Estimates of Insulin Sensitivity Using Glucose and C-Peptide From the Hyperglycemia and Adverse Pregnancy Outcome Glucose Tolerance Test

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Abstract
OBJECTIVE - To determine if glucose and C-peptide values obtained as part of the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study could be used to estimate insulin sensitivity during late pregnancy. RESEARCH DESIGN AND METHODS - A total of 78 women enrolled in the HAPO study were recruited for this ancillary study. Venous plasma samples were drawn after an 8- to 10-h fast (time 0) and at 30, 60, 90, and 120 min after a 75-g glucose challenge, which was performed at 24-32 weeks' gestation. Samples were analyzed for plasma glucose, insulin, and C-peptide. Insulin sensitivity was estimated using the established Matsuda and DeFronzo insulin sensitivity index for oral glucose tolerance tests (ISOGTT). Insulin sensitivity was also calculated from two other commonly used indexes of insulin sensitivity (that for homeostasis model assessment [ISHOMA] and that for quantitative insulin sensitivity check index [ISQUICKI]). A new insulin sensitivity index was calculated using the glucose and C-peptide concentrations at 0 and 60 min to derive ISHOMA (C-pep), ISQUICKI C-pep, and ISOGTT C-pep. These indexes were then correlated with insulin sensitivity estimated from the ISOGTT. RESULTS - The strongest correlation with the ISOGTT was obtained for ISOGTT C-pep (r = 0. 792, P < 0.001). Further, the correlations of ISHOMA C-pep and ISQUICKI C-pep with ISGOTT were also significant (r = 0.676, P < 0.001 and r = 0.707, P < 0.001, respectively). CONCLUSIONS - These data suggest that calculated ISOGTT C-pep is an excellent predictor of insulin sensitivity in pregnancy and can be used to estimate insulin sensitivity in over 25,000 women participating in the HAPO study.