Tslim insulin delivery very slow while bolusing but no occlusion alarms

I was woken up with a high alert yesterday and my BG was like 17mmol (300mg). I thought it was weird as I was good when I went to bed and dinner wasn’t anything out of the ordinary. That site was one day old. I gave myself a manual injection and went back to bed.

I woke in the morning with a good reading 5.4mmol (100mg) and had breakfast only to have a massive post breakfast high 12 mmol (220mg) so I changed my site thinking it’s gone bad. Throughout the morning I was manually bolusing since I was still hovering at the 12 mmol mark. I put a temp basal of 180% and still it didn’t budge. By midday, my Bg was still high and won’t come down so I disconnected my connection and did a 10 unit bolus in mid air to see whether insulin was coming out. Lo and behold I only got like 2 measly drops and it was very slow to come out.

I called support and they troubleshooted and told me nothing was wrong with my pump and I should go and see an endo about my high BG. I told them that didn’t seem right as the insulin flow when I bolused was next to nothing! I suspected my basal was cut off because of this. The only insulin I was getting was from my manual blouses which wasn’t enough without the basal. They still insisted I check with my endo as the troubleshooting guide indicated my pump was working !

I told them I’m going to change out my cartridge, line and site out because this just doesn’t sound right. Sure enough it fixed the problem and my sugar came down.

I’m just a bit concern why the pump didn’t alarm to say there was an occlusion. Even if the line had airbubbles which I couldn’t see, it would have alarmed.

Has anyone experienced this with the tslim? I had it happen once in the past but thought my cartridge was due to be changed as it was on the third day but this one was not.

I haven’t had this happen, of course it’s happened when I eat too much and didn’t count right.

But when I doubt my site or my pump, I will inject my dose with a syringe. And see what happens.

I find that when I run high, I need a lot more insulin to bring it down sometimes double.

It might be a issue with insulin resistance due to high sugars.


I have not experienced this, but would agree may have been issue with cartridge.

Good to know.

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Good for you for sticking with your gut instinct! I hesitated to add my comment since I do not use the t:slim pump but your experience is something I’ve faced many times with other pump set-ups.

Your experiment of asking your pump to deliver 10 units and then disconnecting to actually observe the insulin exiting at the infusion set quick disconnect was a good move.

When you swapped everything out (site, infusion set, cartridge) and that cured the problem, that seems persuasive to me. Theory is fine and it helps us to idealize how a system should work, but when it fails, it requires that you do some pragmatic troubleshooting.

Bottom line was that your body was not getting enough insulin and that drove the BG high. It’s been my experience that when basal insulin is impaired for many hours, it creates quite the deficit that needs to be back-filled before good control returns.

Pump delivered insulin has many failure points:

  • Bad insulin ( not likely)

  • Leak in the cartridge, cartridge/infusion set connection, quick-disconnect, or at the infusion site itself

  • Blockage at the cannula tip caused by tissue inflammation and allergic reaction

  • More generalized poor or non-existent absorption by the margins of the insulin depot (the infused insulin waiting to be absorbed)

  • Air bubbles in the cartridge that interrupt continuous insulin delivery

When I tried the Omnipod, I discovered a pattern of “third-day fade” which led to high BGs. I was not able to resolve this issue and did not determine if it was caused by bent cannula or just a general absorption failure at the subcutaneous insulin depot. This happened so often that I gave up troubleshooting and stopped using the OmniPod.

Your troubleshooting skill is effective and I would encourage you to continue in that manner. The aggressive curiosity you show serves you well.

As to why your pump failed to give appropriate alarms, I can’t offer any good reasons. I do know that designers/engineers struggle to strike a balance between alerting correctly versus alerting too often which can quickly lead to alarm fatigue.

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Adding on to what @Terry4 said, you were getting some insulin although it wasn’t enough. There could be a million reasons for that. I am someone who had years of false occlusion alarms with Tandem and then when I had a bad site, I didn’t get an alarm. I can share some pages from the Tandem manual, but they don’t really explain what you dealt with. If you changed everything out and everything is fine, just move on. For better or worse, nothing about our D-tech is perfect and sometimes you just don’t get an answer.


