Which would you change....Dexcom G5 runs 40 points low

AM Basal testing today has shown me how consistently my Dexcom G5 is running 40 points low. I had incorrectly assumed when I saw big spreads between fingersticks and Dexcom that it was lagging. This would also explain (at least to some extent) why my A1C came back 0.5% higher than I thought it would last week.

My sensor is on Day 1 of the second week (restarted it last night c/o the Faraday bag). But I’ve noticed this 40 point spread for a few weeks now, on other unexpired sensors. I will have to check the manufacture date of the transmitter tonight, but it has 3 weeks of battery life left on it. I do have two spare transmitters…so I’m wondering if the relative (presumed) age of the transmitter could lead to this consistent spread?

I went out and bought a Contour Next meter and strips today to compare to the Dexcom and my Freestyle strips. Contour read higher than Freestyle which read higher than Dexcom.

What would you change? Sensor? Transmitter? Just let it ride and see what it does?


@T1Allison Don’t bother changing the transmitter. Your 3 weeks left on the transmitter is an arbitrary number of days decided by dexcom. The battery in the transmitter should last another 3 months.

How often and when do you calibrate? The G5 requires 2 calibrations per day taken when your Bg is stable. And more calibrations per day than 2 is also not a good idea.

MARD says your 40 point spread is an outlier, but as you’ve seen different Bg meters can have significant differences too.

Your A1c shows that this is a continuing problem for you, so perhaps it’s specific to you and not the hardware?


I really don’t want to be special for any more reasons. :slight_smile:

I only calibrate when prompted and IF I’m level.

It didn’t used to act this erratically. It has also started acting like a slingshot, where if I’m spiking and dipping it’s will overshoot it’s estimation high by a fair amount, and then when I dip it will continue to show the dip action for 30-45 minutes even when I leveled on my own. If I could trust the trend but not the numbers, cool. If I could trust the level numbers but not the accuracy of the trend, cool. But it’s fudging all of it up right now. I’m going back to manual mode (figure of speech for all fingersticks all the flippin time) and watching it with side eye.

My sensors are four months out from expiring and I have newer ones. Might be interesting to try a new one.

I’ve been meaning to ask somebody outside of this site for some time now so as to save face, but… what’s a Faraday bag???

Just a special wallet that keeps the Dexcom magic signals from talking to things that you don’t want it to talk to. I put my old Dexcom receiver in it when I’m tricking the system into a restart without losing the read out on my phone for the warm up.


What’s your process for comparison? How often are you finger sticking??

Thank you, ma’am.

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Well, less than I was since I figured out not to trust Dexcom. Now around 15x a day. But I have several fires I’m figuring out at once.

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As a chronic and borderline obsessive tester myself, can I make a recommendation? Not that you’re any of those things… :smiley:

Could you get yourself settled in with a nice healthy sensor (not first or last day) and just put yourself on a timer and test on cue for 6 or 8 hours?? The longer the better, and the more often the better, but even if you could pull it off every 30 minutes (or any regular cycle), it might help you see what it does through your regular daily fluctuations. if that makes sense. It takes the decision out of it and just puts you on a rote schedule, theoretically catching all differences, delayed or not.

You see how I didn’t wait for permission before laying out the recommendation? That’s a man move.

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@T1Allison, I am not quite sure I understand. Are you sayng that your Dexcom remains 40 points low despite calibration?

Your Dexcom might occasionally lag when you take carbs or insulin. But it should not lag when the derivative of your curve has been close to zero for about 30-45 minutes, even if you are going up or down some. When that is the case (a stable trend, even if not level), do you see a 40 point difference btw your CGM and your fingerstick? And that difference does not change when you calibrate?

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You got it. I’ve calibrated as it asks for them, when level, and it will manage to find a way to become 25-45 points off within an hour even if I really stayed pretty level. And then it continues to just do its own thing.

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What happens if you do a double calibration (if you enter the same number twice consecutively in quick succession)?

I don’t know but I’ll try it!

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Was just going to say the same. But I put in 2 or 3 numbers, usually 2 points higher and lower, within a few minutes. Repeat 5-10 minutes later if needed. But this only happens in 1 out of every 4-5 sensors. I use G4, same sensor, but different trans, so could work differently with G5.

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After a double calibration it should, in general, take the exact number you are entering as its new value. It would be really valuable to find out what happens to your signal in the next few hours. Try to pick a moment when you are sure there will be no.lag, and take several fingersticks if needed to make sure your measurement is solid. This will be a good experiment.

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Have you tested the sensors on different insertion sites?

I ask because I had an issue like this when I put Libre sensors on my thigh but never had this issue with the sensors on my arm.

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My arms were great during my pregnancy in 2013 (G4), but they won’t read right anymore. I tried thighs and abdomen last year with no luck. My back has been foolproof until recently.

You have to be careful with this method: with the G5, if your CGM is low, it will not calibrate to the number you give but some way to that number. But, in the second calibration following another (that is not a double calibration), if you are within 10% of the new Fi gerstick it will not asjust any more.

Example: say your CGM reads 102 but your stick is 122. If you calibrate, your new CGM value may be 111 (as always you need to wait until the NEW CGM value, not the one that shows right after your calibration). So, if you stick again and get, say, 120, your next calibration will do nothing.

On the other hand, a true double calibration overrides your CGM averaging and forces it to the number you give it. That is the only way I know to force a specific value. If it does not calibrate to the number you give, that means your sensor is shot and Dexcom will replace it.

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Just tested and double entered. For the moment it accepted my value. I’m currently two hours post-snack at a bg level peak. Will probably do some finger sticks overnight to check it against Dexcom. Dexcom was reading 30 points low just now, even though I had calibrated it before dinner when level (when it was 40 points low at the time). It stayed accurate for all of 1.5 hours before deviating again.

Maybe this is how Dexcom makes people happy. Show them what they want to see! :yum:

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It’s already shaved 15 points off the reading and I know that’s not accurate.

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