Calculated A1C’s are based on a lot of averages and assumptions so a good ballpark figure but should not rely on it if total accuracy is required.
Actual Lab A1C - These figures can vary as much as +/- .5% on accurate equipment. There are 2 types of equipment, certified and non-certified. The non-certified is supposed to be within +/- .5% but since each machine is not calibrated and certified the results can be off by even more. These units are usually used in small doctor’s office and again are a good ballpark figure but not necessarily accurate.
Calibrated and certified equipment. This equipment is normally used in large hospitals or clinics that specialize in diabetes management and will always be between +/- .5%
The actual number is not nearly as important beyond bragging rights as the trend. If visit after visit your A1C rises, then your management is getting worse. If the A1C keeps dropping then you are improving your management as long as these comparisons are being done at the same facility each time.
Dexcom started forecasting an A1C and then stopped because of these differences and went to their GMI which is in the same range of A1C, higher or lower depending on where your A1C is being tested. Comparing GMI with peers is a lot more accurate than A1C, because GMI is based on the same algorithm for everyone using Clarity report at Dexcom. It is like all of us going to the same Dexcom lab to get our A1C (GMI)
I go to Joslin Clinic in Boston and my A1C and GMI is always within .2%, tomorrow I will go for another check up there and will let you know if any difference.
US still shows GMI as first index to the left in the Clarity report header
I went to Joslin today and they have the professional version of the Dexcom Clarity report vs the Home Version we get. On the Home version, we still get the GMI. On the professional version the GMI has just been dropped from the pro version a few days ago. This begs the question as to how long GMI will still be available on the Clarity Home version.
My memory may be fooling with me but I seem to recall that it was originally labeled A1C, back when I had a G5, and then for a while they stopped listing it at all. When it came back it was labeled as above. That led me to assume that they didn’t want to call it A1C because technically it isn’t. It’s an average of your CGM readings, not the result of an A1C test. So they don’t want to look like they’re telling you the result you should expect from your Dr, just where your CGM readings put you. My GMI has always been lower than my actual A1C, though they’re a lot closer now that I’m using the G6 than when I had the G5.
My last A1c was also quite a bit lower than my GMI.
I don’t really know what the GMI is using to estimate A1c, but it’s definitely not a simple average blood glucose value. Or if it is, then it’s not the generally accepted linear equation that is used to estimate A1c based on average blood glucose because when I input my average bg into the calculators online I get a lower estimated A1c than my GMI
Since the GMI is so mysterious, it seems like a worthless measurement. There’s no way to aim for a better target because I don’t know how it’s measuring/evaluating my blood glucose. I also don’t know if what it is measuring is worth addressing.
Both the A1c and GMI number, with the expression in tenths of a percent, seem to infer a precision that is just not there. Both of these systems draw from users who align with the magic number in a bell curve fashion. Some people match perfectly, some are off on the low side, and some diverge to the high side.
I noticed for years that my A1c did not match up with my CGM average. My CGM average predicted an A1c about 0.5% less (absolute) from the eventual number. I discovered recently that I had iron deficiency anemia and that causes A1c’s to measure about 0.5% high.
I don’t know why Dexcom abandoned its A1c number in favor of the GMI. The GMI also represents a range of experience, again in a bell curve fashion. Some people are near the bell curve central average and some are gathered under the skirts.
As humans we love to compare our situation with others but, both the A1c and GMI are not useful in this way. Both of these indexes can be useful in comparing only with ourselves and watching for a trend. Comparing my GMI to your GMI is just not valid in the way we would hope it to be. Same is true for A1c.
@Terry4 I disagree about A1C. I think it provides invaluable data of how someone is doing. If you are at 10 or at 6.5 etc.
For one, not everyone has a CGM, a lot don’t. But I think one of the biggest benefits is having a low A1C and then being able to refine how you are doing once you have brought your A1C down. But I think A1C is the first step.
I mean really, if you have a 10 A1C, you are not going to be able to even be able to deal with TIR at what numbers??? “I’m not even in those numbers in the first place to be able to stay there???” So I think it’s the best judge to say your numbers need to come down and then you start working on staying TIR and SD numbers.
So I think A1C is a valuable tool to judge at first of how someone is doing. And face it if we’re on this board we probably care more. But no CGM, what the heck are you talking about? CGM and a good percent of people out there just want an acceptable A1C. And maybe it’s all they can aim for if they have other issues going on.
For the life of me though, what the heck with the GMI number lol!
Hi @Terry4 Sorry if I misunderstood what you were saying but I got it from this comment.
“As humans we love to compare our situation with others but, both the A1c and GMI are not useful in this way. Both of these indexes can be useful in comparing only with ourselves and watching for a trend. Comparing my GMI to your GMI is just not valid in the way we would hope it to be. Same is true for A1c.”
The A1C measurement is a compared measurement with everyone else in the first place? Automatically when you have a A1C it is judged with everyone else’s numbers?
Ergo you are at a good level, you are at a bad level. But I’m now thinking you meant once you know it, it is only valuable to you to know if you are getting better or worse??? Or ???
Here is my conspiracy theory: Dexcom tried to predict A1C and failed. Instead of declaring defeat, Dexcom changed the name to GMI. I have no use whatsoever for GMI. Thankfully I don’t have to ask Dexcom to abandon it. I can just ignore it.
A1C is a good sanity check for me. I don’t try to lower my A1C, because the easiest way to do that is banking lows. This also sums up my criticism of A1C. I can improve my A1C by doing something bad.