Diabetes NewsJanuary 2010

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Association of A1C and Fasting Plasma Glucose Levels with Diabetic Retinopathy Prevalence in the U.S. Population

Diabetes Care • November 2009

Tests of glycemia and their thresholds for diabetes diagnosis is an area of long-standing debate. The presence of diabetic retinopathy is arguably the best criterion from which to compare glycemic measures because it is specific and early clinical complications usually related to diabetes, and it represents a specific and relevant clinical endpoint for judging an alternative test. For those reasons, diabetic retinopathy has served as the basis for diagnostic criteria of type 2 diabetes and provides the rationale for the American Diabetes Association’s recommendation of a threshold of fasting plasma glucose (FPG) of 7.0 mmol/l to define the presence of diabetes. However, an analysis of three recent population-based cross-sectional studies suggested that there may be considerable variation across populations and that the association of FPG with retinopathy prevalence may be more of a continuous relationship than previously thought.

A1C levels are considered as an alternative diagnostic tool for diabetes diagnosis. Unlike FPG, A1C does not require an overnight fast, is not affected by short-term lifestyle changes, and has less variability within individuals than FPG. Nevertheless, few studies have examined the prevalence of retinopathy across the spectrum of A1C levels, which could assist in the designation of ideal A1C diagnostic cut points.

The newly released National Health and Nutrition Examination Survey (NHANES) 2005-2006 incorporated a multiple-field retinal photograph examination, presenting an opportunity to reassess the selection of glucose and A1C cut points for diabetes diagnosis. The objectives of the study were to examine the relation between levels of A1C and FPG and prevalence of retinopathy in the U.S. population and to compare the ability of both measures to differentiate people with and without retinopathy.

Objective - To examine the association of A1C levels and fasting plasma glucose (FPG) with diabetic retinopathy in the U.S. population and to compare the ability of the two glycemic measures to discriminate between people with and without retinopathy.

Research Design and Methods - This study included 1,066 individuals aged >40 years from the National Health and Nutrition Examination Survey, A1C, FPG, and color digital retinal images were assessed. Retinopathy was defined as a level >14 on the Early Treatment Diabetic Retinopathy Study scale. Joinpoint regression was used to identify linear inflections of prevalence of retinopathy in the association between A1C and FPG.

Conclusions - Based on the latest nationally represented sample, the analysis examined the association of the A1C with retinopathy and provides new information on defining cut points for diagnosing diabetes. While the A1C and FPG levels of 5.5% and 5.8 mmol/l provide start points at which retinopathy prevalence increases most precipitously, A1C appears to discriminate between the presence and absence of retinopathy at least as well as FPG and offers some advantages over FPG.

The steepest increase in retinopathy prevalence occurs among individuals with A1C >5.5% and FPG >5.8mmol/l. A1C discriminates prevalence of retinopathy better than FPG.

Reprinted with permission from the American Diabetes Association