Incorrect calibrations w/in the first 24 hours of putting in a new CGM

i finally found a great spot to insert my dexcom sensor. around my hip on my back. i do this like i am a contortionist. but it it seems to be the most comfortable and most accurate

yesterday evening, my sensor fell off. i had to put on an entirely new sensor, similar spot. my readings (#s) were off by more than the recommended allowance for both HIGH and LOW.
i called Tech Support and they told me that w/in the first 24 hours or so, the dex readings sometimes need a chance to calibrate properly. so, here it is, the next morning around 10am, and i am having the same problem. and even when i recalibrate the proper BG, my dex reads inaccurately. as well, after all these re-calibrations, my direction arrow disappeared.
well, the direction arrow just came back on. PHEW.

do i leave the dexcom sensor on for the rest of the day to see if it works itself out, or do i change it and start all over again?

I need as much help as possible. anyone out there have this problem or know about these things? Dexcom is sending me a replacement sensor, so its not about the money of replacing it. its really just the not knowing along w/ the inconvenience.

I have this issue most weeks. All I know is that it works itself out, you don’t need to (and probably shouldn’t) calibrate more than the two times per 24 hours. Less is more in this instance.

I wish it didn’t take so long for the sensor to work properly. I am always loathe to change my sensor because I know that night I will get very little sleep because the damned low alert will keep going off even though I’m not low at all.

But I still prefer to have my CGM. 12-24 hours a week of insecurity is better than no security at all, IMO.

thanks for your response. it is 2:30 in the afternoon, here in Brooklyn NY. after too much frustration, i took the defective sensor off and replaced it. it went in fine; no discomfort, etc. i was so frustrated w/ the other one (aforementioned). i was getting readings of 114 when my finger stick read 164. them my dex alarm went off, and (w/in literally 2 minutes) and it read 187^^. BG stayed the same. so its done now. all i can do is wait out the 2 hours and start over. i really really hope this sensor works. i am getting exhausted and going broke from over- finger sticking,using up test strips, and wasting tegaderm (all very expensive and sometimes test strips are paid OOP just like the Tegaderm)
daisy mae

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I just saw some of the thread in the chat room with your probs with your sensor Daisy_Mae. Not pleasant at all. Especially when we rely on the CGMS so much. I vowed I wouldn’t become so addicted to it, but since starting back in December. I’ve only taken a break for a week. I have no insurance, so rely right now on handouts (expired sensors) from Americans. I have 2 more left now, of sensors (they expired last May) … and so far, they’ve managed to hold on for an avg of 3 weeks … sometimes I get 4 (e.g. where you have yours now on your upper back … that one lasted the longest … and you don’t even know it’s there!!!). So, hopefully within a few hours, you’ll be back to “regular programming” and what Tamra11 is saying is true … I heard it from a few other Dexcom users … over calibration can F it up … so I try to only stick to the 2 calibration alerts … but when my Dexcom and vampire machine are really off (like now … I was showing 8.6 and meter was showing 13.3 … then I correct). Ducking fiabetes right?


I suggest pulling it at the earliest possible moment. Call DEX tell them of the issue and they will send a replacement this is fixable.


I know you mention being very lean a few times. I’m pretty lean myself and I know if I don’t get it in a spot with enough fat, the readings will be off and there is really nothing that can be done to fix. It is usually not a sensor issue(only my personal experience), but a placement issue. The only place I barely have enough fat is on my stomach. You might need to find another placement spot.

thanks for your compassionate input. i have finally found a spot for sensor insertion: if i can contort myself, and put the sensor on my back (near enough to the “love handles”, which i don’t really have) so far, so good.
Daisy Mae


One thing to keep in mind with the Dexcom sensors is that they’re using a certain algorithm to come up with the correct readings, since they aren’t really measuring blood glucose. So, when you callibrate, the Dexcom may still take some time to “correct” itself, which is partly why over-callibration can mess it up. I notice that sometimes when my callibration is way off the sensor actually becomes even more inaccurate and gets a lot more “chaotic” for about half an hour before it settles down again, and I think it’s just the algorithm trying to incorporate the new data and readjust.

