CGM limitations and precautions

H

helli

Well-Known Member
Relationship to Diabetes
Type 1
Moderator Note: This helpful reply was copied from another thread as it details some of the commonly experienced limitations of continuous glucose sensors.

MarinaDE said:
My blood sugar has been in perfect range for days now, but although I haven't changed anything, I'm starting to get very short periods of hypos while I sleep and this morning I had a hypo alarm.
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If these are very short periods of lows and occur overnight, they are more likely to be compression lows - when pressure is applied to a sensor, it will report a false low. The reason I say this is more likely to happen at night is because that is when we could lie on our arm and squash the sensor. This could happen more in the hot weather because we don't sleep as well and toss and turn ... at least I do.

I am not sure if anyone shared with you the limitations of CGMs. As Libre is the most common one, these are often reported as "Libre limitation" but they are problems with physics, The only difference is how the manufacturers have chosen to deal with them.
Anyway, enough waffle, here they are with apologies if you already have this
  • Some of us find that our bodies do not like have an alien object inserted into our arm. It take a day or two to "bed in" a new sensor. As a result, the first 24 to 48 hours after inserting a sensor could be more random. Some of us insert a new sensor the day before activating it to overcome this.
  • Compression lows. Take care where you place your sensor and try to avoid the part of your arm that you lie on. If you get a low alarm in the middle of the night, check it with a finger prick before treating.
  • "Normal range". CGMs are designed to be most accurate at "normal" BG. This is around 4.0 to about 8.0 and, if you see a value outside of this range, it can exaggerate the high or low. Again, the advice is to check with a finger prick before treating.
  • Extrapolation. CGMs read interstitial fluid which will react to changes in BG about 15 minutes slower than blood. This is where I know some CGMs differ. Libre handles this by extrapolating the current trend to "predict" the current reading. If your trend changes direction in the last 15 minutes (e.g. when treating a hypo), the prediction could overshoot. Libre will correct this when it has "caught up". But at the time, it will seem like you are going higher or lower and taking longer to recover. Again, another reason for double checking with a finger prick.
  • Faulty sensors. Not every sensor will be checked in the factory so there are some faulty ones in circulation. If you read social media, it will seem as if all sensors are faulty but human nature is to complain when things go wrong and say nothing if things are ok. The most common "fault" is inaccuracy. Therefore, it is a good idea to check the accuracy against .. .yes, you have guess it ... a finger prick. I tend to check once a day when my levels are stable and in that "normal" range I mentioned above. Don't expect exactly the same numbers (meter standards allow 15% inaccuracy and both could be out by 15% in opposite directions) but it is useful to give yourself confidence your current sensor is in the right ballpark ... and staying there each day. If it is out by a lot or if you get a "sensor failure" reported, in the UK, Abbott are pretty good at replacing sensor either via an online form or by calling. I do not know what that is like in Germany.
  • Third party apps. Libre is "factory calibrated". Some of us find "factory man" does not represent them well. There are third party apps like Juggluco, Shuggah, xDrip+, Diabox, ... which allow calibration against a finger prick. These were very popular when Libre required scanning as they also converted the sensor into a rtCGM (real time CGM which did not require scanning). Some people still prefer them as they also allow things like integration with a smart watch. However, they may not update LibreView which could be an issue if you share that with your endo.
I think that is all and I have probably waffled for too long on something you may already know.
 
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Thank you @helli, it's still as useful today after 2+ yrs with CGM as it would have been if someone were to have explained all that on day 1 with CGM! I found the Abbott series of short videos useful to introduce the basics of L2, but inevitably not spelling out the limitations! I didn't find the Dexcom material as good.

Libre 2 and my body turned out to not be good friends and as well as lots of L2 failures I was rarely getting interstitial readings that were near actual BG. But the trend arrows justified persevering with L2 as major assistance to my daily BG management. I subsequently was moved to Dexcom One which was better for reliability but still not very accurate. I'm now self-funding Dexcom G7 as a personal trial and that (with one exception) is extremely good.

