CPR‐6 min), increment of CPR (ΔCPR), secretory unit of islet in transplantation index (SUIT) and C‐peptide index (CPI) were compared between the group requiring insulin (insulin group) and the group not requiring insulin (non‐insulin group). A receiver–operator characteristic (ROC) curve was made, and optimal cut‐off point and likelihood ratio were determined for each index.

Results:  All indices of CPR were lower in the insulin group compared with those in the non‐insulin group. Likelihood ratios at the optimal point of FCPR, CPR‐6 min, ΔCPR, SUIT, and CPI were 2.0, 2.1, 1.6, 2.3 and 2.8, respectively. Optimal cut‐off point of CPI was 1.1 ng/mg. Sensitivity and specificity at optimal point of CPI were 61 and 78%, respectively.

Conclusions:  The advantage of CPI of the indices of CPR to select insulin therapy to achieve good glycemic control was shown, but limitations of the predictive abilities of the indices using CPR should be taken into account. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00096.x, 2011)