T:slim X2 Control-IQ "artificial (IMO)" basal limit of 3u/hour

I have a t:slim X2 with Control-IQ and I’m fine tuning my personal profile in an attempt to get better overnight control in sleep mode.

I am attempting to use the t:connect mobile app on my Samsung Galaxy S9 but I have discovered a disturbing error in the data being displayed.

My profile setting from Midnight to 5am currently has a basal rate of 3.4 u/hr, correction factor of 1u:5 mg/dL, carb ratio of 1u:3.8 g, and a target BG of 110 mg/dL. Sleep schedule is on everyday Midnight-6:00 am. I have been increasing the basal rate from 2.9 to 3.0 to 3.2 to 3.4 and have old profiles on the pump documenting those changes.

The t:connect mobile app, however, shows my (base) basal rate for this current profile period as 3.0 u/hr. I have uninstalled the app and reinstalled it but it still shows 3.0 u/hr.

I’m not sure what data to trust. Is the pump providing bogus data to the app? Is the pump not using my profile data? Is the app not accurately displaying the data the pump provides? Any other possibilities?

Has anyone else seen similar discrepancies?

The settings on your pump are the only settings that matter.
Your ap only reads the settings from your pump.
I’m not sure why it’s not right. Unless the Bluetooth has lost connection. Mine does that sometimes.

You can turn off Bluetooth on your phone and back on. That should force it to push the data to your phone.

Also, you can run sleep mode easily after you corrected your last meal. You can also run it all the time, but you will need to make your own corrections.

If you are not eating sleep mode is definitely the place to be

Check to be sure your pump is set to am when it’s am. I had mine set 12 hours off one time when I was on a Medtronic pump. Took me a long time to figure it out


While I don’t know the answer to your question, I’m going to make a couple of comments and suggestions. I will make reference to the (very long) t:slim Control-IQ manual at:


I don’t know whether you need to be logged into Tandem, but it is a very long document that is probably best viewed on a laptop or tablet rather than on a phone.

Have you compared your t:Connect settings to what your pump is showing? Note: If you hit the bolus status icon in the upper right of your t:slim, it will you the Current Status screen including basal rate, correction factor, carb ratio and target BG.

If your pump and t:Connect disagree, that would seem to be a t:Connect bug and should be taken up with Tandem. If they agree, then it may be something else …

On the warnings page for Control-IQ, I note that 3.0 u/hr is an upper limit when you are not getting a CGM signal from page 246 of the User Guide:

Also, on page 265, it talks about the maximum rate of insulin delivery:


To be candid, I do not know how to display or calculate the maximum rate of insulin delivery, but I wonder whether that is being set to 3.0 u/hr for you. That should be a question that is easily answered by Tandem Tech Support.

Best of luck … and stay safe!



Thank you for that reference and I believe that Control-IQ is limiting the basal rate to 3.0 even when the pump is receiving CGM readings.

I’ve just spent over an hour on the phone with Tandem Tech Support and they have no clue about what is going on. Even if they did have a clue, I’m screwed anyway because if it is a “feature” or a “bug” in the firmware, I (we) will never see a fixed version.


Here is a reference to someone that was comparing Control-IQ to loop:

About 1/3-1/2 of the way through this summary, they talk about what they believe to be true about the 3.0 u/hr upper limit … that would seem to be consistent with your observations.

As to Tandem never addressing this issue, my guess is that “suggestions” that are made to Tech Support rarely find their way to the engineering team. I would suggest trying to identify their Vice President in charge of software development and write a very nice, but detailed letter to them explaining why their current configuration is limiting the effectiveness of their algorithm in your case. If other high-basal rate users, did the same, it may have an impact on future releases. While Tandem has not demonstrated frequent bug fix releases, I am confident that there WILL be Control-IQ 2.0 … or whatever they choose to call it … and it would certainly be nice if there were more “knobs” that would allow you to set a Max Basal Rate when using Control-IQ.

Stay safe!


