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

Okay. I’ve been here long enough to throw out the big one… I will prepare myself to be told I’m wrong (and that would be okay), to be told everyone’s already doing it (then maybe someone will show me how), or the most likely response, crickets. I get crickets ALL of the time.

I’m not sure where to start. So then with this… this forum is different than the other groups I’ve come into contact with, and you all seem way more proactive and, well, smarter than the average bear. When I talk about this with most people, they don’t have a clue what I’m talking about, so I stop talking. But you guys might know more than those other people (or more than I do).

Eric, you mentioned the other day how no two 85s are the same. Well, I also know this, but I use the raw data from my Guardian sensor, the ISIG, in order to see what exactly is happening behind that 85… I’m not sure if any of you use your calibration factor for anything, but I calculate mine in order to see what’s happening in the interstitial fluid alongside what’s happening in the blood. So using that calculation, I can see if my 85 is dropping or rising. I can see whether or not that 85 needs a tweaking or if it’s okay to go walk around the block. Sometimes that second number only confirms what I already know to be true, but SOMETIMES it gives me critical information to aid in making the best choice in action. So when I’m finishing exercise, I might see I’m an 80 with a CF of a 5.6… I know I don’t need to treat yet and might not ever need to. If I saw an 80 with a CF of a 3.2, I need 1-2 tablets.

I don’t know if anyone has any idea at all what I’m talking about, but I figured you guys were worth a shot. I seem to have a second number that enables me to control my blood sugar almost like a I would use the thermostat to control the temperature. When I see a blood sugar of a 90 and a 6.3, I know I need to get up and use one of those small pockets of movement, whether it be to get up and do some housework or jump on my trampoline for 3 minutes… go take a 5 minute walk… But a 90 and a 4.3 would mean I’ll drain out very quickly if I do anything that requires any kind of effort or exertion at all. So I test my blood sugar sometimes 12-20 times a day, and I calculate my CF every single time.

I’m not sure why I’m posting this… Maybe just for some traction. I have been using this number every day, all day, for going on 6 months, and it gives me twice the information I get in just a BG. I just heard about “sugar surfing” maybe 2 weeks ago, and I fell in love immediately because it’s what I do… with my calibration factor.

Too much too soon?? Did I just get moved from “good crazy” to “bad crazy”? Was I ever in the good crazy to begin with? :smiley:

Just looking for thoughts… but none will be okay, too. :slight_smile:


I understand the basis your idea, but how are you calculating your CF from your BG reading every time? Maybe I am missing something.

I do something that might be similar. Not sure if this is the same thing you are talking about.

I know that my CGM is always behind my BG number. So if my CGM is 30 points higher than my BG number, I know I am dropping more rapidly than if my CGM is only 10 points higher.

Is that at all the same thing you are referring to?

How do you get a CF from your CGM reading?

I was planning on bothering you with all of this, but I thought I’d try to get the group to bite to give you a break. :grin:

So with Medtronic sensors, we have a visible ISIG value— it’s the interstitial signal. I think all sensors use them, but most are not visible. For instance the Libre doesn’t show it. I THINK, and I might be mistaken, that you can see this in other ways… maybe night scout??? I’ve never used that, but I thought I read somewhere it was available.

So I take my current bg and divide by my ISig. It gives me a number, the calibration factor, that represents the amount of glucose in the interstitial fluid. This is where my understanding and ability to communicate it falls down to elementary school level… From what I understand, we have two compartments, blood and interstitial fluid. Our glucose moves between the two in a push-pull relationship. So by looking at my BG alongside my calibration factor, I can see what my current blood sugar is but also where I am in the process. Is my glucose pushing over into the interstitial fluid? If so, I can get ahead of it by moving, or at least I can try to contain it… is that pull from the interstitial fluid underway? If so, is it what I want? If I’m starting high, then yes, if starting low, then no…

