Measures of β-cell function during the oral glucose tolerance test, liquid mixed-meal test, and hyperglycemic clamp test

Objective To evaluate clinically useful measures of beta-cell function derived from the oral glucose tolerance test (OGTT) or mixed-meal (ie, Boost) tolerance test to assess insulin secretion in comparison with the gold standard, the hyperglycemic Clamp (Hyper-C) test. Study design We hypothesized that OGTT/Boost-derived measures are useful estimates of beta-cell function and correlate well with insulin secretion measured by the Hyper-C test. This study was designed to assess the correlation between the ratio of the early incremental insulin/glucose responses at 15 and 30 minutes (Delta I-15/Delta G(15) and Delta I-30/Delta G(30)) of the OGTT and the Boost test with insulin secretion measured during the Hyper-C test (225 mg/dL). The same indices were evaluated using C-peptide. A total of 26 children (14 males, 12 females; mean age, 9.9 +/- 0.2 years; mean body mass index = 22.1 +/- 1.2 kg/m(2)) underwent a 2-hour Hyper-C test (225 mg/dL) and 3-hour OGTT and Boost tests with measurements of glucose, insulin, and C-peptide. Results Correlations between Hvper-C- and OGTT-derived measures of insulin secretion were stronger for the 15-minute index than for the 30-minute index of insulin secretion and stronger for C-peptide levels than for insulin levels (r =.7, P < .001 for first-phase C-peptide vs both OGTT and Boost, Delta C-15/Delta G(15)). Conclusions In children with normal glucose tolerance, C-peptide rather than insulin level measured after 15 minutes of the OGTT or Boost test provides a reliable estimate of beta-cell function that correlates well with Hyper-C-derived insulin secretion.