Background. New clinical practice recommendations include hemoglobin A1c (HbA1c) as an alternative to fasting glucose as a diagnostic test for identifying pre-diabetes. The impact of these new recommendations on the diagnosis of pre-diabetes is unknown.

Methods. Data from the National Health and Nutrition Examination Survey 1999-2006 (n=7029) were analyzed to determine the percent and number of US adults without diabetes classified as having pre-diabetes by elevated HbA1c (5.7% to 6.4%) and by impaired fasting glucose (IFG; fasting glucose: 100 mg/dl to 125 mg/dl) criterion, separately. Test characteristics (sensitivity, specificity, positive and negative predictive values) using IFG as the reference standard were calculated.

Results. The prevalence of pre-diabetes among US adults was 12.6% by the HbA1c criterion and 28.2% by the fasting glucose criterion. Only 7.7% of US adults reflecting 61% and 27% of those with pre-diabetes by HbA1c and fasting glucose, respectively, had pre-diabetes according to both definitions. HbA1c used alone would reclassify 37.6 million Americans with IFG to not having pre-diabetes and 8.9 million without IFG to having pre-diabetes (46.5 million reclassified). Using IFG as the reference standard, pre-diabetes by HbA1c criteria has 27% sensitivity, 93% specificity, 61% positive predictive value and 77% negative predictive value.

Conclusion. Using HbA1c as the pre-diabetes criterion would reclassify the pre-diabetes diagnosis of nearly 50 million Americans. It is imperative that clinicians and health systems understand the differences and similarities in using HbA1c or IFG in diagnosing patients with pre-diabetes.