It's estimated with cgms. Not a true reflection they can read both higher and indeed a lot lower. Cgms measure interestrial fluids rather than blood. Other factors such as compression lows where sensor can read lower come into play even when not laying directly on said sensor fluid restriction near the area will also have an effect. Cgms have best continuity when in range they are less accurate with low and indeed higher readings.
It is also why CGM manufacturers state clearly before rely on device for medical decisions to check against both lows and highs and indeed if you appear to feel different than how you would expect to with the reading it gives.
Cgms despite quirks are awesome for showing trends but a little accurate than finger prick.
There is error margin for all test results if take that into consideration alongside the error margin for cgm date you'll find it gives a closure figure than otherwise read at first glance
