ISIG and Calibration Factor: Using my Sensor Raw Data as part of Blood Sugar Management:

I know most of you are Dexcom… as I said before, you’re a sharp group. :smiley:

Yes, I’m waiting for Eric to tear it up… I’d like to say I’m ready, but I have a feeling that doesn’t exist.

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I am not much of a Dexcom guy. Pretty much anyone here is more Dexcom than me.

I don’t put a whole lot of stock into what it says. I like to play a game with it. I always guess my BG, then do a BG test on a meter, then look at the Dexcom. Who is closer to the meter reading - me or the Dex. It’s not even a close competition.

I think you will get far better Dexcom advice from anyone else here.

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So I now have to wonder whether Medtronic’s use of the term “calibration factor” involves SG or does it involve BG. I haven’t found a definition direct from the horse’s mouth, but I may have found a decent proxy in a paper that used the Medtronic Enlite 2; in the acknowledgments section they credit four Medtronic employees for the algorithm, training, and some data analysis, so I suppose this paper defines “calibration factor” same as Medtronic. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4991614/

The paper contains this statement: “Sensor glucose levels were calculated based on a calibration factor estimated from the slope, a linear regression, with intercept set at 0, of plasma glucose and filtered sensor current with a fixed sensor delay time of 10 min to account for delayed BG shifts between the IV and interstitial compartments.” They also say “Calibration factor was 7.692 ± 3.786 mg/nA·dL (target = 1.5–20 mg/nA·dL).” Combining these two, I think they are saying SG = CF x iSig, rather than BG = CF x iSig.

I apologize for being petty with the investigation of how Medtronic defines calibration factor, because the actually useful information is that Nickyghaleb has described how, by keeping track of the relationship between iSig and fingerstick BG, she can get information that is additional to the SG, and that helps her make better treatment decisions. And that’s what actually matters.


Ooh! I love these. :smiley:

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This is interesting to me.

The sensors were recalibrated 8.3 times/day;
Overall performance of the Medtronic Enlite 2 CGM in the post-transplant population was reasonably good with “no risk” or “slight lower” risk by SEG analysis and high CGM-YSI agreement by CEG analysis; however, frequent recalibrations were required in this intensive care population.

We are told again and again by the Dexcom overlords, “Don’t over-calibrate!” And things like, “Too much calibration makes it less accurate!”

But on these intensive care patients, it says, “…frequent recalibrations were required in this intensive care population.” And they calibrated over 8 times a day.

Yes, I know it is the Medtronic CGM and not Dexcom, but it’s essentially doing the same thing.

So is over-calibrating not really as bad as they tell us?


I need to go back and reread what you’ve written, but i’ll add quickly that when it comes to whether or not calibrations or even “BG Required” BG entries (exclusive to 670G users but be careful that in asking you don’t get their pump thrown at you), that calibration factor that’s being considered is the one calculated from the SG. I ALSO believe it’s part of the calculation, the SG/ISIG value, that determines the size of the micro bolus. I know you guys don’t care about the auto mode micro boluses, but i fully understand the size of about 90% of them because it makes sense based on the SG/ISIG calibration factor. That’s part of the problem. Calculate the two side by side, the SG calibration factor and BG calibration factor, and you see a completely different picture. If they’re running their delivery based on that SG one, it can take forever before it reflects what’s really happening. It’s also why i think people get micro boluses at very low blood sugars.

Now i’m going in 12 different directions. I have been calculating the heck out of all of these things and tracking them closely for almost 9 months. It’s taken me a REALLY long time to understand what I was seeing. Then it took even longer to trust it. Then I threw myself in completely and just went with it. It guides me as often as I’ll let it. Unfortunately, it won’t make me not diabetic, so it’s only worth what i am willing to do with it, but isn’t that true of life.

I was so nervous posting last night that my skin hurt. I said i was looking for traction but wasn’t even sure what I meant. I still don’t know, but I’ve gotten it. :smiley: I think it’s the kind of questions that help me with my journey— even the ones that challenge it. Anyway. Thank you for responding. :slight_smile:


I mess with my calibrations in a shameful way. I do it when i’m testing the calibration factor stuff. This is absolutely not meant to be used as guidance in any way, but I’ll just say I sometimes calibrate 4 times in a row because i need that factor to get caught up so i can get insulin. I’ve never seen my accuracy take a hit or any other problems with the sensor OUTSIDE of the normal, very annoying delay.

The other day, I pushed it to the limit, and I even knew I was, and I finally saw my sensor just be OFF for a couple of hours. But what I was doing was so far outside of anything we’d do normally while calibrating, that it didn’t surprise me. I think it was 5 calibrations in a row, and I was testing both the 35% rule as well as the 2-8 calibration factor rule and was giving it vastly different numbers consecutively. I think I finally really just confused it. Even having done that though, it got itself together and was great the rest of its life. So… that’s what i’ve got.

