Weird Nighttime Dexcom Readings?

So I'm doing a trial of Dexcom this week, and yesterday was my first day.  I was pretty impressed with the accuracy so far... until bedtime.  Then, it started looking less and less accurate... until it woke me about a BG of 350, which I tested in a hurry to see... 115?  Double test... 111?!  WTF?

It looks like, after a bunch of up and down readings, it lost reception somehow (no idea if it was showing the reception symbol or the ???), then popped in with only that 350, then it went to ??? for an hour or so (I was just dosing after that 350, so I periodically peeked to see if the ??? was still there), then came back online an hour before I got up and gave me more steady, probably accurate readings.  And it's been fine this morning.

So, Dexcom, I say to you:  ???

I've read somewhere that sleeping on the sensor can cause problems, but would it cause innaccurate readings?  I figured it would just cause poor reception.  I put this trial sensor on the side of my stomach where I almost always sleep, so there was a good chance I was lying on it at the time.  I may have to arrange a pillow so I don't roll over tonight, and see if it's more accurate when I'm not on top of it.

all CGM systems have issues of sensor drift over time.   I am biased, I don't like the accuracy of our finger tests, let alone the accuracy of any CGM currently available.

soooo, couple questions for you:

When was your last calibration with respect to going to bed?  about when did you eat/take fast acting insulin with respect to your last calibration?

also,  did you happen to take anything before gonig to bed?  CGM and even finger test strips are sensitive to interferences from acetaminophen (Tylenol), uric acid, ascorbic acid (Vitamin C), maltose, galactose, xylose, lactose, and (not under your immediate control) oxygen.

sorry for answering in the form of new questions...       =)

I'd calibrated only a couple hours before going to bed (second calibration of the day), and I hadn't eaten or taken anything since about 7:15 PM because I'm using this week with a CGM as an opportunity to more easily test my basal rates.  OK... I did sneak one string cheese (listing zero carbs), but that was a couple hours before bed, probably 5 or 6 hours before the false 350.  Hmmm.  And diet soda (sweetened with Splenda), but I'd had that throughout the day without any confusing results from the CGM (and I can't think why it would).

The more I think about it, the more I'm pretty sure I was sleeping with my weight on the sensor most of the night... except for that hour or so before I got up when the Dexcom numbers looked pretty normal.  I definitely did not have my weight on it then.  Do you think that could be what influenced the inaccuracies?  Seems odd.

well okay so no calibrations during a time when your bs could be changing, and no OTC drugs or vitamines. 

IMO the next thing is to see if it happens again.   There doesn't seem to be any proveable answer anyway.  as far as interstitial fluid monitoring goes, it simply doesn't work for some people.

good luck and I hope it was just a "fluke".


E - the first 24 hours of a new sensor are pretty full of shenanigans, in my experience.  It sort of has to swim in your juices for a while, to get used to you, before it becomes the extremely accurate CGM that we all love.  What you've described isn't unusual, but it doesn't happen every time with a new sensor, either.  It's imperfect technology, and I doubt it's anything that you did.  When I sleep on my sensor, I just get that 'out of range' signal and it doesn't take any readings until I roll over. 

Hope this helps - keep us posted on if the accuracy improves or continues to be lame.  :)


I second everything Kim said. The first 24 hours can be brutal. I've never had much problem with sleeping sensors, and i roll all over everywhere all the time. give it another night and see how it goes tonight.