Most of our sensors follow one of two models at one time or another: they are right on and stay right on, or they are 15-25 off, and see to be off again exactly that much at every calibration.
When our calibrations are very far off (that sometimes happens after a pressure low, if the sensor has been displaced enough), we sometimes simply reset the calibration by doing a quick double entry. But we rarely do that when we are somewhat off (say 25 off). For that case, we did figure out a method that gives us very good results. I have been hesitant to discuss this technique, but our calibration results are much better with it, so I finally decided to post what we do.
We only apply this method when we are 100% sure of our BG fingerprick numbers, which means, for us, that we have taken multiple fingerprick measurements over several hours. If we are reliably certain that we are, let’s say, 25 off, then we expect the next calibration to result in a starting error of about 12 (since the Dexcom typically halves an error when the real numbers are higher). So we add the expected error amount to the calibration we enter in the Dexcom, resulting in an error that is half the originally expected one.
Here is an example that happened this morning. A pressure low around 3:00am resulted in a signal that was 40 off: we fingerpricked twice to validate. We were tempted to reset calibration, but, in the end, decided to enter the real measurement. It resulted in the CGM being 20 lower than reality. A fingerprick a couple of hours later confirmed that the CGM was 20 low.
At wake-up time, we hand-tested again. The CGM gave us a read of 101, but the manual BG read was 127. We expected a post-calibration error of about 13 ([127-101]/2). So, instead of entering 127, we entered 140 (127 + 13 = 140) as a calibration. The CGM eventually calibrated to 122, which is 5 off.
Note 1: when I write that the CGM calibrated to 122, I don’t mean the number showing right after the calibration, but the “real” calibration which shows up a few minutes later.
Note 2: this technique could easily be mis-used. We ONLY use it when the error has been confirmed (a) by several manual BG measurements and (b) over several hours. It frequently happens, for us, that a CGM can be off for an hour, but back on later on.
[EDIT] Note 3: If we fully wanted to eliminate the mathematical error, we would add twice the expected error. But there is significant noise on BG measurements always, and I figure I do want to take advantage of Dexcom averaging measurement errors over a long period of time, so we only “cheat” by adding the error once.