These things are all based off of “average” use for type 1 users, as that is what Control-IQ is currently approved for. They’re just aren’t a lot of Type 1s with a basal more than 3 units/hr. And given how fearful the entire medical industry is of insulin and hypos, the limits are unfortunately expected.

I know it will adjust to more than 3, though. I’ve seen it do so occasionally, but always when I was trending upwards.

That said, Tandem’s major financial outlook right now is dependent on branching into the type 2 market. I listened to an investor talk from last month (not their quarterly investor call), and they said that only about 5% of type 2s in the US use an insulin pump, I can’t remember if that’s 5% of all Type 2s or just the insulin dependant ones… but there’s a massive market just waiting to be tapped. Their current patient base is about 10% Type 2, all of whom are using the pump off-label. They’re hoping they can take all of the data they’ve accumulated on Type 2 usage to the FDA and get their current approval expanded to include Type 2s. That would be the fastest route. If the FDA demands specific clinical trials, though, it won’t be until 2022 that we see more geared towards Type 2. The speaker said that they’ve just been unable to schedule a meeting with FDA to get the Type 2 process rolling because the FDA has diverted most of their manpower to Covid workings.

I don’t know if you’re ( @bsmorgan ) Type 1 or 2, my mentioning of the Type 2s is because they tend to have much higher insulin needs than the “average” Type 1 user. So designing pump updates to be inclusive of the Type 2 market would definitely have to include loosening up those upper limit parameters. They’re also already working towards approved use of concentrated insulins. For instance, many with high needs are already using u200 instead of u100, which means the pump is delivering twice as much insulin than it thinks it is, effectively raising that 3 unit cap to 6 units. But that takes some masterful readjustment of your settings and retraining, so you don’t over-deliver.

We don’t know a lot about what’s in the Control-IQ update that’s already pending FDA approval and expected to launch this summer. They’ve promised “improved algorithm” and “more customization options”… along with the long-awaited mobile bolusing/remote control! Hopefully that will include a fix to this 3 unit problem. The more time they can keep their patients in range, the more favorable their system looks. If they consistently see 3 units being a problem, it is most definitely already on their radar. Just make sure you’re giving your pump a voice and sharing your data with them, either by connecting it to your computer via the device uploader, or by using the t:connect app, so current problems can be identified.

I started as a Type 2 30+ years ago but have progressed to total insulin dependence so I call myself a Type 1. A term I just recently discovered is LADA which is probably what I am.

I started using a Medtronics Guardian CGM and progressed through a 630G, a 670G, and am now using the t:slim X2 with Control-IQ since October 2020.

My insulin needs have always been pretty high with an average daily usage of 120-130 units.

What would really make things easier would be to use U-200 insulin in this U-100 pump but the only U-200 insulin I’m aware of is the Humalog® U-200 KwikPen. Filling a t:slim X2 cartridge is complicated enough (compared to Medtronics) so I’m not sure how that would work using a pen.

I am having to change infusion sets every other day so in addition to moving my pump settings away from these artificial limits, I’d be able to have my infusion sets last for 3-4 days.

@Robyn_H, can you point me to any information on those using U200? As a retired engineer, I’d be up for the challenge of adjusting my settings.

I’m sorry, I don’t know of any particular resources for it off-hand, but Google behaves really well if you use quotes in your searches, like “Tandem” “u-200”. I know how it works, though.

For the time being, concentrated insulins only come in the pens. I don’t think Novolog has a U-200 option, but the newest insulin on the market, Lyumjev, does. Lyumjev has a much faster onset than Humalog or Novolog, shorter duration, and seems to be stable in the T:slim and not cause the problems Apridra and FIASP do. Unfortunately, some insurance make this difficult. You need a cooperative doctor to write your script for subcutaneous corrections, or something similar, because it’s not approved for pumps yet.

