Repeatability of Insulin Sensitivity and Glucose Effectiveness From the Minimal Model: Implications for Study Design

To determine the repeatability of insulin sensitivity measurements generated by the minimal model, we subjected 11 normal men to 3 frequently sampled intravenous glucose tolerance tests (FSIGTs) over the course of 12 days under conditions of fixed diet and limited physical activity. FSIGTs were analyzed by the minimal model using a full, 30-sample data set, as well as a reduced, 12-sample data set that has been proposed for population studies. Minimal model insulin sensitivity index (S_i) calculated from the 30-sample data set averaged 0.8 ± 0.083 × 10⁻⁴ min⁻¹ · (pmol/1)⁻¹ (range 0.10–1.64 × 10⁻⁴ min⁻¹ · (pmol/I)⁻¹ with an average interday coefficient of variation (CV) of 20.2 ± 3.2% (range 6–44%). Glucose effectiveness (SG) was slightly less repeatable, with an average interday variability of 25.1 ± 8.8%. The mean CVs for first-phase insulin secretion (20.1 ± 3.5%) and total insulin secretion (21.4 ± 3.2%) were similar to the CV for Sp Mean insulin sensitivity calculated from the 12-sample data set (0.82 ± 0.08 × 10⁻⁴ min⁻¹ · (pmol/1)⁻¹ was not significantly different from the mean calculated from the full data set (P = 0.37, paired Student's t test). However, the mean CV of Sx calculated from the reduced data set tended to be greater than that calculated from the full data set (27.7 ± 5.4% vs. 20.2 ± 3.2%). Power calculations using the observed variances from the full and reduced data sets indicated that the full sample protocol is preferable for small studies designed to detect small changes in S_i, whereas the reduced sample protocol is appropriate for studies with large numbers of subjects or large anticipated changes in S_i