A1C not a reflection of glucose control

Looking over the posts here, it seems that many people have higher HBA1C results than they are expecting from their Dexcom Clarity numbers.

I’m actually in the opposite boat. My Dexcom Clarity numbers have historically been pretty high (average of 160 to 200) but my HBA1C tends to always run at 7.0 or lower. My most recent A1C was 6.1 even though my Dexcom Clarity average for the prior 3 months was 171.

The variable of “how long do red blood cells last in blood or glucose bond to hemoglobin” (or something like that) perhaps has an impact, entirely separate from glucose control, which could rise with older patients. Could age be a factor here? I’ve read that A1C tends to rise in older patients (links below). I’m 37 years old. My doctor is 35 and she says the same thing, her average blood sugars are 138 and her A1C is 5.4 → much lower than the 138 average would suggest.

Just thinking out loud. Maybe my control hasn’t been that great over the years and I’ve benefitted from some other factor that keeps my A1C low. Clearly the amazing Dexcom technology with the accuracy getting so spectacular is making these things a lot easier to really dig in on.



Interesting article, though the question you’re raising about age and A1C doesn’t seem to bear out in my case. I’m 64, dx’d in 1983, and my A1C is actually the best it has ever been. I cracked into the upper 5’s last summer. I think that has more to do with starting Jardiance, which is off-label for T1 but has hugely helped with attenuating my post-prandial spikes, which makes everything easier.

You’ll note of course that the Clarity report avoids using the term “A1C” and calls it something like “Glucose management indicator.” I think even with A1C it’s best to think of these things as trend indicators rather than absolute values, i.e., compare Dexcom to Dexcom and A1C to A1C.

I think my experience is closer to yours. In that my “Glucose Management Indicator” number on clarity is always marginally (0.3-0.5) higher than my actual A1C from the lab.

I recall a couple of years ago Dexcom called their number an “Estimated A1C” or something like that. But since then they’ve realized that while it’s similar in scale to A1C it’s not actually the same. So now they call it a “Glucose Management Indicator”

Whereas for me the Dex number is consistently more optimistic than my tested A1C, which is always a disappointment. Maybe that’s why I decided I wouldn’t compare them against each other.

There’s actually a considerable scatter in average BG vs A1C. Folks use those conversion calculators like they’re accurate to multiple decimal places but really the relationship is much looser. And it’s possible, even likely, that the scatter isn’t in the measurement of the A1C, but in the presumption that everyone glycates and destroys their blood cells at the same rate.

Look at the scatter graph below. A BG average of 130 could be an A1C anywhere from very low 5’s to very high 6’s.


Interesting that no one mentions that there is a ±.5% variation between different labs using different equipment. The smaller labs also have cheaper equipment that is not calibrated and could be off by even more. At least with Dexcom, we are all basically all with the “same lab” as they use the same standards and algorithms for everyone. So it is certainly fair for all of us on Dexcom to compare to each other’s Dexcom results but then trying to match to lab results is a futile exercise.


I’ve gotten my A1C tested in different countries and different labs… always trends very low vs. Dexcom for me. I think it’s something else in my case.

do you mind sharing your age? No worries either way. But I am wondering if younger patients have A1C which trend lower vs. what you expect from clarity #s while the opposite is true for older patients.

It’s the problem with giving a precise value for an imprecise measurement. Fingersticks are kinda the same: “Oh, my BG is 123!” we think. When actually it’s 123 +/- 20 points or whatever, depending on your meter and other factors. You can’t always state the fudge factor, and leaving it out is just a kind of shorthand, but after a while you take the shorthand for the thing itself. Kinda makes me almost think the old Chemstrips idea was more “accurate” in that it didn’t pretend to give a precise value down to the decimal range.

