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Novel Insulin Sensitivity Index Derived from Oral Glucose Tolerance Test

Division of Endocrinology and Metabolism, Department of Medicine (S.S., W.S., A.T., C.R.); Department of Pathology (W.C.); and Epidemiology Unit (A.G.), Faculty of Medicine, Prince of Songkla University, Hat-Yai, Songkhla 90110, Thailand

Address all correspondence and requests for reprints to: Supamai Soonthornpun, M.D., Division of Endocrinology and Metabolism, Department of Medicine, Prince of Songkla University, Hat-Yai, Songkhla 90110, Thailand. E-mail: ssupamai{at}ratree.psu.ac.th.

The euglycemic hyperglycemic clamp is generally regarded as a reference method for assessing insulin sensitivity. However, this method is laborious and expensive. The oral glucose tolerance test (OGTT), the most commonly used method for evaluating whole body glucose tolerance, has often been used to assess insulin sensitivity. In the previous studies the correlation between the insulin sensitivity index (ISI) obtained from the OGTT (ISI_OGTT) and those obtained from the glucose clamp (ISI_Clamp) may not be satisfactory. This is because the glucose clamp study is designed for measuring peripheral glucose utilization, whereas plasma glucose responses during the OGTT are the results of peripheral glucose utilization and hepatic glucose production. Based on this problem, we developed a new equation, ISI_OGTT, [1.9/6 x body weight (kg) x fasting plasma glucose (mmol/liter) + 520 - 1.9/18 x body weight x area under the glucose curve (mmol/h·liter) - urinary glucose (mmol)/1.8] ÷ [area under the insulin curve (pmol/h·liter) x body weight], which would represent peripheral glucose utilization only. We tested our equation with ISI_Clamp and also compared with others. Thirty-three healthy volunteers (16 males) with normal glucose tolerance underwent a 75-g, 3-h OGTT on the morning of d 1 and a glucose clamp on the morning of d 2. Their mean (±SD) age and body mass index were 30.8 ± 8.3 yr and 22.0 ± 3.9 kg/m², respectively. The mean (±SD) glucose disposal rate and ISI determined by glucose clamp were 27.46 ± 16.55 µmol/kg·min and 7.39 ± 2.72 µmol/kg·min/pmol·liter, respectively. Pearson’s correlation coefficient between our ISI_OGTT and ISI_Clamp was 0.869 (P < 0.0001) which was stronger than those corresponding values calculated from HOMA, QUICKI, Belfiore, Cederholm, Gutt, Matsuda, and Stumvoll, the respective values of which were 0.404, 0.434, 0.643, 0.533, 0.584, 0.734, and 0.508. In conclusion, the ISI_OGTT derived from our equation is more suitable than others in assessing insulin sensitivity in subjects with normal glucose tolerance. Further studies in subjects with impaired glucose tolerance and diabetes mellitus should be performed to confirm the validity of this equation.

This work was supported by a Faculty of Medicine research grant from Prince of Songkla University and by Glaxo SmithKline (Thailand).

Abbreviations: AACglu, Area above the glucose curve; AUCglu, area under the glucose curve; AUCins, area under the insulin curve; BMI, body mass index; BW, body weight; FPG, fasting plasma glucose concentration; ISI, insulin sensitivity index; ISI_Clamp, insulin sensitivity index obtained from glucose clamp; ISI_OGTT, insulin sensitivity index obtained from oral glucose tolerance test; OGTT, oral glucose tolerance test; PPGC-without insulin, postloading plasma glucose concentration without insulin; Uglu, glucose appearing in the urine.

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