Estimated A1c by Libre 2 app

I wear a Libre and find that the estimated A1c is always lower then a lab blood test, anywhere between 2 and 5mmols/mol difference. Others have reported it the other way round.

I did wear Libre2 for well over 3 months, it said mine would be 40mmol/mol and it ended up being 42. Another 3 months has passed and I'm due another blood test on Friday, I've moved to Dexcom One+ which has 81 days of data, GMI is saying 5.6% which is 38mmol/mol so we'll see how accurate this is.

> Just how does this device come to this ? Obviously some sort of algorithm based on average glucose levels I would assume.

Correct. If you are interested in the algorithms, you can see and run my test code at https://dotnetfiddle.net/wlamqb (Hopefully I haven't made any mistakes)

> Just how accurate is this??

Everybody is different. I've seen some people who said their results have usually been spot on. or at most different by 1mmol/mol but others not so much. The difference can be caused by inaccuracies in the Libre's measurements, but also by differences between how long your blood cells live compared to the 'average person'.

> as anyone been able to check this app by also having a Blood test?

I've checked my last four blood tests and the Libre estimates are always lower for me. The first three differences were either 6 or 7 (which is quite a big difference, but since it was consistent was still helpful and and I had just got to the point where trying to keep the estimate on the libre below 41 so that my true HbA1c would be under the 48 target - but then my last blood tests difference was only 3.

So now my current estimate is 42 mmol/l , so I am not sure if my real HBA1c will be 49 mmol/l or 45 mmol/l !

Looks like @sgm14 's code is accurate, however GMI and HBA1c may rarely line up. You have to take into account variability in sensor readings, compression lows, the inaccuracy in the first 24hrs of new sensors warming up. The only data it can base it on is your average glucose over a time period, where as HBA1c is more glycated hemoglobin average of 2-3 months and factors with genetics and lifespan of red blood cells can affect it.

It gives a good indicator but I wouldn't expect GMI to match HBA1c

I found it to be so far out (more than 20% lower) that it was just another useful metric to track but not an indicator of hba1c.
That said, time in range is far more important than hba1c but a single blood test once every 6 months s a lot cheaper for the NHS.

There are additional reasons the result cannot be that accurate (measuring different things in different fluids for a start and subject to different sources of error) but my experience is that its pretty close, enough as you say to be reassuring and motivating..

My recent hba1c came in at 39 which is 5.7 and my libre 2 estimated 37 which is 5.5 so not too far out.

My own experiences of the Libre is a story of joy and happiness and i've had it over 5 years.
My latest result of 39 is the best i have had in 50 years.

Good luck

Tony