The considerable disagreement in the definition of impaired glucose tolerance (IGT) by National Diabetes Data Group (NDDG) and World Health Organization (WHO) criteria was explored in two independent representative adult population samples in Israel (n = 1119) and the United States (n = 1783). Five categories of nondiabetic glucose tolerance were defined according to fasting plasma glucose (FPG) values (mM) and 1- and 2-h plasma glucose values (PG1 and PG2, respectively) after oral glucose load: 1) normal by WHO and NDDG (FPG <6.4 mM, PG1 <11.1 mM, PG2 <7.8 mM), 2) Normal by WHO, nondiagnostic by NDDG (FPG 6.4–7.7 mM, PG1 <11.1 mM, PG2 <7.8 mM), 3) normal by WHO, nondiagnostic by NDDG (FPG <7.8 mM, PG1 >11.1 mM, PG2 <7.8 mM), 4) IGT by WHO, nondiagnostic by NDDG (FPG <7.8 mM, PG1 <11.1 mM, PG2 7.8–11.0 mM), and 5) IGT by WHO and NDDG (FPG <7.8 mM, PG1 >11.1 mM, PG2 7.8–11.0 mM). Established markers of abnormal glucose tolerance were also measured, including glycosylated hemoglobin A1, insulin response, plasma triglycerides, serum uric acid, and rate of hypertension in Israel as well as rates of hypertension, peripheral vascular involvement, family history of diabetes, and history of cholelithiasis in the U.S. Accounting for potential confounders, levels of these markers in both national samples were similar in categories 1 and 2 and in categories 3–5. Levels were significantly higher in the latter three categories than in the former two. It is therefore recommended that categories 3–5 be considered as constituting IGT. Group 3, with normal PG2 but abnormal PG1, cannot be identified by the WHO oral glucose tolerance test (OGTT), which omits PG1. Therefore, OGTT in studies relating to the nature and associated risks of IGT should include a midtest glucose sample.

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