For those newish to CGM there is a different thread in this Pumping and Technology Section titled "What CGM do you use?" - link given below. This does provide helpful dialogue from regular CGM users about their CGM experiences and could be useful  after getting started on CGM and once feeling a bit more adventurous!


Also possibly useful to people newish to CGM or just diabetes are the NICE Guidelines that provide the "rules" [from which GPs, Hospital Specialists and the relatively new (1 July 22) Integrated Care Boards (ICBs) that replaced Cost Centre Groups (CCGs)] for the diagnosis and care of T1 and T2 diabetes. They include guidance for when CGMs can/should be prescribed.

NG 17, for Type 1, last updated Aug 22:
The National Institute for Health and Care Excellence
https://www.nice.org.uk › ng17
Type 1 diabetes in adults: diagnosis and management | Guidance

NG 28, for Type 2, last updated June 22:
The National Institute for Health and Care Excellence
https://www.nice.org.uk › ng28
Type 2 diabetes in adults: management | Guidance

These were last reviewed in 2022 and have been progressively widenening the availability of CGM. NICE Guidelines are pretty strong regulation for the NHS, but they are not manadatory for ICBs to adopt all the guidelines if funding provision is not immediately available. Now, in Sep 23, my regional ICB has not yet fully implemented details of NG17; still under review apparently - even though some of the details are cost neutral (eg Dexcom G6 vs the newer G7). Presumably it is the wider potential availability across the ICB that is causing an overall cost increase, which needs funding review.

Anyway, some of this is drifting away from the useful easy-read "limitations of CGM".
 
helli said:
[*]Third party apps. Libre is "factory calibrated". Some of us find "factory man" does not represent them well. There are third party apps like Juggluco, Shuggah, xDrip+, Diabox, ... which allow calibration against a finger prick. These were very popular when Libre required scanning as they also converted the sensor into a rtCGM (real time CGM which did not require scanning). Some people still prefer them as they also allow things like integration with a smart watch. However, they may not update LibreView which could be an issue if you share that with your endo.
[/LIST]
I think that is all and I have probably waffled for too long on something you may already know.
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Hello, I’ve just logged into my librelink account. I can say with the use of one of the mentioned 3rd party apps, my historical flash scanning with librelink was patchy to say the least. My history reflects this.

But since the recent upgrade on the librelink app making it a CGM and running in tandem with the 3rd party app on my phone. I can verify reports are more consistent to librelink from the sensor. Flawless, even. :)
 
Hi ,I started using Freestyle libre 1 approx 4 years ago and changed to Libre 2 which I use presently and up until a few months I had several failures, I realised that my the skin on my arms was to tough as the probe needle etc on the sensor was folding and sometimes bending thus not allowing it to penetrate my skin , so then I started using a skin cream which has been successful and so far I have had no more failures.
 
Tradesman said:
Hi ,I started using Freestyle libre 1 approx 4 years ago and changed to Libre 2 which I use presently and up until a few months I had several failures, I realised that my the skin on my arms was to tough as the probe needle etc on the sensor was folding and sometimes bending thus not allowing it to penetrate my skin , so then I started using a skin cream which has been successful and so far I have had no more failures.
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Not sure if you are aware but there is quite a heavy duty stainless steel needle in the applicator which fires through the sensor during the application process carrying the sensor filament through your skin with it and into the tissue and then there is a spring also in the applicator which immediately draws the needle back out. I would be surprised if that needle wouldn't go through rhino skin as it is quite substantial (I take them apart to recycle what I can and put the needle in my sharps bin), so I doubt it was your tough skin causing the issue, but always good to keep skin moisturized and supple as long as you ensure there is no moisturizer on the skin when you apply the Libre as that can result in adhesive failure. Moisturizers are fundamentally an emulsion of oils and water with other things added and anything greasy on the skin, even if it was applied yesterday or the day before can leave a slight residue.
 
