Despite its standing as the most validated and widely used measure for average glycemic control over time, the relationship between hemoglobin A1c (A1c) and glucose concentrations is not completely understood. The purpose of this DirecNet study was to utilize continuous glucose monitoring data to examine the relationship between A1c and glucose in type 1 diabetes.
Research Design and Methods
48 youth enrolled in studies of Navigator continuous glucose monitor were encouraged to wear the Navigator sensor at home continuously. A1c was measured at baseline, 3 months, and 6 months. Sensor glucose data were directly transmitted via the internet, assuring that essentially all glucose values were analyzed.
Subjects had a median of 112 hr/wk of Navigator data in the first 3 months and 115 in the second 3 months. The slope of mean glucose over the previous 3 months vs. A1c was only 18 mg/dL per 1.0% A1c. Individually, there was substantial variation in the relationship between mean glucose and A1c. A1c was not associated with glucose lability after controlling for mean glucose. Measures of an individual’s rate of glycation were moderately correlated at the 3 and 6-month visits.
As the chemistry of glycation would predict, we found no evidence to contradict the simple hypothesis that A1c directly reflects the mean glucose over time. There is, however, substantial variability in individual mean glucose concentrations for a given A1c. Transforming reliable A1c values into calculated mean glucose values would, when applied to an individual, introduce substantial error.