The second thing is that part of the degree to which HbA 1c represents the average blood glucose depends on a lot of factors, and some of these factors are things that we can do absolutely nothing about because we are born with them. African Americans tend to have higher HbA 1c levels than do whites for the same glucose. That difference is as much as 0.4. An HbA 1c of 6.2 in African Americans gets you a 5.8 in whites for the same average blood glucose. Similarly, Native Americans have somewhat higher HbA 1c, although not quite as high as African Americans. Hispanics and Asians do as well, so you have to take your patient’s ethnicity into account.
The second has to do with the way that HbA 1c is measured and the fact that there are many things that can affect the measurement. An HbA 1c is dependent upon the lifespan of the red blood cell, so if there are alterations in red cell lifespan or if someone has anemia, that can affect HbA 1c. Certain hemoglobin variants, for example, hemoglobin F, which is typically elevated in someone with thalassemia, migrates with some assays in the same place as thalassemia, so the assay can’t tell the difference between thalassemia and hemoglobin F. There are drugs and other conditions that can also affect HbA 1c. You should think about HbA 1c as a guide, but no number should be considered to be written in stone.
Dr. Conlin . I can imagine that this would be particularly important if you were using HbA 1c as a criterion for diagnosing diabetes.
Dr. Aron. Quite right. The effects of race and ethnicity on HbA 1c account for one of the differences between the VA/DoD guidelines and those of the American Diabetes Association (ADA).
Dr. Conlin. Isn’t < 8% HbA 1c a national performance measure that people are asked to adhere to?
Dr. Aron . Not in the VA. In fact, the only performance measure that the VA has with a target is percent of patients with HbA 1c > 9%, and we don’t want any of those or very few of them anyway. We have specifically avoided targets like < 8% HbA 1c or < 7% HbA 1c, which was prevalent some years ago, because the choice of HbA 1c is very dependent upon the needs and desires of the individual patient. The VA has had stratified targets based on life expectancy and complications going back more than 15 years.
Dr. Conlin. Another issue that can confuse clinicians is when the HbA 1c is in the target range but actually reflects an average of glucose levels that are at times very high and very low. How do we address this problem clinically?
Dr. Aron . In managing patients, you use whatever data you can get. The HbA 1c gives you a general indication of average blood glucose, but particularly for those patients who are on insulin, it’s not a complete substitute for measuring blood glucose at appropriate times and taking the degree of glucose variability into account. We don’t want patients getting hypoglycemic, and particularly if they’re elderly, falling, or getting into a car accident. Similarly, we don’t want people to have very high blood sugars, even for limited periods of time, because they can lead to dehydration and other symptoms as well. We use a combination of both HbA 1c and individual measures of blood glucose, like finger-stick blood sugar testing, typically.