By Gary Scheiner MS, CDE
Most people with diabetes are familiar with the hemoglobin A1c (or simply A1c). For years, A1c has been the “gold standard” for assessing overall blood glucose control for a period of two to three months, similar to looking at a baseball player’s batting average for an entire season. However, A1c has a major limitation: many people with diabetes just don’t know what the number means on a practical level.
The Science Behind A1c
The A1c tells us the percentage of red blood cells (the cells in our blood that carry oxygen) that have glucose stuck to them. When blood glucose levels are normal, approximately 4 to 6% of red blood cells will have glucose attached. The higher the blood glucose over a period of time, the higher the percentage. Red blood cells live for an average of two to three months before they are broken apart and replaced with new ones. So, an A1c measurement gives us a good estimate of how high the blood glucose has been over the past two to three months.
Why is A1c so important? The Diabetes Control and Complications Trial (DCCT) and United Kingdom Prospective Diabetes Study (UKPDS) showed that A1c is closely linked to the risk of developing diabetic complications. Essentially, the higher the A1c, the greater the risk of developing eye, kidney, nerve and heart problems. That is why the American Diabetes Association recommends keeping the A1c as close to normal as possible, and certainly below 7%.
Not A Perfect System
Not all labs worldwide measure A1c the same way. With inconsistent “normal” ranges, a 7% at one lab could be the same as a 6.5% at another. Efforts have been made to ensure that the vast majority of labs use the same standards as those used in the DCCT study, but there is still potential for inconsistency. Recently, the International Federation of Clinical Chemists proposed entirely new methods for measuring and reporting A1c levels, which would certainly add to the confusion.
Also, most people with diabetes think in terms of their daily blood glucose measurements, measured in mg/dl or mmol. Exactly what does an A1c of 8.2% mean? After all, it is much easier to strategize to “keep blood glucose below 140 mg/dl” than it is to “keep A1c below 6.5%”. Clearly, if A1c could be reported in terms that are consistent and understandable, it would make life easier for both patients and health care providers.
eAGer For A Solution
Dr. David Nathan at Harvard Medical School, along with a team of international colleagues, formed the A1c-Derived Average Glucose (ADAG) Study Group. Their mission was to define exactly what A1c means in terms of average glucose, and verify this relationship across different patient populations. More than 500 individuals, most with type-1 or type-2 diabetes, were included in the three-month study. Unlike previous attempts to correlate A1c to average glucose, Dr. Nathan had participants perform frequent fingerstick blood glucose measurements (both before and after meals) and wear continuous glucose monitors intermittently throughout the study. In fact, each subject collected more than 2,500 glucose values over the course of three months. This, he believed, would yield a truer overall picture of the estimated average glucose level (eAG) than had been obtained in previous research.
The results (reported in Diabetes Care 31:1473-1478, 2008) showed a very strong linear relationship between A1c and average glucose readings in high, low and normal ranges. Specific formulas were developed for converting A1c into estimated average glucose (eAG) levels in both mg/dl and mmol. For example, the study found that people with an A1C of 7% have an estimated Average Glucose (eAG) of 154 mg/dl.
Dr. Nathan found that the relationship between A1c and eAG was similar for men & women, type-1s and type-2s, whites and Hispanics, and young and old (although children under 18 did not participate in the study). There was a slight tendency for blacks to have lower eAGs than whites at the same A1c level, but this will not be considered until further studies are performed.
Keep an eAGle Eye Out
So what will come from all this? Most lab reports now include eAG (expressed in mg/dl or mmol) along with A1c values. That way, you and your healthcare team can talk in language that everyone understands. And when you perform blood glucose monitoring at home, you’ll have a better idea of where you stand in terms of reaching your overall goals.
In the meantime, if you would like to calculate your own eAG, here’s how:
If you measure blood glucose in mg/dl (as is the norm in the United States), take your A1c, add 46.7, and divide by 28.7. If you measure your blood glucose in mmol (as is the norm in Canada and many European countries), take your A1c, add 2.59, and divide by 1.59.
If that’s more math than you can handle, the American Diabetes Association has a handy A1c-to-eAG calculator at: http://professional.diabetes.org/glucosecalculator.aspx. All you need to do is enter your A1c and the program does the rest.
Editor’s note: Gary Scheiner is owner and clinical director of Integrated Diabetes Services (www.integrateddiabetes.com), a private practice specializing in advanced training and management of children and adults on intensive insulin therapy. His team of CDEs (all of whom have Type-1 diabetes) provides consultations worldwide via phone and the internet. Gary is author of several books including “Think Like A Pancreas” and was named 2014 Diabetes Educator of the Year by the American Association of Diabetes Educators. Feel free to submit questions and comments to gary@integrateddiabetes.com
Photo credit: Pixabay
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