I did not mean to impugn your calibration skill. The Dexcom adjusts its level up or down by lining up with the calibrations. After a couple of days, in most cases, the static behavior of the Dexcom should line up with whatever meter you use, with some readjustments necessary every day. So, if the calibrations are done by the book (when your BGs are really flat, clean and warm hands, in-range etc.), then it should not be statistically off by much when you are in-range. It will typically be further off when you are far from the calibration values.
On the other hand, your calibrations will not change the dynamic behavior of the Dexcom:
- it will always be "behind" the real BGs by 15-20 minutes in general in reflecting the full change
- it will always average transient peaks, so finger pokes will always show more extreme numbers when you are moving up or down fast.
At least that's what I have found both in the research I have done and in the results we have had. Because of the math smoothing done by the Dexcom, I typically trust the Dexcom in most circumstances more than finger pokes after a couple of days, as do some other members of this forum. But, as always, YDMV:-) If my son's Dexcom was always off by 15 in one direction, the first possibility I would look into would be to figure out how my calibrations could result in a steady error. Of course, it is totally possible that, for some reason, you and your daughter are in this situation without calibration issues.