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

OBJECTIVE: The Expert Committee on the Diagnosis and Classification of Diabetes 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 best measure of long-term glycemia and an important predictor of microvascular and neuropathic complications, we evaluated the distribution of hemoglobin A1C (Hb A1C) levels in individuals who had undergone an oral glucose tolerance test to determine how well 2-hour values could identify those with normal versus increased Hb A1C levels. DESIGN: A cross-sectional analysis of 2 large data sets was performed. We cross-tabulated 2-hour glucose concentrations on an oral glucose tolerance test 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 Hb 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 that reflect long-term glycemia, we conclude that the 2-hour glucose concentration criterion on an oral glucose tolerance test for the diagnosis of diabetes 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 subsequent development of specific diabetic complications (e.g., retinopathy).