Relationship of Glycosylated Hemoglobin to Oral Glucose Tolerance: Implications for Diabetes Screening

The oral glucose tolerance test (OGTT) for diagnosis of diabetes is inconvenient and requires a great deal of patient cooperation. Glycosylated hemoglobin (GHb), an index of long-term glycemic control, could offer several practical advantages over the OGTT for diabetes screening. We evaluated GHb as a screen for diabetes in 381 adults from a population with a high prevalence of non-insulin-dependent diabetes (Pima Indians). All individuals underwent a standard OGTT (75 g) and were separated into one of three groups: normal (N), impaired glucose tolerance (IGT), or diabetes mellitus (D) based on World Health Organization criteria. HbA_1c, a GHb, was measured by highly precise high-performance liquid chromatography (interassay C.V. <4%). The normal range for HbA_1c was 4.07–6.03% based on the 95% confidence interval for a nondiabetic, mostly Caucasian population. Compared with OGTT, HbA_1c was highly specific (91%); an elevated HbA_1c usually indicated D or IGT (sensitivity = 85 and 30%, respectively). A normal HbA_1c did not, however, exclude a diagnosis of D or IGT. Based on previous epidemiological studies relating plasma glucose to chronic diabetic complications, GHb as measured in this study would properly identify the vast majority of subjects at risk. Long-term studies are necessary to determine the actual risk of complications in individuals with persistently normal HbA^1c and D or IGT (based on OGTT).