Hi , I read your email with great interest, it was a representative from Abbot the Freestyle people who suggested that I used some cream to soften my upper arms as I had approx 1 in 5 failures at that time and I wasn't aware there is a needle in the applicator so how do you think the filament folds over at the tip or sometimes bends in the middle which shortens the total length of the filament, and another thing to take into consideration I have had to change my injection sites after nearly 60 years injecting I have had nurses comenting during vacinations how tough the sites were , I now avoid my thighs as they are a problem, there are no lumps also just tough , thank you for your comment as I have learned something I am always interested,
Gordon.
 
The sensor filament can get kinked if you don't hold the applicator securely against your arm when you insert it or when you lift the applicator off afer insertion and it doesn't come away cleanly and the sensor gets lifted slightly as you pull the applicator off because the adhesive hasn't had time to bond fully. Or the sensor adhesive starts to fail later in it's life (a hot shower and a bit of some careless scrubbing can easily do it). Or when it gets caught on clothing or other inanimate objects like door frames where it doesn't pull it off completely but causes the sensor to lift a bit and then gets pushed back down and perhaps you might put some tape over it to hold it secure in the hope of keeping it going. Or even a failure of the application system itself perhaps because the needle and filament are misaligned by a tiny fraction, perhaps due to wear and tear of the machines manufacturing them. The tolerances are incredibly fine so it would not take much for it to not fully align and affect the insertion.

Injection sites do get hard over time. The skin and subcutaneous tissue gets damaged by being regularly pierced and insulin injected and I suppose it becomes a bit like scar tissue. I believe it is called Lipohypertrophy.
 
Tradesman said:
Hi ,I started using Freestyle libre 1 approx 4 years ago and changed to Libre 2 which I use presently and up until a few months I had several failures, I realised that my the skin on my arms was to tough as the probe needle etc on the sensor was folding and sometimes bending thus not allowing it to penetrate my skin , so then I started using a skin cream which has been successful and so far I have had no more failures.
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Hello, that’s interesting you mentioned this filament (or probe?) bending.
I notice this on sensor removal. Either bent at the base at 40/45 degrees or a “crock” in the middle?
I put it down to how I tore off the sensor. My personal experience with these sensors has been positive from day one. Libre 2 with the Bluetooth CGM feature has been a great improvement.
OK. I did have one die on me a couple of hours before it was meant to.

Best wishes.

rebrascora said:
The sensor filament can get kinked if you don't hold the applicator securely against your arm when you insert it or when you lift the applicator off afer insertion and it doesn't come away cleanly and the sensor gets lifted slightly as you pull the applicator off because the adhesive hasn't had time to bond fully. Or the sensor adhesive starts to fail later in it's life (a hot shower and a bit of some careless scrubbing can easily do it). Or when it gets caught on clothing or other inanimate objects like door frames where it doesn't pull it off completely but causes the sensor to lift a bit and then gets pushed back down and perhaps you might put some tape over it to hold it secure in the hope of keeping it going. Or even a failure of the application system itself perhaps because the needle and filament are misaligned by a tiny fraction, perhaps due to wear and tear of the machines manufacturing them. The tolerances are incredibly fine so it would not take much for it to not fully align and affect the insertion.

Injection sites do get hard over time. The skin and subcutaneous tissue gets damaged by being regularly pierced and insulin injected and I suppose it becomes a bit like scar tissue. I believe it is called Lipohypertrophy.
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Not my experience. But I’ve seen photographic evidence of that “hole punch” on the Libre applicator being left behind in the centre hole of the Libre?
And you are correct. The applicator does need gentle coaxing to let go of the sensor. I’ve devised a technique of pushing the blue shroud back on the sensor, keeping the applicator as square to it as possible, drawing back & listen for that dry scape of plastic? So I know it’s about to “give.” (And checking the adhesion.)
I find all this easier placing the Libre inside the arm.
 
It is worth noting that on the inside the arm the blood vessels are nearer the surface so there is more chance of a "bleeder" applying it there, plus it is not the location recommended by Abbott of course, but each person needs to decide how rigorously they feel they need to follow that guidance.

For me Libre works brilliantly, but I think developing a good application technique is very important. I warm and dry my site after washing and towel drying with a hair drier which I think ensures it is completely dry and the warmth seems to help the adhesive bond. I also hold the applicator against my skin for a count of 100 seconds before I wriggle the applicator free. Again, this gives the adhesive a little longer to take hold before I tug at it at all and then I wriggle it to free with a slight twisting motion rather than trying to pull it straight off, because the adhesive is less likely to allow twisting than a straight upward lift when it is still trying to bond with the skin. I also use an elastic arm band with a platic watch face which fits over the sensor and applies slight pressure around the whole circumference of the sensor. It also acts a visual reminder as to which arm my sensor is on when I get in the shower because there were occasions before I got the strap where I simply forgot and scrubbed the sensor loose/off under a hot shower because I simply couldn't see it or feel it. until it was too late!
 
rebrascora said:
It is worth noting that on the inside the arm the blood vessels are nearer the surface so there is more chance of a "bleeder" applying it there, plus it is not the location recommended by Abbott of course, but each person needs to decide how rigorously they feel they need to follow that guidance.

For me Libre works brilliantly, but I think developing a good application technique is very important. I warm and dry my site after washing and towel drying with a hair drier which I think ensures it is completely dry and the warmth seems to help the adhesive bond. I also hold the applicator against my skin for a count of 100 seconds before I wriggle the applicator free. Again, this gives the adhesive a little longer to take hold before I tug at it at all and then I wriggle it to free with a slight twisting motion rather than trying to pull it straight off, because the adhesive is less likely to allow twisting than a straight upward lift when it is still trying to bond with the skin. I also use an elastic arm band with a platic watch face which fits over the sensor and applies slight pressure around the whole circumference of the sensor. It also acts a visual reminder as to which arm my sensor is on when I get in the shower because there were occasions before I got the strap where I simply forgot and scrubbed the sensor loose/off under a hot shower because I simply couldn't see it or feel it. until it was too late!
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You are quite correct again regarding Abbott recommendations on positioning. I can’t say I’ve had that many bleeders. Where mine gets planted. (Through the hole?) Pretty clean underneath when removing too. I also use a hair dryer after a wash which includes a little exfoliation on the site, and again with an alcohol swab. (I get my own since they stopped providing them.)
Those straps are great had them back in the days of Bluetooth bridges? Didn’t like the idea of hanging a device by double sided tape to the Libre. ;)
 
Hi there, I've been reading your comments with interest, it's good to get different opinions on these sensors , personally I have applied mine on the instructions we were given at the clinic from the Abott rep and nurse but I have only one bleed since using them and I allow 2 days from using cream on my arm until the next sensor replacement.
 
Satan’s little helper said:
You are quite correct again regarding Abbott recommendations on positioning. I can’t say I’ve had that many bleeders. Where mine gets planted. (Through the hole?) Pretty clean underneath when removing too. I also use a hair dryer after a wash which includes a little exfoliation on the site, and again with an alcohol swab. (I get my own since they stopped providing them.)
Those straps are great had them back in the days of Bluetooth bridges? Didn’t like the idea of hanging a device by double sided tape to the Libre. ;)
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Yes, I too am a fan of exfoliating in general and I always do it during the washing process before drying and applying a new Libre. I want that adhesive stuck to clean fresh skin if it is going to be on there for 15 days, so old dead skin needs to get scrubbed off before application, rather than just a simple wash. My skin in general is so much better since I started using exfoliating gloves in the shower.
 
