Background

Diabetes mellitus is a chronic disorder characterized by insulin deficiency, hyperglycemia, and high risk for development of complications of the eyes, kidneys, peripheral nerves, heart and blood vessels. The disease is highly prevalent, affecting approximately 26 million people in the U.S. (1). The disease is also expensive, in terms of both human suffering and dollars; appoximately 1 of every 5 health care dollars spent in the U.S. is spent treating patients with diabetes, and over 1 of every 10 dollars is directly attributable to diabetes (2). In the U.S., diabetes is the most common cause of new cases of blindness in adults, kidney failure, and nontraumatic limb amputation (1).

The landmark nine-year Diabetes Control and Complications Trial (DCCT), completed in 1993, showed that the risk for development and progression of the chronic complications of diabetes is closely related to the degree of glycemic control, as measured by hemoglobin A1c (HbA1c) (3). The DCCT also provided a large body of data relating HbA1c values to mean blood glucose. Thus, the DCCT results had set the stage for establishing specific diabetes treatment goals using HbA1c as an index of mean blood glucose. However, the fact that HbA1c assay methods had not been standardized among laboratories prevented optimal use of the test; studies had clearly shown the advantages and feasibility of standardizing HbA1c assays (4-9).

Because of the positive impact standardization of HbA1c determinations would have on the care of diabetic patients, the American Association for Clinical Chemistry (AACC) Standards Committee established a HbA1c Standardization Subcommittee in April 1993. The goal of the subcommittee was to develop a plan for HbA1c standardization that would ultimately allow individual clinical laboratories to relate their HbA1c assay results to those of large-scale studies such as the DCCT, where relationships of HbA1c values to mean blood glucose and to risks for developing chronic diabetic complications have been established.

Although the DCCT was completed in 1993, the HbA1c assay systems from the study (10) were still in place as part of another National Institutes of Health-sponsored long-term diabetes study called the Epidemiology of Diabetes Interventions and Complications (EDIC). In order to initiate a standardization program in a timely fashion, the subcommittee recommended that the DCCT assay method (11) be used as an interim reference method while studies were being performed to evaluate candidate reference methods and to develop purified HbA1c standards. Standardizing HbA1c results to DCCT values would allow individual clinical laboratories to provide diabetic patients and their health-care providers with test results that could be related directly to both mean blood glucose values and risks for development and/or progression of chronic diabetes complications.

Early efforts to standardize GHB values among clinical laboratories by using a "universal calibrator" proved feasible with some assay methods (4). Later studies showed, however, that such an approach, although relatively simple, did not work for a number of existing methods (7). It was found that materials prepared for use as calibrators, quality control materials, and proficiency-testing samples are often subjected to preparative processes that may cause them to yield results that differ appreciably from those of patient specimens, i.e., matrix effects. Therefore, since an important goal was to allow standardization of most existing and future assay methods, it was proposed that, for most assay methods, standardization to the DCCT reference could be performed best at the manufacturing level, where the most appropriate materials and standardization format for each method could be determined. It was also proposed that verification of method standardization should be based on fresh sample comparisons with the DCCT Reference Method.

The NGSP HbA1c standardization program began in 1996 to implement the recommendations of the AACC subcommittee. The NGSP approach to HbA1c assay standardization is modeled after the Cholesterol Reference Method Laboratory Network program (12). The Cholesterol program is based on performing split-sample comparisons with the cholesterol reference method and thus provides a means for manufacturers to establish traceability to the National Reference System for Cholesterol. For HbA1c standardization, a network of reference laboratories is calibrated to the DCCT Reference values. A Central Primary Reference Laboratory (CPRL) set the initial calibration and is responsible for monitoring back-up Primary Reference Laboratories (PRLs) and Secondary Reference Laboratories (SRLs). The SRLs work directly with manufacturers to standardize their methods and provide comparison data for method certification, in order to achieve the desired end result of comparability of fresh sample results with the DCCT Reference. Proficiency testing (PT) data (from fresh sample surveys) is used to assess the effectiveness of method standardization (13).

References:

1. Centers for Disease Control and Prevention. National diabetes fact sheet: national estimates and general information on diabetes and prediabetes in the United States, 2011. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 2011.
2. American Diabetes Association. Economic Costs of Diabetes in the U.S. In 2007. Diabetes Care 2008;31(3):596-615.
3. DCCT Research Group. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. N Engl J Med 1993;329:977-86.
4. Little RR, England JD, Wiedmeyer HM, et al. Interlaboratory standardization of glycated hemoglobin determinations. Clin Chem 1986;32:358-60.
5. Little RR, Wiedmeyer HM, England JD, et al. Interlaboratory comparison of glycated hemoglobin results: College of American Pathologists (CAP) survey data. Clin Chem 1991;37:1725-29.
6. Bodor G, Little R, Garrett N, et al. Standardization of glycated hemoglobin determinations in the clinical laboratory: three years experience. Clin Chem 1992;38:2414-18.
7. Little RR, Wiedmeyer HM, England JD, et al. Interlaboratory standardization of measurements of glycated hemoglobin. Clin Chem 1992;38:2472-78.
8. Feichtner M, Ramp J, England B, et al. Affinity binding assay of glycated hemoglobin by two-dimensional centrifugation referenced to hemoglobin A1c. Clin Chem 1992;38:2372-79.
9. Weykamp CW, Penders TJ, Frits AJ, et al. Effect of calibration on dispersion of glycated hemoglobin values as determined by 111 laboratories using 21 methods. Clin Chem 1994;40:138-44.
10. DCCT Research Group. Feasibility of centralized measurements of glycated hemoglobin in the Diabetes Control and Complications Trial: A multicenter study. Clin Chem 1987;33:2267-71.
11. Goldstein DE, Little RR, England JD, et al. Methods for quantitating glycosylated hemoglobins: high performance liquid chromatography and thiobarbituric acid colorimetry. In: Clarke WL, Larner J, Pohl SL, eds. Methods in Diabetes Research, Vol.2.: Clinical Methods. New York: John Wiley, 1986:475-504.
12. Myers GL, et al. Standardization of lipid and lipoprotein measurement. In: Rifai, N, Warnick GR, eds. Laboratory Measurements of Lipids, Lipoproteins and Apolipoproteins. Washington, DC: AACC Press,1994:177-205.
13. Little RR, Rohlfing CL, Sacks DB. Status of HbA1c measurement and goals for improvement: From chaos to order for improving diabetes care.Clin Chem 2011; 57:204-214.