Revisiting the oral glucose tolerance test criterion for the diagnosis of diabetes

OBJECTIVE: The Expert Committee on the Diagnosis and Classification of Diab etes retained the 2-hour glucose concentration on an oral glucose tolerance test of greater than or equal to 11.1 mmol/L (200 mg/dL) as a criterion to diagnose diabetes. Since glycated hemoglobin levels have emerged as the be st measure of long-term glycemia and an important predictor of microvascula r and neuropathic complications, we evaluated the distribution of hemoglobi n A1C (Hb A1C) levels in individuals who had undergone an oral glucose tole rance test to determine how well 2-hour values could identify those with no rmal versus increased Hb A1C levels. DESIGN: A cross-sectional analysis of 2 large data sets was performed. We c ross-tabulated 2-hour glucose concentrations on an oral glucose tolerance t est separated into 4 intervals (< 7.8 mmol/L [140 mg/dL], 7.8-11.0 mmol/L [ 140-199 mg/dL], 11.1-13.3 mmol/L [200-239 mg/dL], and greater than or equal to 13.3 mmol/L [240 mg/dL]) with Hb A1C levels separated into 3 intervals (normal; < 1% above the upper limit of normal; and greater than or equal to the upper limit of normal + 1%). RESULTS: Approximately two thirds of patients in both data sets with 2-hour glucose concentrations of 11.1 to 13.3 mmol/L (200-239 mg/dL) had normal H b A1C levels. In contrast, 60% to 80% of patients in both data sets with 2- hour glucose concentrations greater than or equal to 13.3 mmol/L (240 mg/dL ) had elevated Hb A1C levels. CONCLUSION: Since Hb A1C levels are the best measures presently available t hat reflect long-term glycemia, we conclude that the 2-hour glucose concent ration criterion on an oral glucose tolerance test for the diagnosis of dia betes should be raised from greater than or equal to 11.1 mmol/L (200 mg/dL ) to greater than or equal to 13.3 mmol/L (240 mg/dL) to remain faithful to the concept that diagnostic concentrations of glucose should predict the s ubsequent development of specific diabetic complications (e.g., retinopathy ).