The Notorious E.A.G. (Estimated Average Glucose)

A1C as an indicator of diabetes control is not an easy concept to explain to patients. The measurement—expressed as a percentage—does not intuitively relate to the blood glucose measurements that patients perform on home glucose monitors. This discrepancy may make A1C targets difficult for patients to translate into action.

The results of the A1C-Derived Average Glucose study (ADAG), published in 2008, affirmed the existence of a linear relationship between A1C and average blood glucose levels, albeit not without controversy (see here and here). In light of the study results, clinicians can report A1C results to patients using the same units (mg/dl or mmol/l) that patients see routinely in blood glucose measurements.

The American Diabetes Association (ADA) and the American Association for Clinical Chemistry have determined that the correlation (r = 0.92) in the ADAG trial is strong enough to justify reporting both the A1C result and the estimated average glucose (eAG) result when a clinician orders the A1C test, as discussed below.

What is the A1C-Derived Average Glucose (ADAG) Study and why was it conducted?

The A1C-Derived Average Glucose (ADAG) Study was sponsored by the ADA, European Association for the Study of Diabetes (EASD), and International Diabetes Federation (IDF). The objective of the study was to define the mathematical relationship between A1C and estimated average glucose (eAG), and determine if A1C could be reliably reported as eAG, which would be in the same units as daily self-monitoring.

How was ADAG conducted? 

507 people, including 268 patients with type 1 diabetes, 159 with type 2 diabetes, and 80 people without diabetes were recruited from 10 international centers. A1C was measured using a combination of continuous glucose monitoring and frequent finger stick glucose measurements similar to the way in which people with diabetes check their diabetes control at home by self-monitoring of blood glucose (SMBG). By comparing the measurement of A1C with the average glucose levels, study investigators were able to derive an equation so that A1C levels can be interpreted accurately as an average glucose level or eAG.

Source: Nathan et al. Diabetes Care. 2008

What is estimated Average Glucose (eAG) and why is this measure important?

The ADAG Study established that A1C does represent average glucose over time. With that relationship demonstrated and defined, clinicians could thus report A1C results to patients in the same units that they use for self-monitoring (i.e., mg/dl) which should benefit clinical care.

Source: ADA’s eAG/A1C Conversion Calculator

Why is the chart in ADA’s Standards of Care showing a correlation between A1C and mean glucose levels slightly different from the correlation published with the ADAG results (as above)?

The chart published in the ADA’s Standards of Care is based on a study that analyzed data collected during the DCCT, which included quarterly A1C tests and 7-point glucose measurements in 1,400 type 1 diabetes patients. The ADAG Study involved people with type 1 and type 2 diabetes as well as people without diabetes, and took advantage of the development of continuous glucose monitors, as well as patients making traditional glucose checks, to generate a much larger pool of data. Both studies showed a linear relationship between A1C and average glucose; the ADAG study presents a more refined and accurate formula describing that relationship, and the ADA’s Standards of Care adopted the new correlation.

The relationship between A1C and eAG is described by the formula 28.7 x A1C – 46.7 = eAG, which is available here.

Many patients who practice SMBG already see an “average glucose” on their blood glucose meters. Is eAG the same thing?

No, an eAG value is unlikely to match the average glucose level shown on a person’s meter. Because people with diabetes are more likely to test more often when their blood glucose levels are low (e.g., first thing in the morning, and before meals), the average of the readings on their meter is likely to be lower than their eAG, which represents an average of their glucose levels 24 hours a day, including post-meal periods of higher blood glucose when people are less likely to test. One advantage of using eAG as a measure of glucose control is that it will help patients more directly see the difference between their individual meter readings and how they are doing with their glucose management overall.

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