I have been working on fixing some cholesterol subfractionation issues and in doing that my A1C has dropped to its lowest ever since being diagnosed 3+ decades ago. I will be 72 this year and my May 19, 2020 A1C was 5.3 and it is always dead on with Clarity Glucose Management Indicator

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Yes, I remember changing labs (insurance company change) and my A1C dropped by 0.5 right there. It was consistent within either lab but there was a step discontinuity at the lab change.

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I am 52, T1 for almost 40 years. Not sure whether that is “older” vs “younger” :-).

I googled a bit to see if A1C to average bg relation might have been studied as a function of age. I found a lot of studies that just looked at A1C vs age for all kinds of populations, but they did not include any measurement of average bg.

A man with one clock knows what time it is. A man with two clocks is never sure.


I guess my age question was more about whether younger people have an A1C which is lower than their blood glucose averages whereas older people have an A1C higher than their blood glucose averages.

Definitely not making a statement about younger people having better control… just that perhaps for younger people the A1C results makes it seem sometimes like the control is better than it truly is.

It doesn’t mean that older people will have worse A1C outright… especially if the blood glucose control improves dramatically. Does that make sense?

To be clear, this is just 100% speculation.

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Ah, I see. Missed that distinction–thanks for clarifying.

A1c increases with age for non-diabetic populations. They aren’t very specific about what types of diabetics are included in the study. So, without more information, I would say that, in general, yes, this is obviously true.

If you have concerns about your a1c accuracy, you can track down the lab that did the test and ask what equipment they use. Then, you can google for the accuracy of that machine. I have done that.

Ever seen this documentary? This was the woman behind doing a1c testing at Walgreens. The Inventor: Out for Blood in Silicon Valley (2019) | Official Trailer | HBO - YouTube

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I’m 33, and my A1c has consistently been about .5% lower than my average bg would predict it to be.

This used to bother me because I worried there might be something wrong that is making my red blood cells die off. My doctor ran a CBC and a few other tests, and everything came back fine. Since my average bg and standard deviation are in good shape, my doctor said I should stop worrying about the discrepancy.

It would still be nice to have an explanation because studies have shown that glycation is the primary cause of complications. Since the A1c is a measurement of glycation, I do think it is important tool.

We discussed this topic quite a bit in this thread:

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I’m 63 and my A1C is 5.1%. Clarity has always said I was higher than I test.

But one thing I do when I calibrate my Dexcom is I calibrate it to run about 5 points over what my number really is when I am at 90-105. So my tests always come in lower than what my Dexcom says. If you let your Dexcom run lower than your actual BG level then your tests will come in higher than what Clarity says.

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I think it does reflect glycemic control, but not quite as specifically as you’d expect. As an aside, an endo of mine mentioned to me that it is a lot less science than I think. We try, but there is always variations.

  • There are Type 2 diabetics in that study, outnumbering Type 1’s
  • In studies of Type 1 I’ve seen decreasing average A1C with age
  • The synopsis mentions that A1C increases with age, even in the non-diabetic population, possibly because of reduced red blood cell count, among other possibilities
  • Other factors affecting A1C: Some of the factors that influence HbA1c and its measurement

Well, I think the age thing is just a natural thing as we age. And older is when most people with diabetes are diagnosed. There are so many things going on in the body as we age.
And with all things dealing with diabetes, each is different.
My CGM reports are pretty much right on target with the lab results. Both have been in the same range, never more than .1 or .2 difference, most times it’s the same. My labs have always shown my CBC numbers to be normal, which is probably why I never seem to have a problem. While I have been doing the diabetes thing since 1970 and I am in my late 50’s, I am doing better than I ever have mainly due to all this new technology. And do I worry about the “getting older” line? Sure! But I can’t change anything about aging but I am going to just keep do what needs to be done to try for a healthy retirement when I get there. And I no longer sweat the small stuff and for me numbers are a small thing. Do I feel good? Can I do all the things I want to do? Can I enjoy my hobbies? Do I enjoy my family and friends? Those are the things that matter, not the numbers.