I use it everyday. Yesterday I did the last day of my “dawn phenomenon” experiment and was in auto over night. It always wrecks my morning blood sugar… so I was a 224 according to the guardian, a 234 according to the Libre, and a 194 according to meter. I decided I would head out for a run as i would normally do it later (with temp basal), but I might as well just ride the high and bring it down… kill 2 birds… when I calculated my CF, I saw it was a 5.4. From what I’ve observed so far, a 5.4 can really mean a pretty quick drain with heavy activity so even a 194 wasn’t going to stick. I had a banana and went out the door before my blood sugar had time to rise any more. I returned at a 90 and coasted an hour before needing to get a glucose tablet. I don’t know if this is a good example because it looks so ugly with those high numbers, but I use it for a million reasons…

Oh, I see what you are referring to now. In your initial post, I saw “CF” and just immediately thought “correction factor” instead of “calibration factor”, because that is the abbreviation I see so much of.

Yes, your post makes sense. But I just gotta get you pulled over to the Dark Side. I have been working on @daisymae for a year to convert her. It is so much easier and more liberating. image

At some point, give me two weeks and I know I can make you a convert.

There is a little bit of The Force in all of us:

daisy mae



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So you’re saying there’s an easier way…

I’m trying to use the force… I am. :grin:

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Sorry, don’t mean to hijack your thread.

You are doing experiments with auto mode. If you want, sometime, I can give you 3 steps toward using The Force. A different kind of experiment.


First of all, you can hijack my thread whenever you like. :blush:

Second, you didn’t.

Third, I was doing an experiment in auto, but that didn’t have anything to do what I do with the calibration factor, which means I hijacked my OWN post.

Fourth. I’d be happy to accept your mission. Who can argue with just three steps? :thinking:

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Does this make sense? Do you do anything like this? :arrow_down:


All the frickin time. I’ve become the CGM whisperer. :star_struck:


I can also see how fast of a drop I’m at using my calibration factor. A 2.8 is a plummet. A 4.9 is a gentle slope.

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My understanding of the calibration factor and iSig is maybe a little different from yours. First of all, the iSig is just the raw data from the sensor: the measurement of the current produced by the electro-chemical reaction of glucose in the interstitial fluid at the sensor. I think you and I agree on that. But to my understanding, the calibration factor is the quotient (CGM’s estimate of BG) /iSig. To me this contains no additional information from the raw iSig, and in particular, I don’t how how it could contain any additional information about the difference between BG and interstitial glucose.

What I think the calibration factor does really well is give an indication of the “strength” of the sensor. As the sensor ages and gets weaker and weaker, the iSig drops lower and lower, so the calibration factor gets larger and larger. A calibration factor of 3.2 (with a BG of 80) shows that the sensor is really strong and giving a good signal. A calibration factor of 8 means the sensor is older, so we expect the CGM to be less accurate and also maybe the lag behind BG is getting larger as the CGM algorithm tries to smooth out increasing error/noise in the signal.

Ages ago on the Medtronic Sof-sensors, I also watched the iSig, but for a different reason. If I kept track of “what iSig means my bg is ok on this sensor today” then I didn’t lose the use of my CGM when Medtronic turned off the BG display when 12 hours passed without a calibration.

In reading your last post, I’m starting to wonder if you define calibration factor differently from me. Maybe to you the calibration factor is (fingerstick BG)/iSig. In that case there is additional information in your CF that I have not considered.


I would like to see this, but I guess just using the out-of-the-box Dexcom implementation, the calibration factor is not available.

Self-proclaimed… only a self-proclaimed CGM whisperer…


[quote=“bkh, post:11, topic:4099”]
Ages ago on the Medtronic Sof-sensors, I also watched the iSig, but for a different reason
[/quote] ::slight_smile:

I remember using ISIG with the Sof-sensors too. My recollection is that it helped to know if it was a good time to enter a calibration. I think if low, it meant not a good time.
I originally found DOCs due to searching ISIG. Sometimes when I called Medtronics for help, they would ask for the ISIG.

