CGM limitations and precautions

[christine.h]

Moderator Note: This helpful reply was copied from another thread as it details some of the commonly experienced limitations of continuous glucose sensors.


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.

Really grateful.for that information I am on the Libra 3 as the 2 would not work with my phone

 

[TonyB]

Moderator Note: This helpful reply was copied from another thread as it details some of the commonly experienced limitations of continuous glucose sensors.


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.

So much to learn and take in

 

[Miggs]

Moderator Note: This helpful reply was copied from another thread as it details some of the commonly experienced limitations of continuous glucose sensors.


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.

Understand..n no worries re waffle..it isn't at all..just wonder, who sleeps in one position all night..lol??
Honest, if sensor in right arm..it causes soo much lost sleep, trying not to turn that way..or left side, if applicable..best night sleep, when sensor in tummy position..n the little alarms in night, not hi or low, but lost connection, cos , I was sleeping soundly, on the sensor., until the #### thing, let me know it felt a bit squashed!!!!

 

[Miggs]

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!

What cgm do you use?

Just curous, what with the issues that libre seems prone to, and the choice with dexcom 1, how many people are using each?

forum.diabetes.org.uk

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".

Oh my, agree, been pump user for 8 yrs, cgm, just this yr(2024). Soo much important info, Not provided, at training day. I'm still shocked..have spoken to sev T1 nurses re cgm tech..n will continue, to do so. Just wish, they had T1 users of the tech, in on the newbie, training sessions.