Freestyle Libre Sensor: highly inaccurate??

ECDRUM · Jul 31, 2018

BG and interstitial glucose are not likely to be the same when taken at the same time. Interstitial will lag behind BG by 10 to 15 mins typically so if BG is rising it will read higher and if it is falling interstitial will read higher. This is however not a linear correlation and other factors such as activity level and local inflammation at the sensor site for the first few hours after application can all have effects. I find that when blood glucose is abnormally high or low Libre tends to exaggerate the effect in its readings and in these circumstances it is wise to do a BG test. I find the great value in Libre is in showing the direction of travel and the convenience of being able to scan 30 times a day or so to recognise trends and micromanage bolus injections to keep within normal parameters ( in my case I target staying within a range of 4 to 10 with a target average around 7). If I can achieve that this results in an HbA1c of about 6. Without Libre the best I could achieve was 8. It has been a game changer for me but it is not a total or direct replacement for finger prick testing: they measure different things and are not interchangeable.

Listlad · Feb 23, 2019

Scott-C said:
My post was intended to answer your question about why people generally attach a while before activating, not why your particular sensor is dubious.

The American version, which was only approved by their FDA a few months ago, has a 12 hour warm up time for that reason: letting it settle down to accomodate natural repair mechanisms and foreign body responses - looks like they've learned from their European forebears who've been using it for a couple of years now.

I've been using libre for about 18 months now. For the last 5, along with a small bluetooth transmitter which turns it into cgm and lets me iron out the sort of inaccuracies you're seeing.

I've used getting on for 40 sensors. I'm not going to pretend they are perfect. Most are ok, but some are plain sketchy. Abbott have normally replaced those. Maybe you've had bad luck and got a sketchy one.

It takes a few months of using them before you can make a judgment call on whether it's a "good" or "bad" sensor.

The science going on here is hugely complex: an enzyme on the sensor filament, glucose oxidase, is breaking down passing glucose into other chemicals, giving off electrons, which are measured as an electric current, represented as a number, which is then turned into a guess about what that number means as a bg level.

What could possibly go wrong in that situation? Obviously, lots of things.

I'm T1. I need to inject insulin several times a day to not die, and some of the judgment calls I make about those shots, if I get them wrong, might put me in a situation which would make a bad lsd trip seem pleasant.

Despite libre's imperfections, I've figured out ways to use it in ways which make my life significantly easier. Heads up on hypos, for a start, and quite a few other things.

I've got very little patience for non-T1s who use it for a few days and complain about it, "not being the same as my meter". They have failed to understand the complexities of it.

You might just have a rogue sensor, in which case Abbott will likely replace it. If you don't trust it, stop using it.

Am plodding my way through this thread.

Great post. Except I disagree with your (later) assertion that it is overkill for Prediabetics to use this device.

Listlad · Feb 23, 2019

Scott-C said:
There's a few companies working on these things, Abbott, Dexcom, Medtronic, and some others who are not yet on market.

They are all trying to make their products better but are working against huge biological problems which are inherent in all attempts to do what seems like a simple thing, measuring in vivo glucose, but is actually really, really difficult.

You're tag says you're pre-diabetic. If you're not even diabetic at the moment, testing with cgm is overkill. Wait until you're injecting insulin and then worry about it.

Blood sugar levels are a constantly moving target. Yours are still being managed by your pancreas. Mines aren't, so I have to do it myself. Cgm, despite what you perceive as inaccuracies, helps me a lot with that in ways you won't understand until you're in that position.

These things are cutting edge, they still make mistakes, but they're the best we've got for the time being, so we make the most of them.

Overkill for prediabetics ? Anything that will give frequent data points that relate to blood sugar levels will be beneficial to a Prediabetic. Or shouldnt we bothered about that?

Scott-C · Feb 23, 2019

Listlad said:
Overkill for prediabetics ? Anything that will give frequent data points that relate to blood sugar levels will be beneficial to a Prediabetic. Or shouldnt we bothered about that?

If it works for you, go for it!

Reading the thread back, I was getting a tad annoyed with the op's attitude of more or less writing libre off as a waste of time and money after his huge experience of, erm, three sensors, despite several posters, including myself, pointing out ways of dealing with that, such as replacement, calibrating in his head, and calibrating through add ons and apps.

Listlad · Feb 23, 2019

Scott-C said:
If it works for you, go for it!

Reading the thread back, I was getting a tad annoyed with the op's attitude of more or less writing libre off as a waste of time and money after his huge experience of, erm, three sensors, despite several posters, including myself, pointing out ways of dealing with that, such as replacement, calibrating in his head, and calibrating through add ons and apps.

Okay. Understood.

I can see it would be a useful tool for T2s and Prediabetics. Though I can see it might be particularly useful for T1s.

Joesdad · Mar 10, 2019

I'm a first time poster here.
My son is 6 years old, diagnosed T1 in November 2018, and has been using the FS Libre for nearly 3 months now. Up to now they have generally been fine. On Thursday we changed the sensor 3 times, and the readings were 2-3 mml different to the blood prick readings. On the first one, we put it into the side of his arm and not the back. I read somewhere that if it goes into a muscle it will be inaccurate. So we changed it. The second one, there was a bleed when we applied the sensor, so we thought it was because of that. So we put a third one on, another bleed and again a difference of 2 to the blood test. We called Abbott and they suggested waiting 24-48 hours, after a bleed. So we are waiting, while in the meantime, can’t send him to school and expect teachers to do blood tests instead using a simple scanner. (Before anyone calls social services - He does have a careplan, and we and school keep to it rigidly, but we rely on the libre to make it work. DS's teachers are trained in doing blood checks but we don't want to drive him mad.)

Have any of you ever had this experience? Firstly, how common is it to bleed when applying the sensor? Secondly, does a bleed affect the accuracy of the sensor? And, if bleeding does not affect it, we had three different sensors giving wild results!!

The thing that is confusing us here, is that we've used probably 5 or 6 sensors to date, and they've all been fine. Now suddenly, we have 3 that are faulty?! For us, we don't want to finger prick a 6 year old constantly, the libre has been a lifesaver, but now we feel we are driving on a dark road without any lights, as we are giving food and don't know for certain where his levels are.

deszcznocity · Mar 5, 2020

Joesdad said:
I'm a first time poster here.
My son is 6 years old, diagnosed T1 in November 2018, and has been using the FS Libre for nearly 3 months now. Up to now they have generally been fine. On Thursday we changed the sensor 3 times, and the readings were 2-3 mml different to the blood prick readings. On the first one, we put it into the side of his arm and not the back. I read somewhere that if it goes into a muscle it will be inaccurate. So we changed it. The second one, there was a bleed when we applied the sensor, so we thought it was because of that. So we put a third one on, another bleed and again a difference of 2 to the blood test. We called Abbott and they suggested waiting 24-48 hours, after a bleed. So we are waiting, while in the meantime, can’t send him to school and expect teachers to do blood tests instead using a simple scanner. (Before anyone calls social services - He does have a careplan, and we and school keep to it rigidly, but we rely on the libre to make it work. DS's teachers are trained in doing blood checks but we don't want to drive him mad.)

Have any of you ever had this experience? Firstly, how common is it to bleed when applying the sensor? Secondly, does a bleed affect the accuracy of the sensor? And, if bleeding does not affect it, we had three different sensors giving wild results!!

The thing that is confusing us here, is that we've used probably 5 or 6 sensors to date, and they've all been fine. Now suddenly, we have 3 that are faulty?! For us, we don't want to finger prick a 6 year old constantly, the libre has been a lifesaver, but now we feel we are driving on a dark road without any lights, as we are giving food and don't know for certain where his levels are.

I have this all the time. You are inserting a foreign object to your childs body and the immune system kicks in, hence higher values than finger prick tests. The results settle in after 3-4 days. Also, whenever you take a shower - libre goes crazy. That is why I NEVER give myself any insulin based on Libre reading. I always do a finger prick test and then decide whether I require insulin or not. As for the bleeding part, yes, I do get bleeding occasionally. It does not affect the functionality. Again - you are inserting a sharp foreign object into your childs body. Last thing, after heavy intense physical exercise Libre also can go crazy. Always finger prick to be sure - saves you from going out of your mind. Hope this helps.

kev-w · Mar 6, 2020

With regards to the Libre, I'm about 18 months in using one on prescription, I'm quite happy to bolus from it usually, I swim and train a few times a week and can't fault its effectiveness, I have in years gone by finger pricked a finger on each hand and had 1.5mmol difference on my accucheck Aviva at the same time, and from talking to other T1's find they've seen similar, with regards to bleeding it's hit or miss, I've seen some folk say "bleeders are better readers" but I don't know, it's less accurate when the blood level is changing rapidly as the sensor sits in 'interstatial fluid (sp) which is under the skin on the outside of the muscle so doesn't directly read the capillary blood, and they do seem to like 12 hours to settle from insertion.

Funny really in a non amusing way but my one week old sensor is probably the least accurate one I've had, I've a cold and trending a little high but yesterday I had an alarm go off telling me I was at 18mmol and a fingerstick said 12 and the closest it's been was 2mmol out so I'm going to blag the maker for a replacement and take it off a week early as it's a prescription item.

Neonataldiabetes · Jul 15, 2020

hankjam said:
This would be disappointing as the amount of R&D to bring this product to market would have been wasted. It has been found to be "a good thing" by a lot of forum members.
I would give it a couple of patch cycles and see how it goes.

£64 sounds better than I though it was, I was thinking it was around £100 at the moment. Been thinking about but don't have a smart phone, which would be an additional cost....

I get the full package, there's a scanner which collects all the 24 hour data, you can upload this to a cloud and share with your diabetes team, so no smart phone needed.

Kaycee · May 29, 2021

Ledzeptt said:
I’ve been using the Libre for about a year; so over 20 sensors including 3 faulty ones that Abbott replaced. A few points from my experience:

1. At present, Asda pharmacy are selling sensors for £44 IF YOU can show you have diabetes. Plus they can get them within 2 days (Abbott are taking 3 weeks to deliver.)

2. The first 24 hours of a new sensor is very inaccurate so I don’t activate mine for 24 - 48 hours, else it’s a waste of money.

3. Sensors do not measure blood glucose, but the software tries to predict a similar reading. There is often a 10- 15 min delay behind a finger prick blood glucose (BG) reading. This only matters if BG is falling/rising quickly. Sensors are also less accurate when having/close to a hypo, but at least I get more warning of an impending hypo.

4. Typically, the sensor reading is 0.5 to 1 mmol below my BG reading, so I see this as being on the safe side. I only had one sensor that reported higher readings than my BG meter and that was a faulty sensor.

5. I was in hospital a few weeks ago and had a particularly accurate sensor. It was usually the same or within 0.1 mmol of the hospital’s BG meter (that they calibrated every time) - very impressive!

Generally, I find the Libre to be accurate and “life changing” in managing my diabetes and also reducing stress/anxiety.

Ps. I self fund and have no connection to Abbott.

Hi. My libre sensor is Mike's out. It says 5.2, but my finger prick meter tells me its 10.
7, surely it should be that far out. I don't know what to do. I'm really confused as with everyone saying that the meter isn't a proper reading and could be faulty also the sensor could faulty as well. I mean, which is right.

Tramk · May 29, 2021

Hello, and welcome to the forum!
Suggest you perform a comparison check. Guess you measure your blood sugar by fingerprick about 1,5 hours after meal. As you must give the Libre a little more time, 15 more minutes, and then compare the results. The numbers will probably not correspond, but after some time you will get the picture. If they deviate more than say 2-3 mmol/l during the week, you should probably make a telephone call to Libre. Good luck!

