More Diabetes Answers

Why does the average blood glucose from my meter give me a different number than the eAG?

In a recent post I explained how it is possible to tell you what your average blood glucose level has been for the past 2-3 months by knowing your HbA1c.  Although this is meant to be helpful it can sometimes be confusing.  Let's say you test your blood glucose 3 or 4 times a day.  If you have a memory meter it will probably tell you what your average has been in the last 14 or 30 days.  Let's say that average is 183 mg/dL (or 10.1 mmol/L).  If you look at the table comparing HbA1C to the estimated average glucose (eAG) you would expect that when your HbA1c is tested it will be 8.0%.  but what if your HbA1c comes back at 9.8%, almost two whole percentage points higher than you meter average would have predicted.  What is going on?  The problem is probably not with your meter or with the accuracy of the lab test.  If your HbA1c is 9.8% this means that for the past 2-3 months your average blood glucose level really has been somewhere between 226-240 mg/dL (12.6-13.4 mmol/L).  The average that your meter gives you is simply the average of all the blood glucose tests that you did.  If you only test before meals or when you are feeling low, for example, and are not testing after meals or when you feel that your blood glucose is high then your "meter average" will be an underestimate of what your actual average is; it is not adding in the higher numbers to the average.  On the other hand if your "meter average" is higher than the eAG from your HbA1c test this suggests that there must be times during the day when your blood glucose is low when you are not testing.

So you can really make good use of this eAG number.  If it suggests that your blood glucose levels are higher than you thought from the tests you are doing every day then you should consider testing one or two hours after your main meals to see if it goes high at that time.  If the eAG is lower than you expected then it is likely that your blood glucose level is going low at times when you are unaware of it.  This can often happen during the night when you are asleep so it might be a good idea to test then to see if that is happening.

It is certainly a good idea to discuss your eAG with you doctor and health care team if the results don't make sense to you.

What is the Estimated Average Glucose (eAG)?

One of the most important numbers that you should track if you have diabetes is your HbA1c number (sometimes called glycosylated hemoglobin or hemoglobin A1c or simply A1c).  the HbA1c number tells you what percentage of the hemoglobin protein in your red blood cells has got glucose attached to it.  If the number is high this tells you that your average blood glucose level for the past two or three months has been high.  I explain this in more detail in Chapter 7 of The Diabetes Answer Book.  For people who don't have diabetes the HbA1c will be between four and six percent.  If your diabetes is really out of control more than 10 percent of your hemoglobin will have glucose stuck onto it.  For most people with diabetes the HbA1c is between 6-10% (and if you can safely keep it closer to 6% than to 10% that is great).

Because most people are more familiar with thinking of their blood glucose levels the American Diabetes Association is pushing for laboratories to not only give people their HbA1c result but to tell them what their average blood glucose level must have been for the past 2-3 months to get that HbA1c number.  The reason that this is possible is because of a very well-designed study that was published in the Journal Diabetes Care in August 2008.  Dr. Nathan and his colleagues worked with 507 subjects (268 had type 1 diabetes, 159 had type 2 diabetes, and 80 did not have diabetes at all) at 10 international research centers.  Over a six month period each subject checked their own blood glucose 7 or 8 times a day at least three days a week (before and90 minutes after each meal, before bedtime and once in the middle of the night).  In addition they wore a continuous glucose monitoring system (CGMS) on eight days during the six months.  So each subject had about 2,700 blood glucose tests done during the six month period as well as having their HbA1c checked several times during the same six months.  By taking an average of all these blood glucose tests and comparing them to the HbA1c taken during the same period the researchers found a very strong and straight relationship between the two.  They were able to construct a table which shows the average blood glucose level for each HbA1c.  Ihave shown this for a variety of HbA1c levels in the table below.  So the next time you get told what your HbA1c test result is you can look at this table and see what your average blood glucose level has been for the past 2-3 months.  The middle column is in "American" units of mg/dL and the right hand column give the estimated average glucose in International units of mmol/L.

HbA1c	eAG (mg/dL)	eAG (mmol/L)
6.0	126	7.0
6.5	140	7.8
7.0	154	8.6
7.5	169	9.4
8.0	183	10.1
8.5	197	10.9
9.0	212	11.8
9.5	226	12.6
10.0	240	13.4
10.5	255	14.1
11.0	269	14.9