The Nutrition Debate #228: A1c and your estimated Average Glucose (eAG)

The glycated hemoglobin (Hb) A1c test has become the new standard for assessing a patient’s diabetes or prediabetes status. It is measured in percent, as for example 7.0%, the default ADA target to be met or exceeded in your diabetes care plan. The percent is the amount of glycated hemoglobin on the surface of your red blood cells, that is, the amount of glucose attached to the hemoglobin molecule.

Red blood cells typically live 2 to 3 months, so this is a pretty good way of determining the level of glucose in your blood 24/7 over a 2 to 3 month period. The ADA recommends that your doctor test your A1c level at least 2 times a year and preferably 4. Medicare will pay for 4, unless more frequent testing is deemed “medically necessary” by your doctor.

Prior to the widespread use of the A1c test to assess your diabetes status, the clinician used two consecutive fasting plasma glucose tests that exceeded the ADA standard for a diagnosis of type 2 diabetes. Prior to 1997 that standard was 140mg/dl (U.S measurement). After 1997 the standard was lowered to 126mg/dl. In addition, a new standard for prediabetes was established: two consecutive fasting plasma glucose readings between 100 and 125/mg/dl. If your lab tests are between 100 and 125mg/dl, and your doctor hasn’t told you that you are prediabetic, you might want to ask him/her why?

The difference in units (% vs. mg/dl) has resulted in a disconnect between these two tests. Besides, knowing your fasting blood glucose only tells a small part of the picture of your diabetes health. What happens to your blood glucose after you eat is what really defines how impaired your glucose tolerance is and the degree of insulin resistance you have developed. And that is why the A1c test is a better indicator of your diabetes health than the fasting plasma glucose test. This is particularly true for middle aged women, oddly enough.

So, to resolve the disconnect and the confusion that has resulted, scientists have developed a table that creates a correlation between A1c % and estimated Average Glucose (eAG) in mg/dl. This table is copyrighted by diabeteschart.org.

As you can see on this chart, an A1c of 7% is equivalent to an estimated Average Glucose of 154mg/dl. This is stunning. An average of 154mg/dl means that, if you have an A1c of 7% (the ADA recommended target), your plasma blood glucose is higher than that for many hours each day, especially if your fasting blood glucose is in the 126mg/dl or lower range. Why is that important? Because it is generally understood and accepted that any time your blood glucose is above 140mg/dl, it is slowly but inexorably doing damage to your nerves and microvascular system. It may take years, but damage is being done.

My own A1c’s have been higher than I would like for the last year or so. I have not been as vigilant as I should be in watching what I eat. I am always scrupulous about breakfast and lunch; but at or after dinner, I have not always stuck to my plan. As a result my A1c’s are now almost always in the high 5s. An A1c of 5.8 is equivalent to an eAG of 120mg/dl, which means if my fasting blood sugars are in the low 100s, my postprandials are at or near 140mg/dl. i.e., borderline bad.

But my doctor, who adheres to the ADA standard, considers my blood sugar “in control.” He is wrong, of course, but how to convince him otherwise? So, I just have to establish my own standard, and hope he goes along with my self-management plan. So far, he’s “on board,” but if at some point we see it differently, I will just have to change doctors.

I currently have a prescription to test twice a day. When I was learning about low-carb eating, my previous doctor (now deceased) wrote a script for 4 test strips a day. But now, I can get by with 2 tests a day. I always do a fasting test and use the second strip for a 2 hour postprandial test or to test at other times.

I sometimes to do a “continuous” (@ 2 hour intervals) test all day. Recently, while on Doxycycline to treat a tick-borne infection, I had these results:  9AM (fasting): 128; 11AM (2hr pp): 120; 1PM (4hr pp, before lunch): 125; 3PM (2hr pp): 120; 5PM (before dinner): 118; and 7PM (2hr pp): 179. Here’s why. For dinner, I ordered 2 appetizers: 6 raw oysters, and then steak tartare with an egg yolk in the center. Then, I helped my wife eat the large order of sweet potato fries that came with her “steak frites.” Of course I should have ordered a side of roasted Brussels sprouts instead.

Result: my blood sugars were stable, if somewhat high, all day long due to my body helping the antibiotic fight the infection. Then, after dinner, I had a huge “spike,” or excursion (in the language of blood sugar monitoring), due to my indiscretion. I love sweet potato fries, but I paid a price for going “off the reservation.”