Blood Glucose Averages


By Timothy Bailey, MD, FACE, FACP


This article will show you how to use blood glucose averages to better understand your diabetes.

How am I doing... on average?

An important reason to know your average blood glucose is that it is closely related to your risk of developing long-term complications of diabetes. As a rule, the closer your average is to "normal" (around 90-130 mg/dL), the better off you are. Most blood glucose meters tell you your average blood glucose.

Your meter probably has more "smarts" than you give it credit for. It stores recent blood glucose values along with the testing date and time. It also may have the ability to tell you 7-day or 14-day averages. This allows you to see your blood glucose average over the last 1 or 2 weeks just by pushing a button on your meter. If you do this every week or two you will get a good idea how you are doing overall.

Figure 1 below shows a meter with a 14-day average of 106 mg/dL. Let's say that the person in this example also has an A1c level that is 7.9%.

That usually reflects a 2-3 month blood glucose average of 180 mg/dL. Why are these two averages different?

 

Figure 1
Example: 14-day average of 106 mg/dL is displayed.



Comparing A1c to 14-day Averages

To help you better understand your A1c result, many labs tell you the 'equivalent' estimated average blood glucose value (see Figure 2).

Figure 2:  A1c Levels and Estimated Average Glucose (eAG)

A1c Level (%) Average Blood Glucose*
(mg/dL)
14 335
13 326
12 298
11 269
10 240
9 212
8 183
7 154
6 126
5 97

Nathan DM et al. Translating the A1c Assay into Estimated Average Glucose Values. Diabetes Care, Vol. 3, No. 8, Aug. 2008


How does this chart apply to you?

The A1c measures the average blood glucose over the past 2-3 months. If you measured your blood glucose at random, many times each day for 2-3 months, the average blood glucose level in your meter and your A1c test would show virtually the same level. In most people, the 14-day average blood glucose reflects the A1c, but is not an exact match. The average is still a good way to see how you have been doing in the past 1-2 weeks. It can show you how changes in your diabetes care, especially your meals, have affected your blood glucose.

The more often you test your blood glucose, the closer the A1c level and 14-day average will be. This is especially true if you test before and after meals, and occasionally at night.

Your 14-day average blood glucose and A1c levels can be quite far apart if:

  • There have been major changes in your blood glucose control, recently. Since the A1c measures 2-3 months, while the 14-day average measures only 2 weeks, if your blood glucose has been much better in the past 2 weeks, the A1c level will not have caught up yet.

  • You test only before meals, and your blood glucose after eating is very high. These high post-meal values are reflected in the A1c number but are not reflected in your meter's 14-day average.

  • You have very low or very high blood glucose levels during the night, but you do not wake up to test. Again, these abnormal values will not be included in the meter's 14-day average, but they are reflected in the A1c.

Think about it this way. If your A1c is high, you know that your overall average blood glucose is high. However, this does not mean that there are problems all day long.

In fact, things like missing a mealtime injection or eating more carbs than you planned can cause spikes, which may only occur at certain times of day. Knowing this allows you to concentrate on solving specific problem situations in your diabetes care routine.

Want to get more specific?

In addition to looking at your overall 7-day or 14-day averages, you can focus on the average blood glucose at a particular time of the day. This is called time-specific averaging.

Looking at time-specific averages is another way to discover trouble spots in your diabetes care. With this feature, you see the average of the last three glucose readings in a 2-hour period that you specify. These periods usually revolve around a meal, such as "dinner". They may be pre-meal or post-meal averages.

Figure 3: Example of time-specific averages for each mealtime

 


Figure 3 shows how this works. In this case, the average glucose level is highest at dinner (189 mg/dL). This means that fewer carbs or more insulin at lunchtime may be needed. This person could work on reducing the dinner average over the course of a few weeks. Later, when the dinner average is in target, she can work on lowering the high bedtime average.

This approach is called "pattern management". With pattern management, you look at how your meals and insulin doses usually affect your blood glucose. Then you make adjustments, either to your food intake, your insulin dose, or both. You work on your specific trouble spots to improve them, one at a time.

In summary, knowing how to get the most out of your blood glucose meter is essential because today's meters have newer, more helpful features "on board". You can enjoy better than average diabetes control by using those averaging functions!


Dr. Bailey is Board-Certified in Endocrinology, Metabolism, and Internal Medicine. He specializes in treating diabetes, osteoporosis, thyroid diseases, and lipid disorders at North County Endocrine. He is in full-time clinical practice and a clinical investigator active in conducting phase II to IV clinical trials. Optimally managing data from medical devices is a long-term interest of his. As a clinical Assistant Professor at the University of California at San Diego, he is active in the education of endocrinology fellows. He is a frequent speaker to both physician and patient groups.

Important note: The content of this article is not intended to be a substitute for professional medical advice, diagnosis or treatment. Do not disregard your doctor's advice or delay in seeking it because of something you have read on this article.

 


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