Estimated average glucose and self-monitored mean blood glucose are discordant estimates of glycemic control

Abstract

Objective: The A1C-Derived Average Glucose study recommended reporting A1C in estimated average glucose (eAG) equivalents. We compared eAG with self-monitored mean blood glucose (MBG) to determine whether eAG is systematically biased due to biological variation in the relationship between MBG and A1C.

Research design and methods: MBG and A1C were recorded from charts of 202 pediatric type 1 diabetic patients at 1,612 clinic visits. Patients were divided into groups with low, moderate, or high A1C bias based on a hemoglobin glycation index (HGI).

Results: The mean +/- SD values for MBG versus eAG were as follows: total population, 194 +/- 34 vs. 196 +/- 36 mg/dl; low-HGI group, 186 +/- 31 vs. 163 +/- 20 mg/dl; moderate-HGI group, 195 +/- 28 vs. 193 +/- 19 mg/dl; and high-HGI group, 199 +/- 42 vs. 230 +/- 31 mg/dl.

Conclusions: eAG underestimated MBG in low HGI patients and overestimated MBG in high HGI patients. Disagreement between eAG and MBG downloaded from patient glucose meters will cause confusion if eAG is implemented for clinical use.

Figure 1

Disagreement between eAG and MBG. Mean eAG and mean self-monitored MBG were compared in all 202 patients in the population and separately by HGI group. Data are group means ± SD. MBG was similar to eAG in the population and in the moderate-HGI group, higher than eAG in the low-HGI group and lower than eAG in the high-HGI group. Dividing the study population into HGI groups automatically produces subpopulations with similar MBG but different A1C. Because eAG is calculated from A1C, it is not surprising that eAG systematically under- or overestimates MBG in some patients.