Bimodality of glycosylated hemoglobin distribution in Pima Indians: relationship to fasting hyperglycemia.

Glycosylated hemoglobin (HbA1) concentrations were determined in 300 Pima Indians aged 15 yr and older. Frequency distributions of HbA1 were unimodal in the 15--24-yr-old age group, but were bimodal in those aged 25 yr and over. The bimodality indicated that the subpopulation with diabetes could be identified by the presence of elevated HbA1 levels. This group was comprised primarily of subjects who also had fasting plasma glucose levels of less than or equal to 140 mg/dl, but subjects with impaired glucose tolerance without fasting hyperglycemia had HbA1 levels that were not significantly higher than those with normal glucose tolerance. The prevalence of diabetic retinopathy was much higher in the subgroup with elevated HbA1 levels and increased with increasing HbA1 level. HbA1 levels and triglyceride concentrations showed only a modest association. HbA1 determinations provided no advantage over fasting or post challenge glucose levels in the diagnosis of diabetes.