CGM wildly off

Update: After 2.5 days, the sensor finally came around and has been reading mostly within 10 mg/dL of finger pricks since then.

I applied a new FreeStyle Libre sensor earlier this week and even after the first 24 hours, it’s reading anywhere between 30 and 60mg/dL lower than finger pricks.

My last sensor consistently read 10 mg/dL higher than finger pricks over the life of the sensor. I overlapped the two sensors for a bit, which helped me spot the mental calibration I need to do. I’m fine doing the math, but when the low alarm is set to 60 (the lowest it’ll go) and goes off, but I feel fine and fingers say I’m actually 110, it’s really annoying. Especially in the middle of the night.

Has anyone had experience getting replacement FreeStyle Libre sensors when the readings are this wildly off? So far all my sensor replacements have been for sensors that ended early, so I’m not sure they’d send a replacement in a situation like this.

I haven’t, but here is a website where you can report it and get replacement.


Thanks. I’m in Germany and have contacted them about other sensors in the a similar way (the German form doesn’t get me to the people that can help me because my regional insurance company has stuff set up differently, so I need to call or send an email).

Just wondering if anyone has had experience reporting really off sensors and being given replacements. If it’s going to be a struggle, I don’t want to bother. If I don’t get to the people that speaks English (and there are some), it’s just too frustrating. My last call I got help from a neighbor and even with a German speaker doing the talking for me, the call took 45 minutes. The calls with an English speaker were just 15 minutes and easy.

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Yes. Anytime my sons sensor doesn’t last the full 10 days I get a replacement without issues. That includes one that is wildly off and I have to change out early.

They have replaced entire boxes (lots) for us at least once because 3 in it were trouble sensors and I didn’t want to use the rest of them in the box.


In general CGM sensors are wildly off.

Dexcom was selling the idea that they are not for a while but recently my G6’s have also been wildly off.

So yes: they really are wildly off, but that’s where we live and we can’t relocate.


@Finn posted about Libre sensors, so experience may be different.


@jbowler I know accuracy varies person to person, but my FreeStyle Libre sensors tend to be consistently within 10mg/dL of finger pricks. My previous sensor read 10 low and the one before that was consistently 10 high.

Nice spot, @MM2 . Yes, I’m using FreeStyle Libre.

@ClaudnDaye - I’m using FreeStyle Libre at the moment, not Dexcom, but it’s good to know they replaced an entire box for you.

Anyway, it looks like this isn’t an issue anymore - this sensor finally calmed down after 2.5 days and is now more or less in line with a finger prick (reading about 10 lower than fingers, which is in line with other FreeStyle Libre sensors for me).


@Finn , see my post on Libre 3 vs Libre 2. I’ve had inaccurate (sometimes largely so) sensors about 20-30% of the time. My current sensor was activated Nov 7. I wore it for about 12 hours before activating, to get the sensor acclimated. Here is a summary of its life:

  • Nov 7- Activated
  • Nov 8 - Dead on
  • Nov 9 - Dead on
  • Nov 12 - 7 pts low
  • Nov 13 - 23 pts low
  • Nov 14- 26 pts low (78 indicated/104 actual)
  • Nov 15 - 42 pts low (115/167) Called customer support, who replaced sensor.
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I had that issue last year and had to stop using it. Mine was reporting up to 100 points higher and I went trhough3 sensors in a few days. Abbott Labs customer support had no idea either. That was with the FreeStyle 2.

I’m trying the FreeStyle 3 this week and so far it’s been pretty close to my actual meter. With the 3, it’s stated right upfront that taking more than 500 mg of Vitamin C supplement a day could throw off the sensor numbers. Make sure that’s not an issue.

Other than that, I heard some medications can have an effect. Hopefully, you figure this out. The concept behind the 3 is important for improving my A1C. The alerts sent to my phone make me more attentive to my bg levels and gives me time to react before I hit 250 or 50.



This was also a Libre 3 and I activated right after applying it, but only because I was overlapping it with a Libre 2 and I wanted to compare the accuracy. That Libre 2 sensor read consistently 10 low, so I had an idea what I was working with.

After 2.5 days, the Libre 3 found it’s bearings and has been reading within 10mg/dL since then. It was just an annoying first few days where it was more off than usual for me.


Sensors (both Libre 2 and 3; this one is Libre 3) are usually pretty consistent for me. This one just had a longer warm up period than most sensors. After 2.5 days it finally came around and has been mostly within 10mg/dL of finger pricks since then.

I take no meds or vitamins that would interfer with the sensor (just insulin, Vitamin D3 and sitagliptin) so that’s not a concern.

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Day 7 with the Libre Sensor.
224: sensor
176: meter

Been having readings that are over 40pts off since last night. I’m betting these Libre sensors have a higher fail rate as they get nearer to their supposed expiration date. That really renders this thing fairly useless, even if Abbott replaces it repeatedly.
$230 for a defective device. Does Dexcoms have these issues?

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I use Dexcom G6 sensors, and they have their challenges too. My experience is they are pretty close to meter readings in the middle range, but not so close at either extreme end.

Each sensor seems to have its own personality. Some will be really “good,” so I’ll do a restart on it to hopefully continue the closeness to the meter and the restarted sensor will be way off! Ugggh!

Most of mine will last the full ten days, but some do seem to get a little wonky near the end.

I still don’t ever give an insulin dose without a finger stick check…so much for “No finger sticks ever” advertising.


This has always been our case as well.

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According to the AndroidAPS web site the things can be corrected -20mg/dL…+40mg/dL and, at least for the freefreefree, that’s all there is.

So I’m guessing that if a sensor isn’t within -20…+40 it’s an instant replacement from the milk guys.

I have had to replace at least half a dozen Libre2 sensors with Abbott, and they’ve been pretty accommodating. Just remind them that you may be making medical decisions based upon the readings from their product and that a malfunction could put you at risk and they will become very interested and very helpful. I can’t speak for Europe, but here in the US the process was not daunting at all. The worst part is the overly complicated packaging they send you for returning the faulty unit, and even that is not so bad. Interestingly, I never had a problem with an Abbott-supplied replacement unit, only with ones I got from the pharmacy.

FWIW, so far I have had much better luck with the Libre3.

Good luck!

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