I went through this in June. I started experiencing severe highs that were unresponsive to aggressive bolusing and had ruled out other potential causes. I got the same responses from Tandem going through all the things only to end up with “check with your doc, it’s not the pump”. Fast forward another week of this crap and correcting with injections which work every time I did the same thing you did and not only visualized the test boluses which seem to be not enough but also because of that i measured them afterward just to see if it was even close to the amount. Now i know it isn’t scientific at all but a 10 unit bolus delivered only gave me 4 units that I was able to pull up into a syringe. And I did that with varying amounts all of which were not even close to the amount that it should have been.
So after much discussion with Tandem they logged my complaint but stated that there had been no other complaints filed for this type of issue. They tried their best to not replace the pump but I persisted and they finally agreed. The other piece of evidence I had was the fact that while this was happening I switched back to my old medtronic pump and my numbers immediately went back in line.
This whole experience has made me consider switching back to MDI and not have to rely on something that can fail.
Hope this helps.


Did it continue to happen after swapping out the infusion set and reservoir?

I also wonder how many reports of this happen but they just don’t know about or just conveniently dismiss it since it’s the easiest thing to do. I did ask whether there has been reports like this from customers and they said no, so obviously your case didn’t get filed or they just didn’t bother to look.

Interestingly enough, that problem resolved with a the new pump but a new problem started with almost daily occlusion alarms with my Trusteel sets. And it was a mix problem between those and the cartridges. Convenient that I had problems with both over the same time frame. 20 site/set changes in 30 days due to occlusions. But that all seems to be resolved now. Just starting to lose faith in Tandem at this point.
I think they are not going to tell other patients of that problem because everyone with unexplained highs would be demanding a pump exchange. If it was proven in a scientific way I think it could be an FDA reportable event and nobody would want that.

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They actually have a system for these scenarios. If you have a situation that isn’t covered by their guidance (rule book with they’re protocols and tests), you can speak with one of their pump trainers, who have the ability to request a replacement without justification if you can establish establish that you’re not just using it wrong.

Unfortunately, because of growing pains and their need to train more sorry staff so quickly, seems like there are plenty of tech support people who’s aren’t trained as well as the hires if previous years. I’ve found that when you’re not happy with the support you’ve received, just call back and talk to someone else. You’re likely to get a different response.


I am glad you posted this. I was on the Tandem pump for a little over three months. I had this issue so many times, I decided to go off the pump and am now back to injections only. I am not suggesting you or anyone should go off the pump, just wanting to validate your issues. Tandem suggested I try different kinds of infusion sets (different angles, steel, longer needles, etc.) that it was my body, not the pump. Convenient for them to assert and if correct, okay, maybe the pump just doesn’t work for me. I found myself having to change sites too often, too many highs for no reason, with or without occlusion warnings. It was the first pump I’d ever used after injecting for nearly 25 years. Not sure if I will go back. My blood sugar is about the same with or without the pump (A1C at 6.2-6.6).

I hope you can figure it out because the pump did offer significant advantages over injections in some ways.

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Tandem pump has a very sensitive occlusion alarm.
I’ve also run into it. It starts happening to me on day 3 because I some allergy to the sets. It gets a little inflamed.
I also limit myself to 6 units bolus at a time because if the sensor issue.

The Steele sets never worked for me because it would hit muscle and clog.
The ones I use are the ones you put in on an angle almost parallel to the skin. I have fewest issues with that

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I’ve noticed that also but feel strange when they look at the notes and say hey you called earlier and this is what we told you.

Yeah this is what I thought too. It was clear to me that insulin’s was blocked up but because it never alerted me, I was a bit worried. And the troubleshooting process with help desk wasn’t making sense, they check off everything on their check sheet and tell me the pump is working and I point out it’s clearly not. All they check for it whether insulin was coming out to pass the test and deem it working as expected. The flow of insulin is not a consideration point. It’s almost like common sense has no room in the troubleshooting because it’s not on their check sheet !

The pump seems fine now.

Just know that the actual pump valve is in the cartridge.
The pump only moves a cam that moves activates the cartridge mechanism.
Since it’s made of cheap plastic and is disposable, it’s the first place I’ll look if there is an issue. Also a good idea to change it out if you are suspecting an issue

I change my Omnipod every 2 days that solves the problem with the third day “fade”.