When I’ve had days where my sensor is really off I actually do callibrate more than recommended, but no more than about four times throughout the day. I find four times (breakfast lunch, dinner, bed) on days when I callibrate and it goes off again within an hour or two, seem to help get it back on track. I wouldn’t callibrate more than this, though. I’ve also done the three callibrations 15 minutes apart thing that Dexcom recommends for when it’s really far off.

Since I pay out of pocket for everything, I’m willing to put up with a lot of issues (hours of ???, a site that hurts or itches insanely, multiple false alarms, readings that are fairly inaccurate…) before I will pull a sensor. I pretty much wait until I have a 24 hour period where I have more blank spots than readings or I get “Sensor Failed” alarm and it just stops working before I’ll pull it.

I’ve heard of people using their breast and their upper butt as sites. Might these be some possibilities where there might be a bit more fat?

thank you for your reply. i have just discovered a place for my sensor to go on my body which neither hurts or is dysfunctional: my tushy /lower back/ by the hip area. so far, so good. what are the “algorithms” you are talking about with regard to the sensor? i know that the sensor is supposed to go into some fluid that reads ( as closely as possible) your BG levels. but i don’t understand how that works either.

would love to hear back from you,
Daisy Mae

My understanding is that there’s an enzyme coating the sensor that generates an electrical current in the presence of glucose. So the sensor detects differences in this current based on whether there is more or less glucose present in the interstistial fluid (this is fluid between cells), and it also uses changes in this current to decide whether your glucose is rising or falling. But it’s not actually measuring glucose nor measuring in mmol/L (or mg/dl) units, so it needs to do calculations to convert the data it gets into something that can be displayed and make sense to you.

So, for the first 24 hours of each sensor (and I’ve heard some say the first 72 hours, though I don’t know how accurate that is), your Dexcom is basically trying to figure out how to match the electrical current it’s receiving from the sensor to the blood sugar readings you’re providing. There is nothing inherent in the actual electrical current that says anothing about what blood sugar level it corresponds to, which is why you need to start each new sensor with two callibrations and make sure you callibrate regularly.

The glucose in interstistial fluid lags behind glucose in the blood by 15-20 minutes. So when you callibrate your Dexcom, you’re providing it with a reading that it “should reach” in the next 15-20 minutes, not with the reading that it should currently be showing (especially if you callibrate when there’s an up or down arrow). So, it nees to make further calculations so that it can adjust so that in 15-20 minutes the reading it displays is accurate accoridng to what your meter said.

At least this is my non-scientific understanding of how things work. It makes sense to me, at least, that the Dexcom would get “confused” and have to re-calculate a bit (and not be able to display things like arrows, or display the wrong arrows) after a callibration that tells it that in 15-20 minutes it should be much higher or lower than it had calculated. Kind of like when you suddenly go off the route your GPS planned onto a street it didn’t know existed, it has to pause and re-calculate everything based on the new information. Others can correct me if I’m wrong or provide more detailed/accurate information.

@Jen - That’s a concise and informative description of CGM operation.

I have had similar issues with some of the Dexcom Sensors. When I called them, they had me do a slow calibration, 3 calibrations over 45 minutes spaced 15 minutes. They also sent me a new sensor in case I wanted to change it out, entirely up to me. I left it in and it did work it self out after a total of about 18 hours ( I learned to change my sensor in the middle of a day instead at the end of a day so that I wasn’t woke up by false lows). I have found Dexcom to be very helpful and have no problems sending out another sensor if need be even to be on the safe side.

thanks for your reply. my sensor wasn’t picking up my BG readings at all…meaning, i would do a finger stick and my BG would be 114, and the dex would say something like 178^^. this happened all day long and into the following day. i did the reclaibration thing, every 15min. but to no avail. finally i called dex tech support and they sent me out a replacement sensor. it arrived over night. i have a few frustrations: 1) re-doing a sensor session; 2) using way too many finger stick test strips (which cost a small fortune) and 3) using up my Tegaderm (which is also expensive). i am glad that dex stands by their product w/ a no-questions-asked policy, and i love that they are always available and very helpful; i guess i just get frustrated easily, trying w/ so much difficulty rotating btw my pump, my sites, and my sensor location. as i have said on many other sights at TuD, i am VERY lean and have tremendous difficulty finding places where i can successfully put my sensor on. but, anyway, i wanted to say thank you for your explanation about the fluid. i have only been on the DEX for about 10 months, and i am still figuring my way around it.