Dexcom G6 had wildly off reading!

I just inserted a new G6 sensor last night and woke up to a high reading that surprised me. I corrected. Just now, later in the morning it read 370 and it did not feel right to me so I triple checked on my meter and my BS was actually 230! I calibrated and now my G6 is showing double arrows down. Has this happened to anyone and what have you done about it? It is crazy off and makes me not trust it at all. Thanks!

I was told by Dexcom, that their readings can be from 20-30%!

When my daughter switched to G6 we had trouble with the first 2 sensors with lots of inaccuracy & threw them out within 24 hrs. When I called Dexcom I also got the 30% info, which really surprised me. They did mail us 2 new sensors for free. The dexcom rep assured me that G6 is usually accurate. Since then most our sensors are accurate from the start, but when it’s not (about 1out of 7 sensors or so it’s not accurate in the beginning) we use a meter & calibrate every time over 1 to 2 days & then it becomes accurate & we no longer have to calibrate. So the first two that we threw out after 24 hrs may have been ok if we held out & calibrated for another day. Except! There was one other time we had to throw out a sensor because it never switched to being accurate, at least not accurate enough for us to use/lean on in any way. I suppose it might have fit into Dexcom’s 30% allowable error, but that’s not usable. We do reset/stretch out sensors to 20 days so if 1 out of 7 sensors needs to be calibrated for a day or two it’s not bad.

Hi Shelly @ShellyJensen100, the “double arrows down” are easily explained; your CGM thought you were at 370 mg/dl one minute and within a very few minutes it was seeing you at 230 mg/dl - “apparently” falling at a rate of greater than 3 mg per minute.

Why it gave you readings of 370 in the first place isn’t as easy [for me] to guess. By any chance, had you eaten a “large” meal during the two hour warm-up? Anything else that was different for you? I’m just fishing and trying to know in case I end up in a similar position. So far for me, with a G5, when comparing my CGM readings and meter readings to lab tests, the G5 is more accurate.

Nothing like this has ever happened with Dexcom for me! I don’t think there was anything unusual with my eating, etc. It seems to be back on track now that I have calibrated a few times.

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Happy to hear that you are back on track @ShellyJensen100.

Generally, I have found Dexcom G6 to be amazingly accurate, but once in a while they seem to be off by quite a bit during the first few hours of a new sensor. Mine have always settled in after two calibrations a couple of hours apart. So I try to change sensors when I know I’ll be up for several hours, and check my blood sugar a couple of times over the first 4 hours. I calibrate only if the blood test is more than 20% different from the sensor.

Diddo Mikefarley. I have had fantastic performance from the G6. On only one occasion did I experience weird results. In that case I knew something did not seem right due to the extreme deviating data, Dexcom sent me a new sensor and I was off to the races.

I am not sure if my experience can really be blamed on Dexcom. After 40 years as a T1, I have a ton of variation in site effectiveness. I have been on CGM since the original Medtronic sensors with the 45 degree insert and C-shaped needle from hell. My perception (could be WAY out in left field) has been all the systems had some variation, but I could correct that typically with a change in site. The risk in this personal perception is that to change site, you have eliminated the potentially errant sensor to identify root cause of the problem.

My point is that:

  • I do what I can knowing my sensory perception and potential limitations
  • Dexcom (Medtronic too) continues to support me when things go wacky
  • My excursions are at an extreme that they are easily identified as abnormal, needing further investigation

The real question I have is what happens when Control iQ is released? Will ControliQ shutdown with large data excursions, or keep dosing boluses? I do not know…

I have a G5 and find day one can be problematic. Don’t know if it’s the same with the G6. I just make sure to keep track and recalibrate if its really wacky. But, don’t forget that time lag between actual blood sugars and readings from interstitial fluid. It seems like forever when you’re upset and frustrated, but for me it’s about twenty minutes or so. I try to insert new sensors early in the day so I have all day to to keep track and can go to sleep pretty sure all is OK.