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I just tried looking that up… i could’ve sworn i had read somewhere there was a way to get it, but i think you might be right. :frowning: I have a notebook somewhere with things I’ve printed out, which all of a sudden feels very shameful because I probably shouldn’t, where I kept that article… It’s one piece of paper among 46,000… So we’ll just assume you’re right.

Maybe this helps


Yes, they are good for that as well, but I am using them differently… I use the calibration factor in order to determine, basically, whether or not “now” is a good time to “…”. I don’t use the SG calibration factor, although that is just a delayed version. And although it CAN show whether or not calibrating is okay, there’s another reason it shows it. Those calibration factors actually represent the amount of glucose in the interstitial fluid… and there are that many dots because that’s a BIG statement for a stay-at-home mom who slept through biology class. But i’ve doing my reading, WAY more than what they tried teaching me however many years ago, and I’m piecing stuff together. And what I DO know is those ISIG values don’t just show sensor strength. They show concentration. There’s another zinger… that statement. I use it day in and day out, and my factors range from a 2.3 (never seen lower) to a 12.5 (and that truly WAS a failing sensor). My average ones range between about a 3.8-6. I can estimate my blood sugar on my way home from a run by multiplying my ISIG by my estimated calibration factor… I’m often a 3.5 by the end of a workout, so if my ISIG is a 18.57, I usually calculate a range and would multiply by a 3.2-3.8 to give myself an idea. So I’d say my range would be a 59-70. In this case, my actual BG would be a 64. I’ve gotten pretty good at knowing my numbers and can usually guess within 5. And I know, that’s kind of like, “cool, trick, but who cares?”, but it’s only one way I can use it. This morning I knew at a blood sugar of a 109 that I needed to bounce on my trampoline for 3 minutes (which I enjoy, so there’s no hit to quality of life there— not that that matters anyway), before starting my second cup of coffee. With a calibration factor of a 6.75, that second cup was going to push me higher than i wanted. I remained at a 106-113 at every check thereafter…

I can explain… I’m living it so either it’s because it’s real, OR it’s because i am completely 100% mentally ill, but I’d think people would’ve addressed that by now (unless my mental illness prevents me from understanding that they’re trying)… :slight_smile:


I’ve got nothing against her… I really don’t. I actually like what she does quite a bit. But somehow I always end up being pointed in her direction. I’ll have to go back again (for probably the 4th time) just to see what she says, but from what I remember, it’s more of the standard use. I think there was some stuff in there she was right about, but I also think there were some things I disagree with.

That’s a lot of thinking… I could save us all the time and just go back again and take a look.

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That just caused chest pain because although I say I don’t mind being wrong, it appears I do…

Agreed again. And in support of that statement, I’ll add that around day 5 I begin to see what I call “inflated calibration factors”, which I take as the beginning of a slow degradation of the sensor. Pretty soon thereafter I will change the sensor because those calibration factors are no longer helpful to me and can actually lead to bad decisions.

Here I’m only partially in agreement. For me, a BG of 80 with a calibration factor of a 3.2 means a thinning blood sugar. I could, in theory, also see a BG of 80 with a calibration factor of 8, which would mean a rise. COULD mean a rise. Yes, you always have to consider your context, and if you have reason to think your sensor is struggling, it could be that as well. But an 8 is not all that uncommon a number with, say, a rebound where i’ve really crammed the sugar.

End chest pain. Yes, I’m defining it differently from you. BG/ISIG. And in that, I believe, there is additional information that you may have not considered… (said only because you said it. Far be it from me to be so bold). :smiley:


So it sounds like you have found additional meaning and use of ISIG. That’s great!

I’ve been using dexcom since 2010, and would guess majority of folks here use dexcom. And you are doing something unique. Give Eric a chance, I’m sure he will come back with something.