Mr_Pot · May 29, 2021

I had a trial of the Libre 2 and discovered my bg stayed constant over most of the night. So I did a finger prick test in the middle of the night when any delay wouldn't make any difference and it read almost exactly the same as the Libre.

Brunneria · May 29, 2021

Kaycee said:
Hi. My libre sensor is Mike's out. It says 5.2, but my finger prick meter tells me its 10.
7, surely it should be that far out. I don't know what to do. I'm really confused as with everyone saying that the meter isn't a proper reading and could be faulty also the sensor could faulty as well. I mean, which is right.

All you need to do is contact the Abbott helpline.
They will ask you a few questions to establish that the sensor is malfunctioning, and then they send out another.
It is very simple and easy.

Peter03 · May 29, 2021

hankjam said:
This would be disappointing as the amount of R&D to bring this product to market would have been wasted. It has been found to be "a good thing" by a lot of forum members.
I would give it a couple of patch cycles and see how it goes.

£64 sounds better than I though it was, I was thinking it was around £100 at the moment. Been thinking about but don't have a smart phone, which would be an additional cost....

£64 sounds exspensive to me, I Don't use the libre anymore but they used to be €35 each I to found the libre inaccurate a lot of the time found it better with xdrip

BigDaddy2 · Dec 31, 2021

The published accuracy of freestyle is +/-9.2%
The contour one +/-8.4%

Bubbleblower · Dec 31, 2021

BigDaddy2 said:
The published accuracy of freestyle is +/-9.2%

According to this forum that is wrong, In Response has made many posts pointing that out again and again.

For many people (especially type 1s) the FSL is completely inaccurate. My time in range was 100%, which according to several studies means my HbA1c should have been well below 40 (which was also predicted by the FSL);

And this is for time in range 3,9-10, mine was set at 3,9-8, so I was even dreaming of an HbA1c of below 30.
But it came back at 47,9, my average glucose was higher than my highest glucose according to FSL.
I would have never believed this if it wasn’t from a state of the art lab.

After that disappointing result I did an 18-point finger prick test and found the biggest differences were postprandial; a meal that didn’t effect me at all according to the FSL send me to the highest peak of the day according to finger pricks.

Mungobean · Dec 31, 2021

I have found the readings can be way out at times (up to 6 Mmoll different). I was getting obsessed with the differences and my DN said to stop stressing about it and to just use the Libre readings; and then if you get a high or low reading, then double check with a blood prick. I find the Libre invaluable, especially for the trends, so long as you are aware that you need to check your lows especially. For me it is far better than getting sore fingers, as my BS fluctuates wildly, and so I was pricking many times per day.

I have found that some sensors are better than others. If you keep a record of your readings and your finger pricks and they are consistently very different, you could have a duff sensor; I have had some duffers and if you contact Abbott and give them your readings, they are very good at replacing the sensors.

I have also found that it helps to attach the sensor for 24 hours before activating it, and for me the readings tend to be more inaccurate at the very beginning and the very end of the sensor’s life.

I have found peace with my sensor use, in recognising that the readings rarely match a finger prick, but they do seem good at recognising trends - so you can see if you are going high or low, which I guess is the most important thing.

Tye1 · Nov 7, 2023

TedTomato said:
I will have to check over the next few days, but if it's giving random figures, higher by 2.5 mmol/L than my other blood testers, then this is useless, either for spot checks or for averages.

I have tested over the last 2 hours, and the sensor is still 2 mmol/L higher, so that's going to impact averages too if such inaccuracy is sustained for long periods.

Thank you for your comments.
I agree with you. The sensor is always, 2. mmol higher than a finger prick, which is nonsense.

Just wait until you go to the gym.

Every time I check my sensor it's as high as 18/19, yet my finger prick is 8.5.
If you, like me try to keep your numbers tight, then throw the sensor into the bin. How it ever got a license is behind me.

Freestyle Libre Sensor: highly inaccurate??

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