A1C Level and Future Risk of Diabetes: A Systematic Review

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Abstract
The use of A1C for the identification of persons with undiagnosed diabetes has been investigated for a number of years (1–3). A1C better reflects long-term glycemic exposure than current diagnostic tests based on point-in-time measures of fasting and postload blood glucose (4,5) and has improved test-retest reliability (6). In addition, A1C includes no requirement for fasting or for the oral glucose tolerance test's 2-h wait. These advantages should lead to increased identification and more timely treatment of persons with diabetes. Recently, an American Diabetes Association (ADA)-organized international expert committee recommended the adoption of the A1C assay for the diagnosis of diabetes at a cut point of 6.5% (7). This cut point was primarily derived from a review of studies that examined the association of A1C values with incident retinopathy, and some of the most influential data were obtained from recently published prospective studies. Retinopathy was chosen as the ultimate criterion because it is among the main complications of diabetes. Identification of the point on the A1C distribution most closely related to future retinopathy will identify persons in the greatest need of interventions for the prevention of diabetes complications. In addition to utility and convenience, A1C could help identify persons at increased risk of developing diabetes. This is an important public health priority since a structured lifestyle program or the drug metformin can reduce the incidence of diabetes by at least 50 and 30%, respectively (8). Ideally, selection of diagnostic cut points for pre-diabetes would be based on evidence that intervention, when applied to the high-risk group of interest, results not only in the prevention of diabetes but also later complications. However, currently there are no trials that can provide data to determine the ideal method for defining cut points. In the absence of such data, expert …