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Dexcom G6 underestimates HbA1c

briansolar1

Member
Messages
19
Location
Highlands, Scotland
Type of diabetes
Type 1
Treatment type
Insulin
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Brexit
Hi. I am T1 and take multiple daily injections and use a Dexcom G6 sensor.
On the G6, my 90 day HbA1c estimate is consistently around 6.4-6.5%.
However when I get a blood sample HbA1c done from the doctors it has been 7.1-7.2% the last year or so.

Has anyone else had that type of discrepancy or thoughts on the reason for the difference?

Thanks for your help.
Brian
 
I don't use Dexcom but these types of devices can only work out the arithmetic average based on your readings.

Plus, the device measures the glucose in interstitial fluids , not in actual blood. Then it does a fancy calculation to interperet the readings and approximate A1c results. If your body does not match their standard model (who's does?) then it will be a bit out.

It is just an "estimate".

Edited to add:- do you still do finger pricks? How close are the Dexcom readings for you?
 
Last edited:
Hello @briansolar1 As per what @urbanracer has said, it's not an accurate measurement.

I also use the G6, but never use their estimate as a guide - instead I focus on time in range which for me is a better indicator as to how I'm doing, I also use their Clarity app to drill in on time ranges and see where I may need to make adjustments too.
 
Another thing to add to @urbanracer 's comments is that the estimate from G6 and Libre will be taking an absolute average over the last 3 months. However, I understand that a true HbA1C is weighted towards more recent levels.
This was very clear for me last year when I was unwell, fighting high blood sugars just before last year's test and my HbA1C was significantly higher than usual and my Libre estimate.
 
I use nightscout and that has very good hba1c calculations my last one was almost the same

Mind you the Dexcom wasn’t far off
 
Hi,
Thanks for that.
Yes I still do finger pricks- useful for the times that the G6 fails, and I take a reading every morning when I wake up to calibrate or check the G6. At that time the G6 is pretty accurate when my blood sugars are often steady- maybe +/- 0.5 mmol/l between the test strips and the G6. But of course the test strips can be inaccurate as well...
 
The stats the Clarity app provide are very informative, particularly where blood sugar is worse at particular times of day. I am finding my 'time in range' is between 75% and 85%.
 

Thanks for that, I hadn't realised that was the case, so in that respect at least the G6 and Libre might be providing a better estimate.
 
The dexcom does allow you to calibrate it against a blood glucose monitor, but if it seems to match your meter then I'd be more inclined to trust it than the hba1c. There are a number of reasons why hba1cs can be inaccurate (ag anaemia) because they assume that you have a certain number of red blood cells that live roughly 3 months. If your body is slightly off the standard model, then the result can be inaccurate. You could try asking for a fructosamine test if you think that might be the case.

Personally I've found the dexcom hba1c estimates to be slightly less than my actual result, but only by a small amount (eg latest was 51 or 6.8 while dexcom was 6.7).
 
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