What Is A1C Measuring?

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Managing diabetes involves a lot of measuring — measuring our insulin doses, measuring our carbohydrate intake, and of course, measuring our blood glucose levels. There are quite a few tools that we use to get a picture of our blood glucose, from continuous glucose monitors (CGMs) to fingerstick tests to that holy grail of diabetes management, the hemoglobin A1C (HbA1c or A1C) test. We’ve all grown accustomed to these numbers and hopefully we all know what kind of target numbers we’re shooting for. According to the American Diabetes Association, A1C should ideally be 7.0% or less. Fasting blood glucose readings should be 80–130 mg/dl. Post-meal glucose readings vary by the person and by our individual goals, but a universal rule is that they should be less than 180 mg/dl (many shoot for a lower ceiling than 180, but only very rarely would the post-meal goal be ABOVE 180).

But what exactly ARE we measuring? Why are we measuring it? And when we use these tools, are we ACTUALLY measuring blood glucose concentrations? The fact is, most of these tests aren’t directly measuring blood glucose levels. They’re measuring various things in the blood that should give an indication of glucose level, but the fact is every single one of these tests is an approximation. That doesn’t mean they’re not useful — they’re very useful, and taken together can help us get a good idea of how we’re doing. And as technology improves, they are increasingly ACCURATE approximations of blood glucose, but they are approximations nonetheless.

Let’s start with A1C. The A1C test is measuring the amount of glucose that has “stuck” to the hemoglobin in our blood. Hemoglobin is the protein in red blood cells that carries oxygen. Those red blood cells generally have an average lifespan of about two to three months, and so the idea here is that measuring the amount of glucose that has stuck to the hemoglobin in those cells can give us a picture of our average blood glucose levels during this time frame. The number we get is actually the percentage of glucose in relation to the hemoglobin. Using a formula, a clinician can get an equivalent estimated “average” blood glucose level, or eAG. A person without diabetes should have an A1C below 5.7% (5.7% corresponds with an “average” blood glucose of about 117). For those of us with diabetes, we are aiming for an A1C below about 7.0% (which corresponds with an average blood glucose of about 154).

While the A1C test is very valuable and has improved in accuracy over the years thanks to standardization of testing procedures, it’s not perfect. As noted, the red blood cells tend to have an average lifespan of three months, but that’s not always true. Their lifespan can be increased or decreased by a number of factors, including iron deficiency anemia, vitamin B-12 deficiency anemia, and kidney disease. And a low A1C doesn’t always mean great control. Remember, the A1C is merely testing for an average. If you’re running a whole lot of severe lows with intermittent severe highs, you might end up with a good-looking A1C, but the reality is that those severe highs are still doing damage, and that A1C is merely being lowered by too many severe LOW blood sugars. As with all the numbers we live by with diabetes, it is the context that really matters here. An A1C by itself can only tell us so much.

Well, that’s all the space I’ve got for this week, but I will continue this entry next week when we dissect the processes behind the numbers on our meters and our CGMs. And then we’ll take a look at how to best combine all this approximated information to get a clear picture of how well we are managing our diabetes.

Want to learn more about the HbA1c test? Read “H-B-A-1-C: What It Is and Why It Matters” and “What Does A1C Stand For?” and see our blood sugar chart.

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