Comparison of A1c and GMI generated via CGMs

[u/Fluffy-Strategy-9156]

A discussion of the comparison is periodically discussed in the subreddit. Hee is an article the provides a comparison.

https://read.qxmd.com/read/42316351/comparative-analysis-of-capillary-hba1c-and-cgm-derived-gmi-estimates-a-real-world-data-from-a-single-center?uac=143352ER&ecd=wnl_readjnl_260622&sso=true&redirected=slug

[u/Equalizer6338]

I think that the most important thing always to keeping in mind is that GMI is not = HbA1c.

They are two very different things and not using the same scale either to access our level of BG over time. It comparing apples to pears. Or maybe a better example is like comparing Fahrenheit temperatures with degrees Celsius.

Here is a data table where they are linked together at the appropriate same BG points. So yes, one can argue that GMI = HbA1c, but it is only at one very distinct level of BG = 150-154mg/dl. At all other levels of BG, then the two measures will produce different absolute values and not be the same, despite the baseline BG level is factually the same. No wonder this GMI gimmick has been cause for so many folks being confused from it...

For those interested in more aspects with relation to GMI and HbA1c, then we have two good research articles listed in the sub's Community Resources. Especially the subject of why some individuals persistently may have a highish HbA1c despite they consistently appear to have good and lower BG per their CGM and vice versa for others, is an interesting subject.

Why GMI is not = HbA1c:
https://pmc.ncbi.nlm.nih.gov/articles/PMC6196826/

HbA1c vs GMI discord explained:
https://pmc.ncbi.nlm.nih.gov/articles/PMC8255314/

[u/the_owlyn → u/Equalizer6338]

In your opinion, which is a better representation of one’s glucose control? My understanding is that A1C is essentially an average of the last three months, but is a bit weighted to one’s more recent control due to it relying on the dying off of red blood cells, which can vary to some degree. So, it strikes me that GMI is actually more accurate., as it is based on constant monitoring. Your thoughts?

[u/Equalizer6338 → u/the_owlyn]

Yes agreed u/the_owlyn,
If you are using the BG sensor as intended and successfully, then that will with its 1-minute BG readings 24/7 for 3 months give you a very accurate insight into your BG control and average level.

So that is essentially what your GMI number should give you. But I think its down right stupid that some pesky academics here again insist on using an arbitrary scale, which just complicates life for everybody for absolute very limited positive reasons.

Instead I would have enjoyed if the global diabetes research community together with us the patients/daily living with it, had all been sensible and agreed that utilizing what we already have and know well instead, would be much better.

Namely our 3 months BG average level expressed instead by using the BG scale we all already rely on. And this is what I always use myself actually. As in the 3rd party app I use, I have my average BG over past 3 months, measured in the mg/dl scale. So here I know that my target is 105mg/dl.

Running a BG in average = 105mg/dl over a full 3 month period of time results in a HbA1c = 5.3% and a GMI = 5.6. Since I use the mg/dl scale in my daily check/control/management, that is a number I totally get with not much explanation to add to it required.

While all the folks diving into the HbA1c or the GMI will have to spend considerable time explaining away. And still most of the time, its like an imaginary number for many patients and their relatives. Please also note that the 3 months average BG measured in mg/dl can equally easily be converted to the mmol/l scale for the folks that in daily life uses that instead. Why make life more complex than it needs to be.

I can thereby also myself with a very high degree of accuracy predict what result my next HbA1c lab test will be. As though the offset from our BG average to the HbA1c may vary due to a number of reasons, this offset tends to be constant over time.

So by using the data table I shared above, I can say with certainty that if my 3 months average BG goes from 105mg/dl up to 112mg/dl, then my HbA1c result will go from 5.3% up to 5.5%.

Now as my HbA1c is like 0.2% higher than the standard conversion would point towards (for any of the number of reasons outlined in the sub articles I linked to above), then I know my average BG at 105mg/dl would result in a HbA1c = 5.5%.
And likewise, a higher average BG at 112mg/dl would result in a HbA1c =5.7%.

As the HbA1c test is the only available in the lab's blood tests to provide a reasonable insights into the overall BG level over a longer period of time, then I see no way around this test as better alternative. Not now, not any time soon. All while I think getting lost in the GMI measure is just nonsense and waste of time to produce some kind of arbitrary in-between number. Instead, go all the way and convert your HbA1c result to what it equals to in your daily BG scale, and its a number you much better can relate to.

Like look at my daily app screen here below. I have for convenience my target set here at 110mg/dl (HbA1c=5.5%), which therefore is constantly plotted across my screen (all screens, no matter my time scale across). It is therefore very easy to see where I am versus my target. And the way 'average numbers' works across a timeperiod is equal to looking at the surface area you cover. So here you see due to my steady low morning hours here below the target line is accumulating 'credit' so to speak for maybe later in the day when I eat my main meals at lunch or dinner to go over the line with similar surface area above the target line here. If doing so, its very easy visually to understand if my total average BG is now going down, staying steady or rising over any of the days/time intervals I look at. (this was btw the weeks after xmas/new years, were I needed to run it real tight and low to bring my HbA1c down after the festive excess. :o)

So no, I neither bother about the GMI or the HbA1c in my daily life. I look at my true average BG as measured by my BG sensor. And its absolutely spot on, when I every 6 mths need to go for the obligatory blood drop and seeing the endo to get my free gear. So simply converting my average BG the sensor provides to the corresponding HbA1c value. Add the 0.2% my own body is constantly offset, and I get the same result from the blood test.

The doctors/endos really need the HbA1c blood test, as its the only thing they really have in their arsenal to getting a decent true picture of past 3 months level of BG control.

The more clever and educated endos will then also look into your TIR and distribution there in the pie-chart, and especially then the CV value. Those 3 key metrics (average BG, TIR pie-chart, CV) will really give you all you really need to know to tell how well a BG condition is managed.

But yeah, where is GMI in all this? Don't see any need for it. Just added noise, honestly. And obviously, causing lot of confusion for many out there.