Reasons for DexCom ? Readings

I’ve been on the Dex for nearly a week and have become a big fan. I have questions, though, about the ??? readings. I’ve had this happen 3 times during the week. Each time for 2-4 hours.

  1. Is that amount of ??? reading a lot compared to you, or a little?
  2. I read that it can happen when the blood sugar is changing rapidly. But I know this wasn’t the case 2 of the 3 times w/ me. So my most important question is if there are common reasons this happens?
  3. Any secrets to “kick start” it when this happens? Some people say enter a BG. Some say don’t. What do you do?


How far away is the sensor from the receiver? When I wear the sensor on my thigh I get more of the ??? readings. Maybe it is related to the distance between the sensor and the receiver?

I have never gotten the ??? when I had rapidly changing BGs and I have been using this for a couple of months now. I definitely have rapid changes in BGs often, that is part of the reason I got this system, hahaha! I have not found a real pattern to the ??? other than I tend to get it more when my sensor is starting to wear out. So maybe that is part of it for you if you are near the 7th day. I can usually get 12 to 13 days out of a sensor before it starts acting up. I never put a BG in when it gives me ???, I was told that could really mess stuff up. Not sure if that is true but I don’t want to risk it. I usually do a shutdown (not a stop sensor), let the unit sit for a few minutes, and turn it back on. That usually clears up the ??? right away.

In DN’s case, we get the ??? around 3am during times of rapid BG changes (spiking higher very quickly), due to growth spurts. The other evening I tracked it normal BG at 1:30 and at 2:15 a.m. BG was 100 points higher (on good, recently tested overnight basals). About two hours after this, if we can’t get BG down, we will get the ??? for a couple of hours. This has happened a few times already overnight. When we know BG is more stable (around 5am), we recalibrate and it has accepted the number. Dex tech support told us not to restart when seeing ???. Though I may try this to see if it works, I am going to try it near the end of the sensor in case I get a sensor failure for doing this. Ironically, post-prandially, when BGs are fluctuating, I have not gotten ???. I have gotten numbers that did not reflect the fingerstick, with rate of change arrows, but Dex is still trying to track.

??? means the receiver is getting a reading it can’t interpret. Reasons can be, in no particular order:

Rapidly changing BG
Badly placed sensor
Bad sensor
Sensor moving in and out of range

With ‘badly placed sensor’ if you’re laying on it or its placed where it gets a lot of pressure from you clothing, like from a belt or waistband, the sensor moves around under the skin. This is my theory and experience, by the way, not official word from DexCom.