I wouldn’t conclude that from this study because of the confounding factors they gave. The SG was driving the closed loop AP algorithm, but they were monitoring the BG with a proper lab instrument. They determined that, especially during fast blood sugar drops, they needed frequent calibration to keep SG close to BG. But they did not show what would happen if a period of frequent calibrations was followed by a lengthy period without calibration: maybe their frequent calibrations would have driven the SG calculations too far in one direction, and a sudden cessation of calibration would cause large errors in the other direction. They weren’t worried about that because they were doing real BG measurements too. In other words, they were trying to minimize instantaneous SG error under a regime of recalibrating and swapping sensor whenever that seemed useful, rather than minimizing average SG error under a regime characterized only by the requirement for calibration at least every 12 hours. Note the confounding factor that they had the patient wearing two sensors, and they switched back and forth from one sensor to the other basically whenever they felt like the other one might work better. And these were patients in critical care, subject to edema, which tends to make their situation (hence calibration requirements) different from the norm. This is all from the section “Recalibration frequency analysis” and the following discussion.

I’m delighted by their statement “This analysis shows that all aspects of sensor performance were significantly worse during periods of frequent recalibration supporting the need for CGM recalibration during these times.” It seems a little bit like circular reasoning, and leaves open the possibility that the frequent recalibration was a cause, not a consequence. Or maybe they could have switched to the xDrip algorithm or a different sensor technology if they wanted good CGM performance: the Enlite 2 didn’t have as good a reputation as the new Medtronic sensor or the Dexcom CGM.


That’s actually a good outcome. I would have expected them to say something along the lines of “this sensor is wack, we’re not going to give you any CGM until you change the sensor.”

This is good stuff. It is a kind of surfing off of the combination of iSig and fingerstick BG. If you can formulate a few simple rules of thumb, I bet some Medtronic CGM users would try it. You’ve stated bits of it here and there in the discussion, but it’s not gathered together and summarized in a clear way. Say simply how to track BGCF, what things you look for, and what actions you take when you see those things. Wait — was that suggestion too arrogant? I just hope your good discovery can get wider use.


Yes, it all makes sense, and we are not doing the same thing they were. I appreciate the whole endeavor they were doing, and I also appreciate your detailed response!

Just threw that thought out there because it did strike me as somewhat ironic in light of the conversations and things I read about over-calibrating.

And now that I see we’re all agreeing that over-calibrating is still on the table as a real threat, then if we could all just ignore my comment back there about the things I do while calibrating… it would be much appreciated. Just pretend like it was never said at all…:grin:

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Not too arrogant. Spot on. But I’ll have to wait until my head, now in the clouds, joins the rest of me again. :roll_eyes:

The thing about it is it’s not easy to explain… I’ve been trying to come up with a kind of “sets” and “conditions” that could, hopefully, be used as the basis of guidelines. I would definitely need more than the 5 minutes available at a time (life with children) to make any headway. I am, slow as ever, chipping away at it. I’m also working with Eric on the basics… those help that “what to do” part. He’s going to teach me to “use the force”, and I’m going to match it with a BG/CF combo, also known (in my head) as a “set”.

I was afraid of what you guys would come back with and hope I’m not mistaken in considering it, pleasantly surprised, a bit of a fan to the flame…


This place is based on no judgement, no condescension, and no reproach.

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I have 375 people in my group now… there are 4 of them I’m willing to share you guys with. I’ve just told them about you. :smiley:

And, yes, I’m figuring that out…


You’ll never get crickets here. :stuck_out_tongue:


Just wait. I can make it happen anywhere. :smiley:

You’ve never found a site like FUD. Good luck! :stuck_out_tongue:


Well, I’ve already figured that out. :upside_down_face:


Just stumbled on this thread. I just got my 670 today. Can you explain the correction factor a bit? I’m confused. Are you meaning how much 1u adjusts my BG? Or something else.

I had the enlite for the last 4 years and this is the first time I’ve ever heard of ISIG, and the BG/ISIG formula. My mind is blown :exploding_head: like I’m wondering if this is why so many of my sensors used to fail


Hi! I would love to give you some thoughts on this, but I’ll have to do it when I’ve got a little time. I’ll try to get back for this sometime this evening—- if all goes as planned. :grimacing:

I also wanted to tell you I have a Facebook group dedicated… mostly… to the 670G. You are welcome to come check it out if you’re looking to mingle with fellow users. With that being said, I want you to know you’ve found the greatest diabetes group right here, so I don’t want you coming if it means you miss this. :smiley:

Here’s the group, but don’t feel any pressure if you’re not interested. Certainly NOBODY ELSE around here does. :roll_eyes: (That’s only a joke). :smiley: