Timothy S. Bailey, MD, FACE
TCOYD Newsletter, Vol. 24, 2008

Hemoglobin A1c (HbA1c or simply A1c) has been the test of choice for the past 15 years to measure the average blood sugar over the prior two to three months. It has been the gold standard to predict the potential for long-term diabetes complications (i.e. eye, kidney and nerve damage). Despite the wide acceptance of A1c there are efforts in progress to improve on it. Each of these improvements has the potential to confuse unwary patients and healthcare professionals, so this article sets out to explain the buzz on ADAG vs. A1c.

The life span of a red blood cell (the cell that carries oxygen and hemoglobin to the organs of our body) is approximately two to three months before it is cleared from the body by the spleen. New red blood cells are made continually by the bone marrow. Whether you have diabetes or not, glucose molecules stick to red blood cells, giving us “glycoslated hemoglobin”. The A1c test measures how much glycoslated hemoglobin a person has. An A1c result under 7% is considered well-controlled diabetes, and a result between 4 and 6% is considered normal for those without diabetes.

Before the publication of the DCCT (Diabetes Control and Complications Trial) results in 1993, there were many methods used to measure average glucose levels. Each of these methods resulted in a different numerical value for the same person’s blood because of the different ways the tests were performed. The National Glycohemoglobin Standardization Program (NGSP), established in 1996, required all labs to report A1c in values that are “traceable” to the DCCT method. Based on the NGSP, an A1c of 6% roughly equated to an average glucose of 135 mg/dL. But no one knew precisely how accurate that average blood glucose estimation was.

The International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) has spent years perfecting a more accurate method of measuring A1c that all labs will now follow. This proposed new gold standard also reflects enhanced understanding of what an A1c of a particular percentage equates to in mg/dL—the units by which we measure our blood sugar using a home glucose monitor or continuous glucose monitoring device. Where an A1c of 6% was understood to be an average of 135 mg/dL, the new standard pins 6% to 126 mg/dL.

Last September [2007] at the EASD meeting in Amsterdam the A1c Derived Average Glucose (ADAG) study results were reported. In the trial, A1c was measured monthly and compared with 1000’s of glucose values determined by continuous glucose monitoring (2 days each month) and frequent fingerstick monitoring (7 times daily at least 3 days per week) over 4 months. The results from the subjects (some with type 1, some with type 2, and others without any diabetes) proved that A1c accurately predicts the average blood sugar levels. Importantly, the calculation was not influenced by diabetes type, gender, ethnicity, age, or smoking status.

The formula is: ADAG = A1c x 28.5-45.4; so an A1c of 6.7, for example, predicts an average glucose level of approximately 146 mg/dL) or 6.7(A1c) x 28.5 - 45.4 = 146 (ADAG). We are starting to see labs reporting three values: • NGSP (“old”) A1c • IFCC (“new”) A1c • ADAG—The A1c Derived Average Glucose has the advantage of being in the same unit of measurement that we use when we regularly check our blood using a home glucose meter or CGM device.

Each of these values will also need to be accompanied by a recommended goal range.

Diabetes and the testing that comes with it is complicated. It is likely to become more so in the future as we gain a better understanding of how diabetes affects the body and we improve the accuracy of diabetes tests. A1c will not disappear, but ADAG will join the list of recommended goals which include targets for fasting and post-prandial (after eating) glucose levels. You and your healthcare professional will need to keep up on the latest information to take full advantage of these advances.

Bottom line? New tools to help you take control of your diabetes are in development and may provide a better indicator of how you are managing it.

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