by Katie Bunker
From the Editor: The following article first appeared in the November 2008 issue of Diabetes Forecast. It will be of interest to anyone who keeps track of blood sugar. Here it is:
So how’s your diabetes? One way to tell is by monitoring your blood glucose with a meter. By testing yourself when you wake up in the morning and before and after meals, you can get a quick look at whether your current regimen is keeping your glucose levels in the range you and your doctor have agreed is best for you. Depending on your particular situation, you may be doing this several times a day, a couple of times a week, or less frequently.
The other main way your health care provider can see how you’re doing is by ordering a lab test called an A1C. Most people with diabetes get this test—which gives a rough average of your blood glucose levels for the past 2 to 3 months—two to four times a year. It’s reported as a percentage, with normal levels between 4 and 6 for people who don’t have diabetes.
Pretty soon, however, another number may show up on your lab chart: the estimated average glucose, or eAG. Sound like the same thing as the A1C? It is, in a way, but the eAG will look a lot more familiar.
Why another number? The new number comes in part from new research. A large international study, called the A1C-Derived Average Glucose (ADAG) study, published in Diabetes Care in August, established that the A1C does, in fact, do what scientists have long believed it does: provide an accurate test of average blood glucose. “The results of the ADAG study should give people with diabetes increased confidence that the A1C…is, indeed, a valid measurement of their average blood glucose,” says Susan McLaughlin, BS, RD, CDE, CPT. McLaughlin, who is president-elect of health care and education at the ADA American Diabetes Association, cochaired the steering committee that is helping spread the word about eAG.
The study used many more glucose readings than prior studies, resulting in more blood glucose values to compare with each person’s A1C. This has allowed the A1C to be easily translated into a number that looks a lot more like what you see on your blood glucose meter: the eAG. So, in addition to being given a mysterious “6.7 percent” as your A1C, you’ll get an eAG of 146 mg/dl, which should be familiar based on the numbers you’ve been seeing on your meter.
In the past your provider may have shown you a chart that displayed what your average glucose was for the past two to three months based on your A1C. The new chart, shown below, is a more accurate rendering of how your A1C and average glucose correlate and will be the one used by the ADA and likely other medical organizations in the future. And making A1C easier to equate with day-to-day readings might just promote more blood glucose testing and improve glucose control, many experts believe.
Here's how the A1C relates to the new eAG:
A1C (%) eAG (mg/dl)
“The patients I’ve started talking to really like [eAG],” says Richard Bergenstal, MD, vice president of medicine and science at ADA, executive director of the International Diabetes Center at Park Nicollet in Minneapolis, and cochair of the eAG committee. “They actually say, `What should I do to get closer to the average glucose that would put me at the lowest risk? When should I test?’ It’s gotten us back into the patient’s logbook and back into the glucose meters, which sometimes never happens when you’re just talking about A1C.”
You and the lab, partners in care: the eAG can also help you easily determine whether or not the numbers you see on your meter are telling you the whole story. For example, your meter numbers might all be in the 90 to 130 range, but the eAG says 164. So what’s going on? It may be that you’ve only been testing at certain times, failing to capture the highs that occur undetected during the day or at night. “What I say to patients is, `Let’s change the testing schedule so we can pick up where blood sugars are going too high.’ Then we can adjust the treatment plan appropriately to lower the high values and improve the overall glucose control,” explains David M. Nathan, MD, of the Diabetes Center at Massachusetts General Hospital and Harvard Medical School, and lead author of the ADAG study.
Still, many diabetes experts who support the use of eAG stress the significance of using A1C too. “The major clinical studies were based on A1C,” explains David B. Sacks, MB, ChB, FRCPath, associate professor of pathology at Harvard Medical School and medical director of clinical chemistry at Brigham and Women’s Hospital. “Many clinical studies use A1C as an indicator of effectiveness of therapy. It’s also used for assessment of quality of management of people with diabetes.” Some providers feel that A1C has always worked for them and their patients—why make a change? They may choose not to discuss eAG with patients. There’s also a concern that the term “average glucose” is misleading, since it’s not a precise measurement of an average. That’s why the number is technically called the estimated average glucose.
“We understand that average isn’t everything, that variability in blood glucose and low blood sugars are very important,” Bergenstal explains. “But by getting people back into their logbooks more often and looking at their glucoses, eAG may help us address those issues more effectively than A1C has in the past.”
Teaching the Test
What the heck is hemoglobin A1C? Ever wondered what the A1C is really all about? First, the general idea: an A1C test measures the percentage of glucose-containing red blood cell molecules present in the bloodstream.
Now, to break it down. Hemoglobin is an important protein in red blood cells that’s used to transport oxygen from the lungs to the rest of the body. There are several types of hemoglobin, the most common of which is hemoglobin A. When glucose molecules combine with hemoglobin A in the bloodstream, the resulting molecule is called hemoglobin A1C. The more glucose in the bloodstream over a given amount of time, the more hemoglobin A molecules become hemoglobin A1C molecules. And of course, with diabetes you want to keep that level of glucose as normal as possible. The proportion of A1C molecules in the blood is measured by an A1C test—that’s why it’s reported as a percentage.
Here’s how David B. Sacks of Harvard Medical School and Brigham and Women’s Hospital describes it: “The way hemoglobin A1C forms is that glucose in the blood goes into the red blood cell and attaches to the hemoglobin. The more glucose in the blood, the more will stick onto the hemoglobin. The red blood cell lives in the blood for approximately 120 days because of the long lifespan of the red blood cell. So hemoglobin is there longer, and more glucose is stuck to it—so A1C is reported as a percentage of total hemoglobin in the blood. An A1C of 7 means that 7 percent of the hemoglobin has glucose stuck to it.” Pretty clever, huh?