My Libre2 is always at least 3 up to 6 mmol/L lower than finger prick

[jackdinn]

Hi all,
Yea I'm sure you will say "You know that there is a lag between the interstitial fluid & blood?" and yea i know but I'm not talking about a lag I'm talking about a -5 mmol/L (yep -5 is quite common) continual difference.
Right at the moment the CGM says I'm level(ish) at just over 10 but the finger prick says im at 15.

It will remain 5 to 6 points lower than my finger prick for the rest of the life of this particular libre tag. The next tag maybe a little better it may be worse.

I have just started this tag, but it has been "soaking" for 2 days already.

(o yea the time on my CareSens was an hour behind the phones time in the first pic, i don't use it much at all and i didn't notice the summertime hour had not been altered)



I was going to post this question last month but my previous tag was actually pretty close to the finger prick (about -2) so i didn't, but this new tag is back to the normal -5/ -6

 

[helli]

These are high numbers. As described in this thread, Libre (and other CGMs) are designed to be most accurate at “normal” BG levels.
When my BG is above 9, I always check with a finger prick because it is like walking through London in the smog relying using a CGM.
The other thing to bear in mind (as the referenced thread explains) is that Libre accounts for the delay between BG and ISR so is not the explanation for a large variation unless the current trend has changed direction.
Finally, finger prick meters only need to be within 15% of accuracy. So the variation could be a combination of the two. For example, looking at your first picture, if your “true BG” is 13, both the Libre and the Caresense meter are within 15% of this.

 

[jackdinn]

These are high numbers

Not according to the libre which i live by. My Libre is now my religion. I continually watch the graph (and i mean continually), trying to catch massive drops (for no reason) before they get down to low and trying to push it back up, or down with micro doses when its high here and there. Its an impossible job, its like sailing an ocean liner down a narrow river, everything you do takes at least 40 minutes to have an effect so it ends up over reacting, so i end up chasing the graph up and down up and down. But its literally terrifying when you see that you just ate your dinner and the graph is "tipping over" the normal upward trend and you know its just about to plummet!
So yea I'm attached to the CGM every waking minute so when its 5-6 point lower than it should be it all become academic. But i cant go back to finger pricking as my fingers wont take nearly 1000 pricks a day (as thats how many time i look at the graph according the the libre data).

I nearly died with ketone acidosis and hyper a couple of years ago, i came out diagnosed as T1 at my age they said was very rare (haa , yea tell me about it)

I still don't know what I'm doing apart from dying if i let that graph drop to far so i watch it like a hawk.

It looks like when i am inside the target range according to the libre I'm actually not.

Its all terrifying.


One of my "normal" days (just a couple of days ago) where the exact same food, exact same novarapid that normally balances out but not this day!, lucky i was watching as it tipped over the top of the food raise. It got to 2.2 (because of course it did, its the Libre). But if i had not started stuffing "Lift tabs" as soon as it started god only knows what would have happened.

 

[helli]

That is why it is recommended to check highs and lows with finger pricks before treating (unless you are obviously hypo).
Please read the thread I referenced and learn the limitations of CGMs, especially if it is your religion. You will continue to be very frustrated with it if you expect it to be perfect.
I also recommend this article to help explain why “exact same food” does not always have the exact same BG affect.
Managing diabetes is an art not a science.

 

[jackdinn]

I have checked this many times before, the Libre graph will remain -5 regardless of the finger prick, next time I'm at 5-6, the finger prick will show 10+

 

[Jasmin2000]

The other thing to bear in mind (as the referenced thread explains) is that Libre accounts for the delay between BG and ISR so is not the explanation for a large variation unless the current trend has changed direction.

I've heard this before but I still don't understand it - do you mean there is no delay to be accounted for when I compare a finger prick to the Libre at the same time? I've always seen my Libre catch up to the finger prick value after 10-20 mins.

 

[helli]

I have checked this many times before, the Libre graph will remain -5 regardless of the finger prick, next time I'm at 5-6, the finger prick will show 10+

This is a different problem to the examples you have given in this thread so far. The examples you gave above can be explained by the limitations of CGMs and the 15% accuracy of meters.
If your CGM is reporting 5 when finger prick is 10+, this is likely to be a faulty sensor and should be reported to Abbott.

