Although fasting blood glucose (FBG) is well-used in mass screening, it is not sensitive to find postprandial hyperglycemia patients. Focusing on another screening test for diabetes mellitus, Glycohemoglobin A_<1c> (HbA_<1c>), this study compared the accuracy of both screening tests for postprandial hyperglycemia using 75g oral glucose tolerance test (OGTT), and discussed the ideal cut off point of HbA_<1c>. Previous study reported that the cut off point of 6.5% for HbA_<1c> is as accurate as that of FBS=7.0 mmol/L. However, the cut off point is too high and therefore, very specific but not sensitive. In this study the authors lowered the cut off point of HbA_<1c> and observed the change of accuracy of diagnosis and attribution to finding new patients of diabetes mellitus. The subjects were 231 males and 323 females who underwent all three tests, i.e., FBG, HbA_<1c>, and 75g OGTT.
From the analysis of the ROC curve, the accuracy of HbA_<1c> is not lowered as lowering the cut off point, unlike FBG. When the cut off point was 6.5%, the true positive and negative proportions of the subjects were 2.4% and 1.1%, respectively. In the study population, 19% of postprandial hyperglycemia who could not be found by FBG was detectable by HbA_<1c>. When the cut off point was lowered to 5.6%, the true positive and negative proportions of the subjects were 2.4% and 1.1%, respectively, and 80% of postprandial hyperglycemia could be found by HbA_<1c>. At the point, the sensitivity, the specificity, and positive predictive value were 0.91, 0.82 and 0.50, respectively. If the cut off point was lowered beyond 5.6%, positive predictive value was rapidly lowered, meaning lowering cost performance.
From the study, HbA1c was robust against the change of cut off point, and therefore, lowering the cut off point did not cause of lower accuracy. The cut off point of 5.6% seemed to be reasonable to find more postprandial hyperglycemia patients.