yesterday i put on a new sensor. as i really needed to rotate, i opted for the right side of my lower back. it went in comfortably, no pain at all. watched some TV in upright position which put a minor amount of pressure against the sensor, but nothing much. all receiver readings were accurate. no problems. However, throughout the early morning, i was getting the LOW alarm over and over. every time i checked my finger stick, i was "w/in range on my sensor"
but after breakfast i started to get wild readings. off by more than 40 points. i recalibrated.
i waited the 15minutes then calibrated again. i did this one more time. i called DEX tech support and spoke to a rep; she said that this was perfectly normal and leave my sensor on unless there were more complications. she also told me NOT to sleep on my sensor as that would cause inaccurate readings and i might get the “hour glass” icon or the ??? during the night. has this happened to you? and, what have you done about it? i think that tonight i should sleep on my other side. all suggestions appreciated. thanks a ton,
Daisy Mae
Yes, this has happened to me. I get tissue compression false lows when I sleep on my sensor. They are characterized by the sharp downward “V” signature. Here are several examples:
Just 1 “V” false low.
Another single “V” but not as deep.
This graph has 4 “V’s”.
Everything contained in each “V” is bad data. You can connect an imaginary line over the top of the “V” that would accurately represent where the real BG line is.
As far as the readings on day 1 of any sensor session, for me it ranges from uneventful good data to wild scribes unrelated to reality. Here’s yesterday’s session start. It was a doozy:
I was fingersticking during this whole period.
3:52 p.m. 121
4:12 p.m. 107
4:30 p.m. 112
4:50 p.m. 103
5:17 p.m. 104
5:43 p.m. 85
6:02 p.m. 123
6:29 p.m. 144
7:36 p.m. 71
7:54 p.m. 77
8:06 p.m. 98
8:22 p.m. 113
8:45 p.m. 109
9:07 p.m. 114
9:47 p.m. 95
10:57 p.m. 69 (I ate a glucose tab because I wanted to go to sleep.)
First days on a sensor need to be treated carefully. You can’t always depend on the Dex but sometimes it settles down and becomes quickly accurate. Yesterday, however, you can see that I was flying blind and resorted to old-fashioned fingersticks.
Yes, I get false lows when I sleep on my sensor. My understanding is that the area gets compressed and the amount of interstistial fluid decreases (as it’s pushed out under the pressure), and so the sensor readings drop. Move off the sensor, the fluid comes back, and readings return to normal.
And, also yes, the first day or two of a new sensor can be very inaccurate. I once had my Dexcom reading HIGH (> 400 mg/dl) and my fingerstick was 11.3 mmol/L (203 mg/dl). I called Dexcom and the person on the phone suggested I calibrate, but also noted that this was “normal” for the first day. As I noted in this post, Dexcom is spending the first 24-48 hours of a new sensor trying to match its data to the data you’re providing it, which is why it’s not considered accurate for the first day or so.
I get them off and on. More with certain sensors, less with my current model, bit if I ma laying on it i have a worse chance of it giving a false high or low.
rick
I sleep on my sensor every single night and have always remained very accurate. the only time it’s ever not accurate is the first day I change a sensor.
I use the freestyle libre, so it’s a bit different, but I get false low readings from about 3am until I get up- pretty much every night.
I’m not sure if it’s pressure on the sensor or just general inactivity (I don’t move at all when I sleep- I’ve had friends wake me for fear I was dead 
)
My Enlites will provide inaccurate low numbers most of the time when I sleep on them. The numbers resume accuracy pretty quickly once pressure is removed. Knowing that, I will often turn off the LOW ALARM for a period of time, during the night.
yes, that completely answers my question. i was getting false lows every night i slept on my sensor. it woke me up almost every hour. did finger stick only to find out i was completely fine. last night i slept on my other side and no alarms went off at all.
thanks a lot. i will deff try this. every time i get those alarms, my husband wakes up too. it drives him crazy crazy crazy!!! and i, naturally do not get enough restful sleep.
All of these responses are I think, in hindsight, what one would expect. Namely, that the behavior would seem to somewhat follow a pattern, but you couldn’t count on the pattern being true for all people. Or even to be true for one person 100% of the time. No?
Both the Dexcom & the Medtronic CGMs use a subcutaneous probe and calibration. The big and likely most often flawed assumption behind the calibration process used to translate the output from the probe into glucose readings is that the only thing which is going to change over time is the relative concentration of glucose in the fluid the probe is immersed in. In particular, you assume that the ratio between BG and interstitial glucose won’t change over time.
This obviously never happens and it is why you are expected to recalibrate over time. Things change. Lots of little changes happen all the time after you insert and after each calibration. Clearly most of these are small enough to not be relevant, but not always.
How big a change there is between the environment the CGM probe was in when it was last calibrated and the one it changes to when you, for example, lay on it is going to depend on a lot of things which are going to vary between different people. So the results of pressing on your sensor are also going to vary.
In my opinion, this is also why you hear the horror stories about sensors being such “complete junk” from some folks but the exact opposite from others. Again, it’s not so much the sensor technology per se as how much environmental “noise” the person with the horror story has subjected a sensor to.
I guess my perspective on CGM sensors differs fundamentally from what some other people assume. Some people seem to think of CGM like the proverbial telephone of my youth. That is, it should “just work” without having to think or plan or tweak anything about it. Just slam it in and you should be good to go. End of story. Any experience other than that is to them “proof” that the CGM system is a worthless device.
I instead think of all the potential there is for unexpected variability and I am amazed that CGM sensor technology works at all! That it can work as effectively and accurately as it is often observed to I find astounding, simply astounding. 
[quote]I instead think of all the potential there is for unexpected variability and I am amazed
that CGM sensor technology works at all! That it can work as
effectively and accurately as it is often observed to I find astounding,
simply astounding.[/quote]
I am with you here. I view CGM technology as truly amazing! It honestly still seems like science fiction to me, similar to iPads and 3D printers and other things that truly were science fiction 15 years ago.
I also do not view CGM as a replacement for my meter, just supplementary information. So if that information isn’t always perfect, who cares? At least it’s there most of the time.
But then, I think a lot of people rely on their CGM more for the alarms than for glancing at it constantly throughout the day to decide when to test and make a decision. If I were to switch to the Libre, which has no alarms, I really would not miss them except for overnight, the one time I do rely on them to wake me if my blood sugar is drifting out of range.