 

[helli]

I've heard this before but I still don't understand it - do you mean there is no delay to be accounted for when I compare a finger prick to the Libre at the same time? I've always seen my Libre catch up to the finger prick value after 10-20 mins.

The algorithm used to convert from ISR to BG will extrapolate the current trend and “predict” the current BG.
If the trend changes in the last 10 to 15 minutes, Libre will correct its prediction when it catches up. But you may see it over estimate highs and lows for this reason.
There are many reasons why your finger prick and CGM may differ and it may appear to be Libre “catching up” when it is another reason.
This is explained in the thread I referenced above.

 

[jackdinn]

I shall post again once I've got my BG down to 5-6 which could be a good while since iv just eaten my lunch and i generally do not push it down aggressively as its just to scarily easy to send it down like a brick.

 

[Bruce Stephens]

I've heard this before but I still don't understand it - do you mean there is no delay to be accounted for when I compare a finger prick to the Libre at the same time? I've always seen my Libre catch up to the finger prick value after 10-20 mins.

I think it's a duff sensor.

So get yourself to something reasonable on test strips (and presumably the Libre will be saying you're insanely low) take three pairs of readings (showing the difference between Libre and test strips) and report it to Abbott as a defective sensor.

 

[jackdinn]

All this talk about "defective sensors" is nice and all but i have been watching these tags and comparing them for a while now. I hope 70% are not defective.

 

[Jasmin2000]

The algorithm used to convert from ISR to BG will extrapolate the current trend and “predict” the current BG.
If the trend changes in the last 10 to 15 minutes, Libre will correct its prediction when it catches up. But you may see it over estimate highs and lows for this reason.
There are many reasons why your finger prick and CGM may differ and it may appear to be Libre “catching up” when it is another reason.
This is explained in the thread I referenced above.

Thanks @helli - that makes sense and explains both the corrections in Libre and the differences between Libre and finger pricks when rapid changes are occurring. My issue is with apparent hypos on Libre that I don't see in finger pricks - usually on a rapid drop that I catch with glucose in water but Libre doesn't know the last bit and will give a hypo reading.

 

[rebrascora]

I have a Caresens and Libre always reads lower than it but generally no more than 1-2 mmols. If it is more than 2 mmols then I woud report it to Abbott customer services and they will usually replace it. Being out by more than 3 is absolutely unacceptable. However I make a point of doing my double checks when my levels have been stable and mid range for at least half an hour and very rarely is Libre more than 1mmol lower. I do wonder if Caresens test strips read high as I bought some Freestyle Optium test strips to use in my Libre reader and they always read very slightly lower by one or two tenth of a mmol, than the Libre sensor, which was quite a surprise. I still go by my Caresens as that is my "official" BG meter and my hypo awareness is pretty well tuned into it and I feel hypos between 4.3 and 3.8 (finger prick). If it is wrong and Libre is right then I am sometimes not feeling hypos until 2.8 which I very much doubt.

Anyway, my main reason for commenting on your thread was that some people find that Libre doesn't suit their body chemistry and Dexcom aligns better for them, so if this is a consistent problem then ask to try a Dexcom One instead. As far as I a concerned, Libre works brilliantly for me and 1mmol out is not a problem, but if it is consistently 3 or more out for you then you really need to try something else.

 

[jackdinn]

@rebrascora thx, yea its a possibility however the NHS do not do Dexcom or any of the others, libre3's just about maybe i think.
Then there's all the hardware and software that i have built and scripted to enable me to be surrounded with my Libre data, graphs, technical indicators and alarms. Taken me half a year to build stuff that mostly works from the LibreLinkUp API, so changing would be a right PITA.

But sure, Of course if that's whats needed.

thx.

 

[Bruce Stephens]

I hope 70% are not defective.

They aren't. People tend to talk about things when they go wrong. My last sensor was just fine (and so were the previous few; I don't remember when the last bad one was). The new one is starting up but I fully expect it to be fine.

