Translating the A1C Assay Into Estimated Average Glucose Values

  1. David M. Nathan, MD1,
  2. Judith Kuenen, MD2,
  3. Rikke Borg, MD3,
  4. Hui Zheng, PHD1,4,
  5. David Schoenfeld, PHD1,4,
  6. Robert J. Heine, MD2 and
  7. for the A1c-Derived Average Glucose (ADAG) Study Group*
  1. 1Diabetes Center, Massachusetts General Hospital and Harvard Medical School, Boston, MA
  2. 2Department of Endocrinology/Diabetes Center, VU University Medical Center, Amsterdam, the Netherlands
  3. 3Steno Hospital, Copenhagen, Denmark
  4. 4Department of Biostatistics, Massachusetts General Hospital, Boston, MA
  1. Corresponding author: David M. Nathan, dnathan{at}partners.org

Abstract

OBJECTIVE—The A1C assay, expressed as the percent of hemoglobin that is glycated, measures chronic glycemia and is widely used to judge the adequacy of diabetes treatment and adjust therapy. Day-to-day management is guided by self-monitoring of capillary glucose concentrations (milligrams per deciliter or millimoles per liter). We sought to define the mathematical relationship between A1C and average glucose (AG) levels and determine whether A1C could be expressed and reported as AG in the same units as used in self-monitoring.

RESEARCH DESIGN AND METHODS—A total of 507 subjects, including 268 patients with type 1 diabetes, 159 with type 2 diabetes, and 80 nondiabetic subjects from 10 international centers, was included in the analyses. A1C levels obtained at the end of 3 months and measured in a central laboratory were compared with the AG levels during the previous 3 months. AG was calculated by combining weighted results from at least 2 days of continuous glucose monitoring performed four times, with seven-point daily self-monitoring of capillary (fingerstick) glucose performed at least 3 days per week.

RESULTS—Approximately 2,700 glucose values were obtained by each subject during 3 months. Linear regression analysis between the A1C and AG values provided the tightest correlations (AGmg/dl = 28.7 × A1C − 46.7, R2 = 0.84, P < 0.0001), allowing calculation of an estimated average glucose (eAG) for A1C values. The linear regression equations did not differ significantly across subgroups based on age, sex, diabetes type, race/ethnicity, or smoking status.

CONCLUSIONS—A1C levels can be expressed as eAG for most patients with type 1 and type 2 diabetes.

Footnotes

  • * A complete list of the members of the ADAG Study Group can be found in the appendix.

  • Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.

    See accompanying editorial, p. 1704.

    • Received March 17, 2008.
    • Accepted May 14, 2008.
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  1. Diabetes Care vol. 31 no. 8 1473-1478
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