The ISIG Equation

I swear, as diabetics, we should be granted a pass out of any math class there is. There are notes everywhere on how to figure boluses when taking into consideration Insulin To Carb ratios, Correction factors.. There are ones on how to figure your insulin on board…. There are ones to figure out how many grams of carb to eat if you’re low… Diabetes is FULL of math and we have to use it every single day. I think back to one night when I was doing my algebra homework and my dad came up to me and said, “I don’t get why you have to do that, it’s not like you’ll ever use it in real life”. Oh boy.

One equation that I didn’t have any dealings with until I had a Medtronic pump is the BG/ISIG product equation. Basically, it looks like this:

BG/ISIG = (1.5 - 20)

When would one use this? Let’s say you test your BG (blood glucose) and you’re 220. You look at your pump, but it’s showing your SG (sensor glucose) as 82. That’s wayyyyy off. Typically, one would simply calibrate and go on and cross their fingers that they won’t be given a Cal Error on the pump. This is where the equation comes in. You would need to look at the Sensor Status screen to find your current ISIG value. Then, just put it in the equation.

*Let me interject here and say that the ISIG value is your Interstitial Signal. Basically, it’s the electrical signal generated when your interstitial fluid reacts with whatever the sensor wire is made of. This signal is then formulated into a number, which we can use to calculate the equation with.*

So, let’s say I looked and my ISIG value was 7.52. I would need to take the 220 and divide it by 7.52 and it would equal 29.25. The product is well above the 20 upper limit, so you should NOT calibrate at this time. If you do, you will more than likely get a Cal Error. What you need to do is just see if the sensor comes back in-line over the next couple of hours and keep checking the BG/ISIG product to see when it comes back in that range. If it doesn’t, it’s probably a sign that the sensor is going bad. I’ve learned that when the ISIG seems to be saying really low, it’s probably a sensor nearing the end of it’s usefulness, whether due to wear time or if the sensor wire has become bent.

Now, the same applies in the opposite direction : If your BG is 98 and your SG is reading something like 78.21, your product is going to be outside of the lower end of the range with a 1.25, so it’s also probably not good to calibrate now either. In this case, anything can cause a high ISIG value / low product, so what I do is try to drink some water. Dehydration can interfere with the sensor’s ability to get a good electrical signal from your interstitial fluid. If it’s not a dehydration thing, it’s some other interference causing the ISIG to read high, like, say, if it’s a brand new site. Those can take a a day or so to “calm down” and get adjusted to the area where you’ve placed it.

From the research I’ve done on the sensor ISIG, the relationship between your BG and the CGM’s SG/ISIG is a linear one… meaning that as one increases, the other will increase. As one decreases, the other will as well. So you can’t expect your ISIG to stay at one relative number (unless you’re doing an awesome job at planking your BG!), but the BG/ISIG product should remain relatively the same through the sensor period. I have found that for me, my best times to calibrate are when the product is somewhere between 6 and 12, and without arrows at all. If my product is outside of that range, I tend to try to look at other things like how secure my sensor may be or how dehydrated I may be or even if I’ve drank too much water in the past couple of hours. Or it may even be something as simple as needing to wash (or re-wash) my hands to try again.

If you want to get super technical with how this works, you can read THIS POST on Children With Diabetes or THIS POST on MyParadigm. I admit, I’ve read through it and made the best sense of it as I can, but I honestly have to give a big bit of the credit to my friend Clifton in the Medtronic Pumper’s group on Facebook for helping me understand it and put it to use. Once I learned how to use the BG/ISIG product when calibrating, I experienced a LOT less Cal Errors and Change Sensor alerts. It’s saved me from pulling sensors that didn’t need to be pulled, as well as helped me learn which ones DID need to be pulled.

10 thoughts on “The ISIG Equation”

  1. Wow, lots of detail! (I’ll need to go back and study this post again, and also look for that Facebook group).

    I generally use the ISIG for simpler purposes. First, since I can’t get arrows or glucose numbers during the two-hour warmup period, I’ll look at the ISIG to get a general idea of which way I’m trending, and if I’m level enough for the first calibration.

    Second, I know my “good” ISIG ranges are in the tens/teens. If my meter BG is in-range but the sensor reading is not, I’ll check the ISIG. If that’s high (or low), I know that I need to wait for the sensor to catch up rather than try to fix the discrepancy by calibrating…because that will ultimately mess things up.

    Fascinating stuff for a math geek like me. It’s a shame it has to be so complex, though.

  2. Ah, thank you!!!! Several times while talking with the Help Line they’ve said we’d go over ISIG equation, but then we’ve gotten sidetracked and didn’t. (Totally my fault – I’m chatty and tend to go all over the place topic-wise). This will definitely come in handy though!!

  3. That’s a lot of number there…the more you know, the more you know you don’t know…or something like that. lol! I need to read more about this. Thanks for the post! 😀

  4. This was most helpful. Recently I had to change out a bad sensor and went over this with the Medtronic Help Line staff but your explanation of what the ISIG value actually tells me is extremely helpful. Fortunately I get very few calibration errors but when I do get one now, i will refer back to this and have written down the Equation. Thanks Sarah!

  5. Exactly what I needed and explained perfectly (and simply for those of us who need to use tech but aren’t mathy/techy folks)! Thanks a bunch!

  6. I was able to export the past month’s data from the carelink site and plotted ISIG vs BG. The average equation ends up more like BG = 4.1*ISIG + 31… I use it if I don’t have a meter handy to calibrate with (aka when the pump is beeping in the middle of the night for hours for a calibration). Your usage might vary, though I do recommend calibrating the equation based on your body, tranmitter, pump and all.

Tell me what you think!