Some people find that Libre doesn't work well for them. Some find that another one (Dexcom, usually) does. A few find that no CGMs seem to work.

 

[Jasmin2000]

I think it's a duff sensor.

So get yourself to something reasonable on test strips (and presumably the Libre will be saying you're insanely low) take three pairs of readings (showing the difference between Libre and test strips) and report it to Abbott as a defective sensor.

Libre and finger pricks correlate at stable BGs, but when rapid changes are occurring Libre can read 3-3.5 when finger pricks are 4.5-6. For me this difference can continue for 30 min despite the Libre algorithm, so there's some technical issue at the low end. This happened with many sensors so it's not a single duff sensor, it's a failure of the tech itself or possibly low glucose pockets in ISF where the sensor it placed.

 

[rebrascora]

Thanks @helli - that makes sense and explains both the corrections in Libre and the differences between Libre and finger pricks when rapid changes are occurring. My issue is with apparent hypos on Libre that I don't see in finger pricks - usually on a rapid drop that I catch with glucose in water but Libre doesn't know the last bit and will give a hypo reading.

Do these hypos show up on your graph afterwards or does te graph not dip into the red. This is typical of the algorithm over extrapolating the downward trend and then changing the graph line wen it gets more data and realises levels have gone back up. If you can prevent the rapid drops then you should see much less of this. The way to do this would probabl;y be to work on your prebolus timing so that the insulin and carbs are arriving in the blood stream at about the same time rather than the carbs releasing glucose into the blood stream and sending your levels high and then the insulin kicking in and dropping you fast. If you can get the timing better, you don't go up as high or come down as fast.

Just as an example.... I used to prebolus the usual 20 mins with NovoRapid for breakfast when I was first diagnosed. If I was starting in range, my levels would shoot up to 15 every morning and then come hurtling back down to about 5 a couple of hours later. It didn't feel very p[leasant and it wasn't doing me any good. I was finger pricking at the time as I didn't have Libre but I did lots of testing and established that I needed 75 mins prebolus time at breakfast with NR to get a relatively small hillock instead of a church spire in BG levels. As a result I changed to Fiasp which is a bit quicker but I still needed 45 mins most morning prebolus time before eating my breakfast (usually yoghurt and berries and seeds) Other times of the day I need less and these lengthy timings for breakfast are extreme and most people would hypo in that time, so you have to start with the timing you currently use and increase it by a few mins each day until you find the balance point for you. Once you can stop the peaks, you can slow down the drops and it becomes more stable. It also helps me that I follow a low carb way of eating so the few carbs I eat don't spike me too high and the glucose release from the protein slows down any drop, essentially buffering my levels as protein releases over about 6 hours, rather than the usual approx. 90 mins carbs release glucose.

Dexcom One IS available on the NHS and a direct alternative to Libre 2 so speak to your DSN if you decide to try something else, but I understand that the hardware and software set up will make you reluctant to do so and your body might not be any more compatible with Dexcom although usually people find one or the other works better for them.

 

[Bruce Stephens]

The new one is starting up but I fully expect it to be fine.

It is.

 

[rebrascora]

Libre and finger pricks correlate at stable BGs, but when rapid changes are occurring Libre can read 3-3.5 when finger pricks are 4.5-6. For me this difference can continue for 30 min despite the Libre algorithm, so there's some technical issue at the low end. This happened with many sensors so it's not a single duff sensor, it's a failure of the tech itself or possibly low glucose pockets in ISF where the sensor it placed.

It is just due to the algorithm. Once you understand that it is perfectly workable within a couple of mmols but if you are talking more than 3 mmols out, that is too much to work with I think and you could end up chasing the dragon's tail, causing a rollercoaster effect.

I personally don't see these sensors that the OP is having problems with as defective as such especially as they are getting a fairly consistent discrepancy, but just that I think their body chemistry doesn't suit the Libre system.

 

[rebrascora]

I can't remember the last time I had a problem sensor. My early difficulties were mostly ensuring that I didn't knock them off. Maybe one sensor in 4 years that has been more than 3 mmols out.