Advanced calibration considerations
-
Be ready to validate that your true BG is flat. When we are unsure of what the true BG is doing, we often fingerpoke twice within an interval of 5 minutes. If the second reading is near enough the first one, then we calibrate.
-
If you are desperate for a calibration but do not have the opportunity for one, look for a “saddle point,” a moment when you expect your BG curve to stall and invert, i.e. a maximum or a minimum. Because your CGM is 10-20-minutes behind, you may get a decent calibration when your CGM shows that you are almost at min/max. Hopefully, this means that your true BG is barely past the max, and, on the whole, neither your true BG nor the CGM measurement vary much at the time.
-
When your BG changes quickly and across a wide range, your CGM may be out of sync with your true BG for a long time. Don’t calibrate at those times! You can find an example of what happens here.
-
In rare situations, your CGM may still lag behind your true BG even when your BG has been stable for a while. I have seen multiple circumstances when a CGM needs 30 to 60 minutes to catch up to a large variation of BG, after the true BG has stabilized. When that happens, if you calibrate during the period of misalignment, your CGM will actually catch up to the true level after you have calibrated, and show a new discrepancy in the opposite direction:( Recently, for instance, we found our CGM to be 60 units off (!), triple-measured manually, calibrated, and found out one hour later that the CGM was off in the other direction by half as much as before.
-
Do not always assume that your glucose meter is right and your Dexcom is wrong. When in doubt, test two or three times. Your Dexcom has been averaging your glucose meter errors (only guaranteed to +/- 20%, remember?) for several days – it could well be closer to the true number than your glucose meter.
-
There are times when, for some reason, your Dexcom is doubting your calibration, or when you are so far off that several calibrations 15 mns apart (as per the previous post) still leaves you far from reality. In those rare cases, a last chance technique allows you to force the Dexcom to go exactly to one specific value. It should be used rarely if ever, since (a) each BG meter has significant measurement noise, and (b) the Dexcom, averaging these errors over several days, efficiently reaches, in general, a better value. If you need to use this technique, here is how:
- average at least two BG meter readings (we often do three to be sure) to get a precise number
- enter the value as a calibration
- immediately re-enter it as a calibration again
- this will force your Dexcom to the value entered. If it does not, your Dexcom system is defective, and you need to call it in to Dexcom customer support.
A Dexcom is an addicting tool. We would never be able to manage BG with the same accuracy without one. But it is also easy to forget how to do it without one! So consider withdrawing from your Dexcom 2-3 days every 6-8 weeks to remember what it takes 
I am in debt to @rgcainmd for many of these thoughts!
Resources
Calibrating the Dexcom G4: video tutorial
Dexcom G5 Getting Started [PDF]
End of wiki ---------- comments start here