Tandem Control IQ issues with rebound BS

So I have been on this pump for several months. Things were going fine with TIR about 75% 70-140 and SD in the 20s. I have been strength training about six weeks so far. All of a sudden my numbers are bouncing around. My SD is in high 30s and time in range has dropped to 65% in past 30 days mostly due to highs. What I can’t figure out is what to adjust?? I raised my basal as in the past that has solved bouncing but now I can’t get control back. Lots of overbolusing to bring down highs which are the result of control turning off my pump. I adjusted carb ratio and sensitivity as well.I did a basal check before starting Control IQ about nine months ago. It was fine.
Do I need to do another basal test? Would fasting all day count? When I do that my basal drops but when I add carbs back in to my diet I need more basal. Suggestions?

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Very interested in seeing what others say here. I don’t have any answers, but in my short time I do see some rebound effects. So far it hasn’t been too bad, nothing going over 130, but it would be nice to see the rebound from a 70 end up more 100-110 range.

I eat one meal a day, so I basically do a fasting basal test every day from about 3am to 5 pm when I eat. If you say you are dropping low when doing that, then for sure your basal is a bit too high. I have found that on my control iq I don’t want my basal to be perfect like it was on a dumb pump, because it will cause me to hang in the higher part of “acceptable” range without trying to bring it down. I’m still trying to find that sweet spot where my basal is just a little higher than it should, so that it brings me down to the lower half of the algorithm without causing me to go too low, thus the rebound.

Yeah, I hear you. I need to adjust my sensitivity as it is bouncing me above 140. Not happy.

I have similar issues. Following
the thread.

Okay, the rebounds can be wicked when Control-IQ suspends your insulin altogether, and there’s little that can be done about it, because your settings don’t matter at all with suspensions. It’s entirely based on Dexcom predictions. It will suspend basal when you’re predicted to be below 70 in the next 30 minutes, and won’t resume basal until Dexcom predicts you’ll be above 70 in 30 minutes… And even then, it does so at reduced rate.

There’s a few problems with that. Dexcom is pretty slow to process whenever your BG is changing directions. There’s some inherent lag between interstitial fluid readings and actual BG. Plus, Dexcom uses a smoothing algorithm to make for pretty graphs. Each data point it displays is computed against the previous 2 readings, essentially averaging them out. So all that delay means that your insulin is likely being withheld for too long.

And the other problem… Being low sucks, and thus we’re impatient. A lot of users are double-dipping and eating the corrective carbs while Control-IQ is already treating the low in the background. Double-dipping equals double the rebound.

If your primary problem is rebounds after suspensions, increasing your basal will probably just make that worse, because you’re just driving yourself lower and Control-IQ has to suspend more. I’ve long since recognized that it’s not worth fighting Control-IQ to try and get superstar numbers. It works best when you give it true settings, or at least nearly true. So yes, basal testing is always a good place to start.

You’ve got a few options to handle the rebound highs from suspensions.

  1. Try to avoid the suspension in the first place. This probably means adjusting some settings. It helps to identify what was happening before the low. It might be your basal rate. If it happens after a high, it might mean your correction factor is too low. It could also be a combination of your I:carb is too low (giving too much insulin), but you’re not pre-bolusing and taking it too late, so you’ve got all this insulin on the backend driving you low but not enough up front with your meal to avoid the high. And of course, if you just plummet after a bolus, your I:carb is too low. Edit: Lows can also be a result of a correction factor that’s too low (too low a number = gives too much corrective insulin), look at your T: connect to see where the extra insulin came from.

  2. You can turn Control-IQ off temporarily, which will force the pump to resume basal delivery. You can turn it back on when you’re out of low territory.

  3. This one is hard for me, but try to learn not to eat the correction carbs unless you’re dropping pretty drastically and think Control-IQ can’t save you. Since I lack that self-control myself, I find i have to actually bolus for my hypo carbs, but usually wait until I feel like I’m out of the hole… Even if Dex hasn’t gotten the message yet.

  4. If you are in the habit of pre-bolusing like we’re supposed to… Control-IQ doesn’t like that! It gets scared when you’re below that 110 target with IOB. One way or another, I think most of us wind up bolusing heavy to essentially replace the basal that was withheld. Some people do it by lowering their I:carb ratio, so they get more insulin. I think of it more like super-bolusing. Since I can’t count carbs easily (nothing I eat has a nutritional label on it), whatever I THINK I need, I automatically add 2 units to it (per meal, don’t stack!). That’s about 2 hours worth of basal for me.

There are 2 different updates for Control-IQ in the works. One of them is small algorithm changes, and the latter one will bring greater customization abilities. Hopefully one of them will address the rebound issue. It may also be improved when the G7 comes out, if it’s any more accurate with it’s predictions.


This is very helpful!

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I only use C-IQ overnight, turning it off when I wake up in the morning. When I turn off C-IQ, I immediately start a temporary basal of +35% for the next few hours. If I see that insulin has been suspended in the within a few hours of waking, I know that I have to be very careful with breakfast.

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As usual, @Robyn_H experience and knowledge beats anything I have heard from any official source. Thank you!!

Some interesting stuff in there. I have decided to run a slightly higher basal rate (0.7 instead of 0.6 during the day, 1.0 instead of 0.9 in the evenings, 1.1 instead of 1.0 early am before dawn phenomenon). I’m trying to find that sweet spot to keep me in the bottom half of algorithm, while also not getting insulin suspended (I’m ok with reducing, I actually want that. Got the technique from Robyn, no shocker there).

Turning off CiQ is an interesting fix, but I think I like the bolus to cover the suspension, just so it’s less settings to mess with.

Related question, when stopping insulin from disconnecting, if I know I will be disconnected for 15 minutes, is it best to give the missing 25% basal as a bolus right before disconnecting, or right after? I guess what is typically faster? The rate a bolus drops you, or the rate your blood sugar rises after suspended insulin?

Well, about to have a real life test case on my rebound today. Fingers crossed.

Edit: Well, today the algorithm worked like magic. I guess this is “kind of” what it feels like to have a real working pancreas?