FUDiabetes

What effect should sensor calibration have


#1

I use the enlite cgm sensor and freestyle libre, when I calibrate my enlite it has no difference on my readings. For example my cgm was reading 97 and my libre 112 when i entered the libre value it had no effect on my cgm value. Is this normal, would think it would increase value toward calibration value. Not really concerned as cgm is lower and graph lines reflect libre lines. Just curious.


#2

@T1john, I am not sure about the enlite calibrating algorithm—with the Dexcom, you would expect to see a change in the next value (but rarely to the number you entered), but that is not 100% true.

I would recommend to not use the Libre values for calibration, though, only a hand glucose meter. Both the Libre and the Enlite use interstitial fluid, so they need correction, when they do, from a blood-tested value like a fingerprick.


#3

I fingerstick a lot and only use than when making dosage desicisions. But libre is really close.i know it lags on rapid rises and drops but try not to have them. I stay pretty close to my target of 90 most the time and inevitably go lower which causes my cgm to alarm low which is so irritating. Guess I could drop low alarm value but would be nice if when I calibrated it would get closer. Even tried multiple calibrations. Guess I may have to just deal with it.


#4

@T1john, what are you doing up at this time of the night? :slight_smile: I find that our Dexcom sensor is right on 50% of the time, and 15-20 low the rest of the time.

I think it is probably on purpose: it would be really bad if your sensor reads high, since you would then get low without knowing about it. I figure that they slightly skew the readings towards lower rather than higher, so that the sensor runs less of a risk to read high.

What we do with a high-reading sensor (even after calibration): we set our low alarm a bit lower. For instance, we may take it down to 65 instead of 75.

Re. your source of calibration: I am sorry to be insistent. I speak as a scientist here, not as a fellow forum member: calibration is needed to link the CGM (and the Libre when needed) to an independent measure of blood glucose. It does not matter, mathematically, if the Libre is “close:” when you use the Libre, you create a dependency loop between your two CGMs that does not improve either’s readings, and that may cause them to diverge from reality with time.

Imho it is an unsound and dangerous practice. I am truly sorry for disagreeing with you so strongly—I would be very concerned if someone read this thread and felt it was a practice they should adopt:-)


#5

Vampire wakes me up at 3:30 every night to draw blood. Can’t go to sleep.
And I agree totaly, don’t trust neither when rising or droping. I use to see when I start changing then fingerstick for accury, probably 10+ times daily. Haunt fluctuated more than 30 point in mabey past 25 hours. Mabey not that long but at least 12. Yesterday kind of blur cat lab twice, ct"s mri, was put out 3times for one test or another, will probably get cussed out if I get caught doing this.


#6

Just want to add, I went into sensor settings and changed low threshold and found a calibrate option, used it and it worked. Before I was using capture event interning bg then selecting save and calibrate which did nothing.


#7

I just wanted to tell you guys I also have used my Libre to calibrate another CGM. I probably did it maybe 3 or 4 times with the Guardian. @Michel, I absolutely agree it’s not a great practice as the Libre can’t replace a regular finger stick, BUT I felt differently about my overnight readings. In general, my blood sugar overnight is relatively stable or a slowly changing one. During periods of stability or relative stability, all of my sensors have provided good values. I would never do this during the day because my blood sugar is prone to more changes, but if it means quieting a demanding Medtronic CGM in the middle of the night, I really do feel fairly confident in handling it that way. I also tried, to the best of my ability, to calibrate right before bed so as not to need to do this often.

That’s it. :grin:


#8

Nicky, I hesitated greatly before answering your post! I was not quite sure how to address it. I know you did not write it as an encouragement to calibrate a CGM with another! I am concerned how some readers may interpret it though :slight_smile:

Before I go further I should discuss on what grounds I am discussing it. I am a control engineer: my field is the study of systems, more exactly the inputs you need to give them in order to obtain outputs you want. Examples are how you guide a missile, how you build an oven to keep even temperature, and how you design a CGM. I am very familiar with CGM algorithms and I have been, on my off time, studying how to improve open source CGM algorithms (I have a PhD in my field and my specialty, Model Reference Adaptive Control, is directly applicable to CGMs). My grounds for discussion are—does it make sense from the CGM algorithm’s point of view, and how bad can it be?

A CGM that needs calibration is designed to use calibration from one type (meaning one should not use several different glucose meters to calibrate the same sensor) of manual glucose meter testing blood. Anything short if that is damaging to the algorithm, sometimes severely damaging. Does it mean that ONE calibration from an inappropriate source could cause trouble? In some circumstances, it can. Saying that it is OK to use a Libre as an input is akin to saying that you have, in the past, set up a pump site without sanitizing the site and nothing happened to you. That may be, but could something bad have happened to you, and is it a recommendation that you should make to others? Even worse, will it encourage others to engage in that habit regularly (which would certainly turn the CGM into a useless device)?

