September 10, 2007

Why A1c "Average" Doesn't Match Meter Tests at Normal Blood Sugars

If you've been working hard to bring your blood sugars down to normal, your next A1c might be a disappointment. That's because many of us find that even if our meters show us having much better blood sugars--those truly in the normal range--we still may get A1cs of 5.5-5.7%. According to the formula most doctors use, these A1cs correspond to an average blood sugar of 118 mg/dl to 126 mg/dl (6.6 mmol/L - 7 mmol/L).

Why you'd get an average of 118 mg/dl when you are spending only 3 hours a month over 140 and have fasting blood sugars under 100 mg/dl seems hard to understand. It has happened to me, and to several other people I know.

Doctors will tell you that your testing must be missing significant highs. But a friend of mine who believed that was the case for her blood sugars which tested in the normal range but resulted in a mid-6% A1c used a CGMS for a month and did not find that her blood sugar was spiking at times she hadn't thought to test. It was pretty much what her meter had told her it was. Something else was causing the high A1c.

The real problem is, as I discussed in an earlier blog posting, that the averages associated with A1cs were derived from DCCT, a study where almost all the participants had A1cs ranging from 7% to as high as they go. So those A1c to mean glucose averages derived from DCCT appear to only hold true at the damagingly high blood sugars that were found in that study. (7% corresponds to an average blood sugar of 170-180 depending on the formula you use. We know from a ton of studies that blood sugars over 140 mg/dl damage all kinds of cells. )

Here's a study that shows why at normal blood sugars, red blood cells live a lot longer, so that those formulas no longer apply.

In this study the scientists measured the lifetimes of hemoglobin cells in normal people and diabetics and found that the cells of the diabetics turned over much faster--as little as 81 days, while normal people's could live up to 146

They suggest that getting better control will cause the cells to live
longer. But when they live for a couple extra months, they will also continue to glycate--i.e. collect the bits of sugar that are measured in the A1c test. Cells that are living longer may collect after 5 months of life the same amount of glucose a person with poor control might collect in 3 months. That doesn't mean they have the average blood sugars as the person who developed that degree of glycation in the much shorter period.

This data also suggests if you lower your blood sugars from good control to great control, control, you probably want to test only every 6 months, not every three because it will take that long for your good control to show up.

This may also explain why many truly normal people have A1cs in the lower 5% range. They may have much longer lived red blood cells than normal. I have one very physically fit friend whose fasting blood sugar is never over 95 mg/dl and who tests no higher than 115 mg/dl after eating a very high carb meal that would put me into a coma, yet his A1c is 5.4%.

Copyright Janet Ruhl 2007. If you are NOT reading this on the content has been STOLEN.


Pem said...

This is interesting. I was disturbed to do a home A1c test a few days ago and get a 6.2. I haven't been testing as much as I used to, but mostly because my test results have been so good. An hour after a normal dinner, 115. Two hours, 101. An hour after a breakfast of eggs, tomato, 1/2 piece toast, and a whole waffle with a tiny bit of syrup (waffle house): 95. I then went out on my bicycle for 60 miles (which is why I had eaten such a breakfast) so didn't test again.

My fasting varies, but this morning it was 90. So why has my A1c gone up from 5.7 to 6.2? I was actually thinking it was because my premenopausal very heavy periods are finally slowing down--I'm not having to make new red blood cells as fast.

I just posted a doctor story on my blog at:

Jenny said...

The home A1c tests are inaccurate. The times I tried the A1c Now test at home, it was always .5% off from the lab. I.e. The lab said 5.7% but the home test would say 6.2%. So you probably are about the same.

And with the kind of test results you are reporting, which are a lot like what I see, which give me 5.6-5.7% A1cs too.