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The interpretation of glycosylated hemoglobin (HbA_1c) levels is not intuitively obvious to many diabetic patients, who are accustomed to thinking in terms of blood glucose levels. In this international study, researchers explored the relation between HbA_1c and average blood glucose levels in 507 people; 86% had stable diabetes, and 14% were not diabetic. Each subject underwent 2 days of continuous glucose monitoring every 4 weeks for 12 weeks (generating 288 glucose readings on each monitored day) and finger-stick capillary glucose monitoring seven times daily on 3 days each week for 12 weeks. HbA_1c was measured at 12 weeks.

A significant linear relation between average glucose and HbA_1c levels was confirmed and is reported in Table 1.

link to table go to: http://general-medicine.jwatch.org/cgi/content/full/2008/828/1/T1

  Thus, each percentage point of HbA_1c corresponded to about a 29 mg/dL increment in estimated average glucose.Comment: The authors propose that laboratories should report the corresponding estimated average glucose when they report HbA_1c results; this move presumably would facilitate communication with diabetic patients. Note, however, that the association between average glucose and HbA_1c levels exhibits some variability: For example, inspection of a scatter plot in this paper shows that, for HbA_1c of 7%, average glucose values for individual patients range from about 120 mg/dL to 180 mg/dL.

— Allan S. Brett, MD

Published in Journal Watch General Medicine August 28, 2008