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My son has had problems with the last 7 G6 sensors. They have said replace sensor now at 4 or 5 days in, never even starting up when he inserts it, or the last one was saying 52 and his meter was 156 (3 times in 1 day). When I called they said that’s to much to be of, change it. I have talked to Dexcom after each issue and they said it is most likely just a bad batch. I don’t know how many are in a batch or how long this will continue, but it is very frustrating. I understand your frustration! We have never had this problem before.

I think all of this happened during the first 24 hours after you started your new sensor. That is not unusual for me. My sensors are not very accurate during the first day, but there is usually much better accuracy by the second day.

Hi @ShellyJensen100. I use the G5, not the 6, and have found it to be very accurate. The few times I have had issues were when I took Ibuprofen (which I believe is not supposed to be an issue with the 6), and a couple of times when I had an issue with the insertion: recently I thought I saw a little blood come back up around the insertion site but I rationalized that it was the lighting in the room; but after some odd behavior with the receiver function of my pump I finally decided it was probably user error (me). When I removed it I found there was blood there; the next one worked just fine.

I find this info pretty interesting. I used to use Dexcom 5, and had regular large discreptancies. 20 points is one thing, but I saw up to 100 points difference regularly. Still liked it. Sometimes it worked well. I now have Medtronic Guardian and do not see that much of a discreptancy. Maybe, it’s site location. I now use arms.

I am very curious because I am considering switching to T slim and Dexcom 6. I do not want to return to large discreptancies again.

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Sounds like a bad sight , this happens once in awhile, just eat more and , probably put a new site

I was a g5 user for about 4 years and recently upgraded to a g6. I keep having readings with the g6 that are more than 30% off on either end. Dexcom won’t help if it is within the 30% range, which I find unacceptable. I never had any of these issues with my g5. Question, does anyone bypass entering the 4 digit number and instead calibrate twice per day like we had to do with the g5? If so, do you find that this helps? I do think that I’ve had a bad batch with my latest order, but Dexcom says that just isn’t possible!

My daughter is a G6 user and we have gotten readings like that,
Dexcom said it can be off by 20% when in doubt always finger stick so when my daughter reads over 300 I always double check. Over all I love Dexcom! Hope this helps and good luck!


The G5 uses the 20/20 rule. When they designed the G6, they incorporated a less accurate 30/30. Using the most extreme scenario within the top end of each sensors accuracy range, a BG reading of 100 can be as high as 120 with the G5, 130 on the G6. It might not seem like a big deal, in my case those two readings are the difference between me doing a correction bolus or not. Why they chose to use a less accurate range from the G5 is beyond me. I suspect it has a lot to do with the introduction of the no calibrations, they needed to show the sensor was as accurate as they were touting it to be to get FDA approval. Raising the range gave them more of an accuracy buffer. Purely speculation on my part.

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@Boymom and @GNB I am also having very inaccurate readings on G6! It was often 100 points or more off. Right now, it’s saying I’m an urgent low (~40), when in reality I am at 120 according to my meter. I’ve had this happen SO MANY TIMES with various censors (different boxes) and sites. When I tried to calibrate it would give me a constant “calibrating” and then just stop working.

I first noticed the problem in Feb/March and ended up not using the G6 for a few months because it was so frustrating. I just refilled my rX with a new batch and having the same issue. This time around I plan to wait the full 48 hours and NOT calibrate before I throw in the towel again.

These constant low warnings that you can’t turn off, drive me and my coworkers crazy.

Please let me know if you find a resolution! Or if you switch to something the works better.


Completely agree that the increase in range was due to no calibrations and FDA requirements. Have you tried to calibrate a new sensor from the startup rather than enter the 4 digit number? I’m going to try this on next sensor change.

Amy, your situation sounds almost identical to mine. It’s so frustrating! Have you tried to calibrate a new sensor from the startup rather than enter the 4 digit number? I’m going to try this on next sensor change and will see what happens. I’m approved doe a tandem talking pump, but have out it on hold until I can figure out the current issues with the g6!!