I could go in detail as to why it can destabilize a CGM to do so—it would be long and involved… There is a saying in software: GIGO, Garbage In Garbage Out, i.e. if you feed an algorithm poor data, you get worthless results. Feeding the output of a CGM to calibrate another CGM does just that. Is it worth it when the cost of doing it right is so low: a fingerprick :slight_smile:

My point here is: it is not OK to use a Libre for calibration. Can you get away with it if you do it very rarely? Possibly, but by doing so you are voluntarily worsening your CGM performance and, in some cases, creating the risk of severe undetected lows. Would you recommend to anyone that they should set a new pump site without sanitizing? There are possibly some rare cases where that would be appropriate, but they are few and far between. And, for sure, it is not something that should ever be encouraged on a regular basis at all. I am concerned that someone who reads this thread could see it as a practice to consider: it can easily destroy the accuracy of a CGM and lead to dangerous treatment decisions. It should be a measure of last resort.


#9

Of course not.

So a little more qualified than I am. My qualifications are: stay at home mom, wearer of CGM, wearer of Libre, diabetes enthusiast. :smiley:

And scientist at heart… to give myself some credit. :smiley:

To this I’ll say I do use one meter, but that’s by pure luck. This may very well be the case, but I’ve never had a medical professional specify it. I use one meter because that is what makes sense to me as far as reliability. Because I do test my fingers many, many times per day, I’ve learned a lot about my meters: their patterns, their quirks… I have one I trust, and I go with that. Whatever its problems are, and there will be some, will at least be consistent from one calibration to the next. My point is, I get it, but I never followed it as a rule.

I’m not sure why I jumped on that particular point. I read your response and was a little confused by the message. I absolutely love FUD, I constantly refer to it in conversations in my group, linking threads where I can, but it is the explorative nature of this site that I have found the most appealing. The responsible exploration and the quality discussion that follows. Not everyone will agree with a lot of what they read on this site as being appropriate or correct or proper. Grab an endo off the street, I’m sure they could find something that could cause them to pass out. Grab a Medtronic employee… there’s more than enough in here to throw a rule book at someone’s head. There’s information about letting insulin bake in a car, about re-using infusion sets, transferring insulin from one container to the next, mixing insulin… IV shots and IM shots and higher basal rates… and the list goes on and on. But to back every one of these topics is that discussion that’s so important. It’s a site of possibilities with enough dialogue to allow for thorough consideration. Of course I didn’t mean to encourage people to calibrate their CGM with their Libre. I said I’ve done it, but I do not make a habit of it. I mentioned it for a couple of reasons… I mentioned it because it was relative and because I did have an explanation that was based on understanding of CGM behavior. There is another member on this site whose CGM does require calibrations at any given hour, and if you’ve never used Medtronic, you might not quite understand that. I just took out my 23 day old Dexcom (which, again, I would not encourage of others), and there were almost 15 days straight where I did not need to calibrate at all. It’s a completely different experience. I was trying to communicate to a fellow Medtronic user that I understood what those 2 am calibration requests could be like and to acknowledge the hassle in them. I understand a finger stick is ideal. That was never the question. For someone who saves their tubing so as not to waste the 10 units of insulin in it, I don’t think it’s because they don’t know it’s ideal to use from a fresh source. I think they’re making due with their situation. I absolutely agree with you that it is not a good practice. It has taken me this long to feel like my Dexcom is somewhat reliable… and even at this stage, I test upwards of 20 times a day. I was only trying to acknowledge the difficulty and to share my own experience. I have done it for the sake of sleep, and if I had not done it, my pump was going to vibrate and alarm until I did. And, as I said above, it was not a thoughtless action but an action selected from my current understanding. Because I do test and log and track these sensors like I do, a behavior that spawns from a desire to understand them, I know my blood sugar is relatively stable at night, and I know my sensors are good when I’m stable. So it was a if you can’t be with the one you love, love the one you’re with kind of solution. Making due. I would be happy to slap all over the statement how I do not practice it because I don’t think the Libre, or any CGM, is always dependable. Not for calibrating, not for treating, not for eating a donut.

I can’t remember if you guys ever used Medtronic, but I don’t think you have. The Medtronic sensors, especially the Guardian, really promote some bad behaviors. Truly. If an overnight calibration based on the Libre sounds dangerous, imagine people fudging values… or imagine people feeding back to their pump their sensor glucose because it is the only way to get it to take. It’s an art trying to get some of these sensors to behave properly, and people are probably shooting themselves in the foot with their efforts.

On this, we 100% agree. For the record, I do not think it’s a safe practice, and I’m sorry I made it sound like I did. I was only engaging in discussion because it’s what I do on this site. I hope I did not fail in that part because it is in the discussion where there lies information.

I’m rambling. @Michel, I mean you no disrespect, and I genuinely hope this isn’t received as argumentative. I think you know how much I enjoy this forum and how much I’ve learned from it. The thing I’ve learned above all else is how to be an open thinker, an explorer, and an engaged diabetic. I would feel terrible if I contributed to another member’s experience in any way that was less than that.


#10

In addition to what she mentioned of Libre, Nicky is also a former user of the 670G, a former user of the Enlite cgm, and a user of Dexcom. In the past 6 months she has tried every pump and also done MDI, and she does almost as many BG checks as I do. And the biggest thing, she is a diabetic herself.

This makes her extremely qualified for this discussion and gives her a far greater understanding of the practicality of all of this.

Her recommendations are sound. At 3am, if you have grown comfortable with your Libre readings, and are comfortable with being flat and stable at night, what she is saying is fine. Always having to to a BG check at that time is not ideal. She is not giving dangerous advice, just a practical workable solution that she is saying can be done at times.

The whole argument is funny. The closed loop pumps will all be using CGM values to automatically make decisions for people at 3am. They won’t make you calibrate from a BG reading at 3am. If you don’t think a Libre value can be trusted at 3am, why would you trust an Enlite value or a Dexcom value to make pump decisions at 3am for you?

Is doing this perfect and ideal? No. Nicky never said it was. But is it workable and practical? Yes. And I think she was clear on that.


#11

Not really an argument, since both sides are bending over backwards not to step on each other’s toes. I appreciate hearing both points of view because you need some back and forth to get the ideas out. I’m glad Michel posted, because as a new cgm user, I didn’t know the importance of using only a single meter, and his info makes perfect sense.

We all need to learn which rules we can bend, and which we shouldn’t. The good thing about the G6 is that it almost never requires a calibration, and maybe Michel can let me know if my assumption is correct, that if the cgm reading is close to finger stick, then there is no reason to calibrate, and it is probably better to avoid it unless required to?


#12

Yes, this is great advice. Honestly, we have been using the G6 for a few months now, and have had 0 calibrations. Now I will say that the Dexcom’s for us have been easier to use than most, but adding external data when it isn’t needed is not a great idea.


#13

So sorry about that! I agree 100% with your post. Imho, much of what makes our strength is our willingness to experiment and share our results. And—I felt that your previous post was also exactly in the spirit of FUD.

Because many of those who read our threads may only read one thread or two (for instance if they landed from a google search), when the topic of the thread can lead a less-informed person to some trouble (for instance how we treat insulin, how we stack…) it is sometimes necessary to add caveats to the thread, so as to make extra sure that nobody will get in trouble if they don’t quite master all the context required. My last post is just that and nothing more—a caveat to make sure that even a casual reader understands the context and the dangers involved. Such a caveat is sometimes needed, and level3 members often add one to threads that may put someone at risk. When that does not happen in the course of a thread, a mod might add one as well.

Of course not :slight_smile:

I only have a long answer for that one :slight_smile:

First, I agree with @Chris on the G5. I have experimented with both calibrating every time, and calibrating only when the CGM is off: both cases have worked equally well, which tells me that the G5’[s algorithm does not need calibration when it is right on the money. By right on the money, I mean roughly within two units.

I also know for a fact that calibrating too often actually worsens the G5 behavior: I have destabilized a G5 by calibrating it 4-5 times in a short period of time.

At the same time, as for us, we still calibrate twice a day including when we are right on, despite the fact that the G5 does not appear to need it. The reason, for me, is to keep my son algorithm-agnostic. It is possible to design CGM algorithms that behave differently if they are not calibrated regularly. For instance, if you were the little guy inside the CGM, and nobody told you anything when you were right on, but they only corrected you when you were off, you may assume that you were not close before (when you were not given a calibration), and overcorrect towards the calibration points that are given to you. Since there is a large amount of noise inside each glucose meter, this would result in worsening the performance of the CGM. As I don’t know what algorithm behavior my son’s next CGM may have (or if Dexcom may someday change its present behavior), I figure it is best to let him develop a habit that will work whatever CGM he has. Sorry for the TL;DR…


#14

No worries. :slightly_smiling_face: