What Is an eAG Test?

If you are pre-diabetic or diabetic, you undoubtedly know your A1c number.

You go in and have a blood test two to four times a year so you and your doctor can keep track of it. The number is expressed as a whole number (e.g., 7.0 or 6.5). This number gives you an average of your blood glucose levels over three months. We can call this "the big picture."

If you are doing daily finger sticks at home, you know what your normal numbers are there as well. Perhaps your waking blood sugar reading is 110 mg/dl, but after a meal it is 175 mg/dl. This number gives you a reading of what your blood glucose is at a specific moment in time so that you know if you have to act: inject insulin if sugar is too high, take in some carbohydrates if sugar is too low. This is "the small picture."

Because these readings look at very different time periods, how do you reconcile the big picture with the small picture?

The Conversion

The American Diabetes Association has come up with formulas to use to correlate the different test results, and the calculated number is called the estimated Average Glucose (eAG). The formulas to calculate the eAG are:

• 28.7 * HbA1c - 46.7 = eAG (mg/dl)
• 1.98 * HbA1c – 4.29 = eAG (mmol/l)

Some examples:

• A1c 7.0 = 28.7 * 7.0 – 46.7 = 154 mg/dl (eAG)
• A1c 8.0 = 28.7 * 8.0 – 46.7 = 183 mg/dl (eAG)
• A1c 7.0 = 1.98 * 7.0 – 4.29 = 9.57 mmol/l (eAG)
• A1c 8.0 = 1.98 * 8.0 – 4.29 = 11.55 mmol/l (eAG)

Why This Matters

How is this useful to us? Suppose that you take three glucose readings each day and the average of these readings is 156 mg/dl. You get your regular quarterly blood test, and the doctor tells you that your A1c reading is 8.0 percent. That is a full point higher than your average readings would lead you to believe it should be. Why? There are a couple of possible reasons:

1. The A1c takes into consideration the totality of your sugar load over the three-month period it is testing. This includes many times of the day that you are not testing, like the middle of the night.
2. It is possible that daily finger sticks are being taken only at times when your sugar is low. By not testing during those times of higher sugar levels, you might be missing opportunities to act to bring your sugar levels lower.