Point: The reporting of estimated glucose with hemoglobin A1c.

Healthcare providers and patients with diabetes evaluate the efficacy of glycemic control by 2 strategies. One strategy involves self-monitoring of blood glucose (SMBG) by patients, with portable meters and continuous blood glucose monitors or sensing devices. Patients use these glucose values for daily decisionmaking to adjust medication doses and/or modify food intake or exercise regimens. Blood glucose fluctuates widely over minutes to hours, depending on food intake, exercise, insulin, and physical and emotional stressors. Values obtained by SMBG, therefore, do not signify average glucose (AG) concentrations. When an estimate of glucose values over time is desired, cumulative results can be downloaded from the patient’s meter in the provider’s office. These data are useful for determining whether current diabetes therapies are appropriate or need adjustment. Unfortunately, a number of barriers to blood glucose monitoring that may exist in clinical practice make it difficult to obtain an adequate amount of reliable data from patient logbooks. Barriers to SMBG implementation, as identified by patients with diabetes and their healthcare teams, include not only physical, financial, cognitive, and emotional factors, but also time constraints and inconvenience (1 ). In addition, patient follow-through may be lacking because of inadequate education or communication between patient and healthcare provider regarding what information is needed and why it is necessary. For this reason, it is important that hemoglobin A1c (Hb A1c) be measured regularly. The second strategy, measurement of Hb A1c, provides a more accurate assessment of long-term glycemia than that obtained from SMBG. The concentration of Hb A1c, which consists of glucose attached to the N-terminal valine of the hemoglobin chain, is relatively stable, given that the mean erythrocyte life span is approximately 120 days. Therefore, the Hb A1c value reflects the integrated glucose concentration over the preceding 8 –12 weeks (2 ). Clinically, Hb A1c measurement is used to assess whether a patient’s glycemic target has been reached and maintained. It also predicts the progression of microvascular complications. Most patients, however, perceive diabetes as a disease of high sugar in the blood and fail to understand the relevance of hemoglobin. To facilitate communication with their patients, many healthcare professionals translate Hb A1c values into average plasma glucose. Tables that convert Hb A1c to AG are available in print (e.g., the Clinical Practice Recommendations published annually by the American Diabetes Association), on Web sites, in hospitals, in doctors’ offices, and frequently in the laboratory coat pockets of members of the diabetes healthcare team. The numbers most widely used in these Hb A1c/AG conversion charts were derived from the Diabetes Control and Complications Trial (3 ). Notwithstanding a fairly large population (1441 individuals) and the merits of this trial, the study was confined to patients with type 1 diabetes and was not designed to measure AG. In this trial, capillary glucose data were collected and recorded only from quarterly 7-point glucose profiles over a mean of 6.5 years, for a mean of approximately 182 values per patient (4 ). Therefore, a prospective multinational study was performed to evaluate the relationship between Hb A1c and AG (5, 6 ). AG was assessed by a combination of SMBG and continuous glucose monitoring, with approximately 2700 glucose measurements obtained for each participant. The results of the study revealed a strong linear relationship between AG and Hb A1c (5 ). The study provided a linear regression equation that allows Hb A1c values to be converted to AG. No significant differences in the equation were observed among individuals for any characteristic, including age, race, sex, presence or absence of diabetes, type of diabetes, or ethnicity (5 ). Analogous to essentially all clinical studies, this study had some limitations, including an inherent limitation to accurately measure AG, the small sizes of ethnic groups, and the absence of children and pregnant women. Nevertheless, the study provides the most accurate means to date for converting Hb A1c to AG. Several publications reveal that only 25%–35% of patients with diabetes know their Hb A1c values (7, 8 ). 1 Department of Pathology, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA; 2 International Diabetes Center at Park Nicollet, Minneapolis, MN; 3 The Nebraska Medical Center, Omaha, NE. * Address correspondence to this author at: Brigham and Women’s Hospital, Thorn 530, 75 Francis St., Boston, MA 02115. Fax 617-278-6921; e-mail [email protected]. Received January 20, 2010; accepted January 29, 2010. Previously published online at DOI: 10.1373/clinchem.2009.138669 4 Nonstandard abbreviations: SMBG, self-monitoring of blood glucose; AG, average glucose; Hb A1c, hemoglobin A1c; CAP, College of American Pathologists. Clinical Chemistry 56:4 000 – 000 (2010) Point/Counterpoint