Tslim Control IQ - How Do They Calculate Insulin On Board

Only recently switched to Tandem Tslim Control IQ. Like it a lot especially in SLEEP mode.

But I can’t figure out what IOB refers to when you are using Control IQ.

Coming from Medtronic pump, I am used to IOB meaning bolus insulin not yet absorbed based on when the bolus was delivered and based on duration period. But when using Control IQ IOB has a very different meaning, Could someone explain what it means?

Would be so helpful… Thanks

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I found some good info here.


@MM2. The article author used to ghost another blog I occasionally read and many here migrated from. When she posted, she always had good advice.

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Thanks, I read it but it didn’t answer my question. I did figure out that if you turn off Control IQ you get the IOB (on the home page) based on the bolus alone.

"What about Insulin on Board (IOB)?

The IOB amount is different than with the previous version because it’s based on boluses from YOU, but also increases and decreases in basal from Control IQ.

With the old version, IOB was strictly from blousing, not from basal.

The IOB display time will not be turned on because on the continual fluctuating amounts from the adjusted basal, the automated correction bolus (if predicted to go above 180 mg/dL in the normal mode), and the bolusing from you (correction and food). Just too many factors that make the difference between bolus and basal too muddled. "

Simply put, the IOB marries the bolus based on your carb input with the algorithm controlled basal fluctuations to create an IOB.

Thanks Elver. I kinda know that. I am wondering what I (anyone) should do with the new IOB amount. A bolus will always get you to target faster than a basal. So maybe this is the amount of bolus you should take to get you to target faster as it includes current BG and all the basal adjustments that the TSlim has made (and are still active). Do we simple ignore it? If so why do they show it?

@RCA221 I’m not on Control-IQ yet, so I only speak anecdotally, but it seems that the IOB calculation is more for your peace of mind and to help decide if you want to supersede the algo with an additional bolus to treat a high. Conversely, the IOB display might prevent you from initiating a bolus and creating a low when the algo has already factored in the high and corrected for it.

My personal take is that the IOB indicator in Control-IQ is similar to doing a fingerstick - it’s a sanity check before you make a decision to either bolus or not, because your decision will affect the algo going forward.

I would be super interested to hear from others if I am way off base, as I hope to make the transition to Control IQ early this Fall. The more I know beforehand the easier it “should?” go, yes?

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My son ignores it.

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I’ve been using X2 a couple weeks. My conclusion is I need to not pay much attention to the IOB. X2 will always use a 5 hour time for it, and I used 3.5 hours in Medtronic pump.

I was over-reacting to IOB, eating carbs, only to then seesaw too high and need more bolus.

X2 seems to hard code 5 hour time, so today im going to let it ride. I’m estimating there is only 10-15% remaining bolus in the last 1-2 hours, and if im heading low, IQ will halt basal so it nets out.

So far I have ignored it too. But I still am curious why it is there. I think it tracks all the basal changes it has made to your profile and it you give yourself a bolus based on their IOB it will get you to target quicker than if you simply let them make basal changes. So if I happen to see there is a large number then I might intervene and give myself a bolus. I don’t think it defeats what they are doing, just speeds it along. I have had it a month and for the first time I can sleep without waking up to give insulin or take a fast acting carb. I think it is miraculous.

I agree we need not pay any attention to it. I also know they have hard coded 5 hours for a duration while Control IQ is turned on. Just curious why it is showing at all if we do not have any interest in it. If you turn off Control IQ you will see that the IOB number changes back to only the active bolus insulin remaining.

I found this explanation. Makes sense.

When Control-IQ technology predicts that your glucose value will be at or above 180 mg /dL 30 minutes in the future, and when Control-IQ technology is either increasing insulin delivery or delivering the Max Basal Rate, the pump will automatically deliver an Automatic Correction Bolus to attempt to achieve the target glucose range.

So it brings out the big guns (bolus) when needed.
All is well as long as cgms data is accurate. But “safer” to let conservative increased basal take care of the little guns.

I never like to be conservative. I ALWAYS give myself a bigger bolus than I need so I don’t5have to wait 3-4 hours to come back to target. In essence I always leave myself room for a snack if and when it comes down.

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Me too.

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Are you talking about the Insulin on Board on the main screen or in the Bolus calculator? I definitely care what the main screen says.

Yes, I can’t find IOB anywhere else. I’m very surprised more people don’t care about it.

I understand generally why they are different but would like to understand it more.

yes to what???, The iob on the main screen or the bolus calculator?

I would like to know how the insulin on board on the main screen works.

Thanks for clarifying. It’s different but I definitely like the information, otherwise you would have no clue what’s on board. So I don’t see how you can just ignore the information…maybe I’m missing something??? I certainly take it into consideration in my day to day activities.

It’s rather strange that they would stick it on the home page if it meant nothing so I’m surprised people ignore it too. I really don’t want to understand how to calculate it but I would like to understand how it’s derived.