You basically just inject the insulin into the cartridge via the pen needle and skip the “suck the air bubble out” step. The pens deliver in the same “units” as everything u-100, they just do it by delivering half the volume. Which means you actually need to inject 600 units worth of “units” into the cartridge, to equal the 3ml maximum, or 445 units would be closer to 3 days worth. 130 units x 3 days + 55 units wasted to cartridge/tubing (would usually be ~15 units in the cartridge and ~12 units to fill 23 inch tubing, but you’ve got to double the waste too) = 445 units. I don’t know what the max dosage is on those pens, you might have to inject in smaller increments until you get the total volume in.
Afterr you fill the cartridge, you want to flick it a bunch, just like a syringe, to knock any air bubbles up to the pigtail (luer lock connection). Then when you fill the tubing, the air bubble gets pushed out before the insulin does, and you still wind up with an air-free cartridge. (This was actually normal fill process for the original T:slim, but people complained about how long it takes to fill the tubing, so they have you extract the bubble manually first now. Sucky trade-off, if you ask me).

As for settings, everything gets adjusted so delivering half as much insulin. Correction Factor and I:carb gets doubled. Basal rates get cut in half. The trickiest part is the lazy boluses. You can’t just look at food and “guesstimate” units without stopping to consider that you only need half of what you did before.

As soon as Tandem gets approval for Type 2s, though, all this will get so much easier. Concentrated insulins will get approved for pump use and have more insurance coverage. Novolog would probably join the game. I assume there will be a toggle option somewhere in the settings that will allow you to indicate what concentration insulin you’ll be using, and you won’t have to lie about all your settings anymore.

According to that talk I listened to, concentrated insulins are the way of the future anyway, because there’s so much emphasis on discreet pumping and shrinking the size of these things. Discretion is a MASSIVE factor in marketing to those Type 2s, because the general population incorrectly blames people for this condition, so Type 2s have more of a tendency to feel shameful about needing insulin. We might all be using concentrated insulins in the near future, though, except those with incredibly low needs.

This is the link to the webcast I keep mentioning. You have to register to watch it, but it doesn’t cost anything. I get all the inside dirt by stalking the investor info.


I think getting a correct diagnosis is important. “Total insulin dependence” does not determine type 1 diabetes. Have you had a c-peptide blood test? T1Ds typically have low normal to almost zero c-peptide. C-peptide is a way to see how much insulin your pancreas currently makes. T2Ds will usually have a higher c-peptide number than T1Ds but could have a low number if their pancreas has burned out after many years.

Another test that will help determine the difference between T1D and T2D is for several different autoantibodies. GAD65 is one of the tests. T1D is an autoimmune disease whereas T2D is more characterized by insulin resistance and the presence of metabolic syndrome. Both lead to hyperglycemia and confusion among both patients and medical providers.

If you are a T1D, then getting that diagnosis will help you gain easier access to things like pumps and continuous glucose monitors. It will also help guide hospital staff if you are admitted to the hospital.

Your total daily dose of 120-130 units is higher than most T1Ds but it’s possible to have T1D and use that much insulin. Even people with T1D can become insulin resistant and require larger doses of insulin.

I think it’s a good idea to get the tests needed to identify have type of diabetes you have.

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@Robyn_H, thanks for that informative reply. I actually think this would work except for that dreaded insurance!

I’m on Medicare so currently my insulin and pump supplies are paid for at 100%. I’m guessing my two chances of getting them to cover Lyumjev U-200 are slim and none.

Oh man. Medicare is a complete mystery to me. My eyes gloss over whenever I read about that bureaucratic nightmare.

I do know that U-200 Humalog is on their primary formulary for subcutaneous injection, though. Maybe your doc could write the script for subcutaneous corrections and you still get some u-100 to satisfy the pump requirements? Maybe Lyumjev is covered with a supplemental plan?

In theory, you could even fill 50/50 with u-100 and u-200, and wind up with u-150 insulin in the cartridge. Adjust your settings by a factor of 1.5 instead of 2. I believe the saline solution and preservatives are the same within the same drug name, but double-check me on that. Would give you a max cartridge fill equivalent to 450 units and a max sleep mode basal adjustment equivalent to 4.5 units/hr. And maybe have it still fly by medicare paper pushers? I think it’s all about how cooperative your doctor is and how the script is written.

I’ve gotta tap out there.