Joe, unlike the medtronic CGM, you can calibrate the dexcom anywhere between 40 and 400, and your BG doesn't have to be flat when you do it. I've calibrated going up or down with no problem. But, the one thing you definitely don't want to do is lie or make up a number b/c then it would miscalibrate and misread all your future readings.
but, if your meter is +/- 20% (TWENTY PERCENT ) wouldn't you calibrate under 100, simply because of your meter accuracy???
example: meter reading of 400, actual blood sugar 480 to 320... what exactly would you calibrate your cgm to? 400? 375? is it even close? under 100 your dealing with +/- 20 points... maybe I am too picky, but when I wear the cgm I expect a "lil bit" of trust and accuracy.
Joe, I totally see what you mean, but that amount of inaccuracy isn't typical for my one touch meter. Usually, if I test twice in a row to calibrate after changing a sensor, I'll get readings like 187 and 182. While +/- 20 is allowed by the FDA, it's not typical. Granted, I don't have a lot of experience with high-high's because I'm rarely over 240, so maybe someone with a higher a1c has had a different experience.
The dexcom people, if you call them, don't recommend waiting until you come down. They do recommend using the same meter for consistency. But, the dexcom is more accurate during lows than highs, so they'll actually suggest an extra calibration when you're high if the readings are way off. I'm not sure which cgm you're on, but dexcom just wants you to be under 400 to calibrate. Even if I calibrate when I'm over 120, it's never affected the accuracy later...
I guess I'm just saying, why add a layer of complexity to the CGM when it's not necessary due to some vaguely potential problem. Worst comes to worst, I could just recalibrate later. In your example case, you just enter what your meter says, not make up a number in the middle of a supposed range. It actually works more smoothly than you'd think!