I have mentioned several times in passing that I think many Dexcom lows, when they come off a steep slope, aren’t really lows. Today I decided to prove it.
My son calibrated his CGM this morning at 8:37 AM and was off by 2: his Dexcom has been very accurate for the past two days (so we can assume that, when stable, his CGM accurately reflects his real BG). Later this morning, he had a major puberty peak that took him to about 250. He injected large quantities of insulin and came down quickly.
When he approached inrange level, he and I decided to start our experiment of the day: he took some carbs in several steps to land as smoothly as he could. At the same time, he fingerpoked every few minutes to be able to follow his real BG. The results are below, and they are shocking
The blue line is his CGM curve: it comes down steeply, crosses 70 swiftly all the way to a “fairly” deep low of 53, then slowly goes back up. His total time outofrange appears to be about 30 minutes.
The red line, on the other hand, is his real BG by fingerpoke. It never goes below 69, and hovers above 80 most of the time, while his CGM is showing mild to mediumlow BGs!
So the CGM lied. The circumstances when I think the CGM will show a false low are as follows:

BG is coming down quickly

You are taking in good numbers of carbs to land
In this situation, I think that (a) either the CGM will not see the ingested glucose coming into the system, or (b) the Dexcom algorithm always wants to make sure the CGM number is below the real number as opposed to above it, and may give a worst case approximation – possibly a combination of the two.
Either way, it is clear from this curve that a truly good landing, when using carbs for the landing, may show as a fairly deep low – so let your endocrinologist know when she is scolding you! Or, even better, show her this post and tell her to sign up for the forum:)