I discovered a calculation on my Tandem X2 pump which I do not understand. I write to see if anyone could explain why and when such a calculation is done. The details: Before my meal I pre-bolused 0.5 U. The pump and I calculated the meal carbs alone required 1.2 U. When I went to take the bolus my blood glucose level required a 0.05 U correction and my IOB was 0.36 U. So by my calculation the suggested bolus should have been 1.2 + 0.05 - 0.36 = 0.89 U. However the pump’s ”Total Units to Deliver” was 1.2 U. It had disregarded both the correction and IOB units and would have delivered about 35% more insulin than I calculated … posing the real possibility of a hypoglycemic event.
My questions are why does the firmware disregard the correction and IOB units? For future reference has anyone researched when are the corrections and/or IOB units disregarded?
It prompts you if you want to let the pump adjust the bolus (either up or down accordingly,) when it sees your BG number. Did you select the check mark or X ?
As far as I’m aware, the t:slim X2 only subtracts IOB from correction bolus calculations, not carb bolus calculations, unless your BG is below target. If your BG is below target, I think it will subtract the IOB from carb bolus calculations as well.
The reason why it suggested 1.2 U, instead of 1.25 U (which it would have to cover both carbs and correction if there were no IOB), is because it did subtract the IOB from the correction portion of the bolus.
I had this same argument with the design team when I first started using the t:slim x2 and still remains one of my peeves.
There are many rules written into the programming, but what it boils down to is that the pump software doesn’t like random pre-bolused, like I have a tendency to do and what you did in this example. It wants a complete snapshot of what you’re eating and where your glucose is when you bolus. It assumes that small pre-bolus you took is working on food you’ve already consumed and it’s not yet raising your sugar, but will in the near future, hence it ignores it. When you go to take the rest of your meal bolus, it’s treating it as if it’s a new bolus for new food.
I’ve essentially learned to disregard the bolus wizard because it doesn’t work with my management style. I was constantly low when I trusted it. Before I had CGM, I would take some insulin while preparing food, to get it warming up, so to say. And then I would sit down at the table with my food, and my glucose meter, and plug all the numbers in to calculate the rest of the dose. The bolus wizard does not like this methodology of partial pre-bolus!
I’ve gotten lazier since giving up on the bolus-wizard, but basal-iq and Control-IQ have helped a lot. Honestly, I tend to over-bolus now and let Control-IQ stop my basal to compensate after the fact.
Since I do not have the Tandem X2 pump, but instead still have the original Tslim pump, I probably should stay out of this discussion. However I do agree with Robyn_H and her method of partial pre-bolus. It seems with my pump that whenever I try to trust the bolus wizard, no matter what I eat, I always end up higher after a meal or a snack than I want to be. I am due for a new pump come October and from what I am reading about all the troubles you all are having, I am not looking forward to the learning curve.
quote=“Mayumi, post:5, topic:82212”]
I am due for a new pump come October and from what I am reading about all the troubles you all are having, I am not looking forward to the learning curve.
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Actually, I LOVE the t:slim X2. In my personal opinion, it’s the best pump on the market right now. It’s not perfect, though. I dislike that I can’t give a partial bolus up front and have it compute correctly in the bolus wizard, but I think it’s an old concern that matters less moving forward. With the CGM integration and automation, it’s rendering itself moot. In hindsight, I think they should have programmed a way to designate a bolus as a pre-bolus. Maybe it’s simply that the pump has finally trained me how to use it ideally.
My other irks are that the autosoft:90 is rotated 90 degrees incorrectly in it’s inserter, making it difficult to use sites on my back side. This exact infusion set is common amongst brands, though, so I guess it’s one of those things people have gotten used to. I also wish there weren’t 20 units of insulin wasted to the cartridge as “dead space” and that the Control-IQ parameters were more customizable.
All-in-all, though, I wouldn’t trade it for anything else available. I LOVE how small it is, and the bright, colorful touchscreen. After all my research, I TRUST in Tandem to take care of me and that my safety is paramount. I FEEL GOOD that it’s rechargeable. I’m COMFORTABLE with the intuitive interface. I’m AWED by the quality of their American tech support, as they’ve always been a pleasure to work with. I’m just positively ECSTATIC about the Dexcom G6 integration and how readily available the X2 makes that information. I’m RELIEVED to know that I’ll never have to wait long for innovation, thanks to the software update ability. But most importantly… I’m HOPEFUL for my future now because of how much my diabetes management has been improved with first Basal-IQ, and now moreso with Control-IQ (even if Control-IQ took a little tweaking to find my sweet spot).
@markeeezy is correct. The pump will only subtract IOB from the correction bolus. In this case, your IOB was higher than your calculated correction bolus of 0.5, so it didn’t include an amount for a correction.
The pump doesn’t track ‘carbs on board’, and it can’t know that the 0.5U you gave as a pre-bolus half an hour ago was to cover the carbs you are inputting now, so it assumes that if you input meal carbs that require a 1.2U dose, that these are new carbs that need to be covered.
Think of it this way - in the absence of tracking COB, if the pump did count IOB against a new food bolus the way you’re suggesting it should:
12:30pm: eat a slice of cake and dose for 30g carb - 2U
12:45pm: that was nice cake, I might have another half a slice - go to dose for 15g and your pump says no more insulin required as you have 1.96U IOB, way more than required to cover the half a slice of cake.