Is HbA1c Misleading and 90-Minute Glucose Tolerance Test a Better Indicator in the Diagnosis of Diabetes Mellitus?

Objective: The present study aimed to investigate the efficacy of hemoglobin A1c (HbA1c) in diagnosing type 2 diabetes and to introduce a new approach to oral glucose tolerance test (OGTT). Methods: This retrospective cross-sectional study included 560 subjects (363 females, 197 males) with a mean age of 46.7±12.9 years. These subjects underwent a standard 75-g OGTT, plasma glucose levels were assayed at 0, 30, 60, 90, and 120 minutes in 232 subjects who consented to these tests, and the area under the curve (AUC) was calculated. HbA1c levels were also measured simultaneously. The receiver operating characteristics (ROC) curve was used to determine the sensitivity and specificity of certain HbA1c cut-off values in diagnosing diabetes in subjects who underwent OGTT. Chi-square test was used for data comparisons, and Pearson’s correlation coefficient was used to evaluate the relationship between the measurements. Results: Among the 560 subjects, 129 had diabetes and 40 of the 232 subjects who were frequently tested for glucose levels had diabetes. The sensitivity and specificity of the HbA1c cut-off value of 6.5% in diagnosing diabetes were 55.0% and 80.9%, respectively. The highest correlation was observed between the glucose level at 90 minutes and AUC and HbA1c (r=0.971, P<0.01; r=0.464, P<0.01, respectively). Conclusion: An HbA1c cut-off value of 6.5% had low sensitivity and specificity in diagnosing diabetes. The plasma glucose level at 90 minutes in subjects had the best correlation with both AUC and HbA1c, indicating that using plasma glucose level at 90 minute is a better approach for diagnosing diabetes.