CGM average blood glucose doesn't match A1C

Auto E

Well-Known Member
Messages
132
Type of diabetes
Prediabetes
Treatment type
Diet only
I do not have type I diabetes, but I am posting this question here because I suspect many of you are CGM experts at this point.

My A1C by blood draw was 6% (42), but my CGM is giving me an average blood glucose of 104mg/dL (5.8mmol/L)! That is WAY lower than 6% - I would think my A1C would be more like 5.2% (34mmol/L). I get that there is variability, I get it that RBCs have varying longevity from person to person, I get that the size of the RBCs can cause variability, but wouldn't you agree that this is very very different than expected? Also note - my diet has been the same for 18 months, so I'm stymied by this.

So, the above paragraph is my primary question. But I do have a second question: my CGM has a GMI (glucose management index) of 5.8%/41mmol/L. That ALSO confuses me, because when I look that value up, the estimated glucose is 119mg/dL (or 6.65mmol/L). Why would they give such a high GMI when they report my average blood glucose at 104mg/dL/5.8mmol/L?

I think I will call Dexcom up this week to ask for help, but insight from this forum would be appreciated as well.
 
Last edited:

Marie 2

Well-Known Member
Messages
2,395
Type of diabetes
LADA
Treatment type
Pump
Lol.......Cgm's are almost always off the A1C number. First forget that GMI number, we think Dexcom tried to do a A1C guess and it didn't work so they changed the name. My last A1C was .7% different or 7 mmol different where as last time it was only .1% or 1mmol difference. My A1C was lower others are higher. No one seems to have a clue how it is achieving it's number.

As for A1C, it will be a lot more accurate to compare your average Bg level against what your A1C level is, that seems to be pretty close. But notoriously CGM's numbers are lower than what your meter number is. I keep mine calibrated within 5 points or .3 mmol's. I also prefer mine to read higher than I am.