Take care with exfoliation. My body scrub contains moisturiser so I avoid using it before applying my sensor.
For me, the most important part of my application strategy is position. As my arms are pretty active (weights at the gym, climbing and kneading bread), my biceps flex often. I need to ensure my sensor is not on the edge of a “flex” or part of my arm that bends. It seems obvious when I write it down but took me a few failed attempts to realise the sensors are rigid and do not flex with my arms. If I placed a sensor wrongly, it start up peeling when I exercise and it’s nothing to do with sweat.
I also learnt that bra straps are the arch-nemesis of sensors. Over time, I got used to dressing with a delicate object on my arm but, at first, sensor and strap came in contact a few times and strap won.
 
helli said:
Take care with exfoliation.
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I exfoliate with a pair of gloves and my normal shower gel rather than a special exfoliating preparation. My shower gel is however supposedly "moisturizing" but I have no problems with my Libre adhesive, following the routine I use above. and it definitely needs me to get my finger nails underneath and grimace as I prise it off after 15 days. It is like a limpet.
 
I always imagine that the kinks (both for sensors and infusion sets) happen post-insertion for me, as both are applied with a needle - in the case of infusion sets it goes through the cannula, so I know they were straight going in!

I did have one or two sensors in the past (can’t remember which variety now! though I think they were Libre1) which failed to start, and on investigation there appeared to be no filament at all.

The tech support people suggested rather than manufacturing error and the filament not being present it was more likely that it had been inserted, but then caught or got snagged as the needle applicator withdrew and got pulled back into the body of the sensor housing.
 
I’m experiencing a few issues with the DexCom 6/iPhone link since the last iOS upgrade - has anyone else had problems?
 
I've recently had problem that the reading on my Dexcom G6 does not update for hours; I have to check the time line before assuming the reading is correct. Sometimes I get a banner alert on my iphone telling me my reading is high/low, but when I open the app that's not reflected in the reading because it hasn't updated. The help desk say they have never had this before and, after trying all the usual remedies, suggested I update to the next iOS. I did this and have had a slight improvement but still having the problem at least twice per day. When I queried whether I should be updating iOS, they assured me the app will always work but there might be periods of signal loss. Mine is a different problem to yours, but interesting that they warned me there might be some connectivity issue. Mine is an old phone so I'm not on iOS 16.
 
Gill64 said:
I've recently had problem that the reading on my Dexcom G6 does not update for hours; I have to check the time line before assuming the reading is correct. Sometimes I get a banner alert on my iphone telling me my reading is high/low, but when I open the app that's not reflected in the reading because it hasn't updated. The help desk say they have never had this before and, after trying all the usual remedies, suggested I update to the next iOS. I did this and have had a slight improvement but still having the problem at least twice per day. When I queried whether I should be updating iOS, they assured me the app will always work but there might be periods of signal loss. Mine is a different problem to yours, but interesting that they warned me there might be some connectivity issue. Mine is an old phone so I'm not on iOS 16.
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Hi, I’ve been having the same problem and suggested to the tech team this might be an app problem, but was told it was a sensor problem. It’s never happened before, but continued with new sensor. I deleted the app and re-installed And there was some improvement, but it’s still happening. I’ve made sure I have latest ios installed and updated ap, but there’s still a problem as you describe. I have an iPhone x
 
Gill64 said:
I've recently had problem that the reading on my Dexcom G6 does not update for hours; I have to check the time line before assuming the reading is correct. Sometimes I get a banner alert on my iphone telling me my reading is high/low, but when I open the app that's not reflected in the reading because it hasn't updated.
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runner said:
Hi, I’ve been having the same problem and suggested to the tech team this might be an app problem,
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I’ve observed this too after upgrading to iOS17. It never happened before that.


Upgrading to the latest version of 17 (I’m currently on 17.2.1) seems to have largely fixed it for me. I can’t remember the last time my graph ‘froze’
 
Hi, if you don’t mind me asking, what IPhonrvare you using? My x only goes up to iOS 16.7.5
 
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