Resistance when filling the tslim cartridge

When i filled my tslim cartridge today, it wouldn’t fill the full amount in my syringe. I’m wondering whether I did something incorrectly and whether anyone has had the same thing happened.

The cartridge is brand new. I filled the syringe with insulin, sucked out the air from the cartridge, removed the air bubbles from the syringe and topped it up so that the insulin level was around 290u, and then removed air bubbles once more.

Then began filling up the cartridge. When I got down to about 20 units, the plunger wouldn’t move any more. I pulled out the syringe to check and no visible air bubbles I can see and insulin would come out as I pressed the plunger. So what went wrong?

It’s a pain that you can’t see the inside of the cartridge.

It happen once and really don’t know what was wrong. Onetime I had an issue when there was a lot of resistance pressing the insulin into the cartridge and that time there was an issue with the needle of the syringe. Once I changed the needle, it worked fine. Anyway I only had two issues, so haven’t really worried about it.


It’s called vial coring, when a core of the rubber (or whatever material) stopper plugs up the syringe needle. It’s more of a problem with bigger gauge needles, hence why diabetics don’t get trained on it. Unfortunately, Tandem recently changed the cartridge needle to a bigger size, and this problem has been popping up more often. Good news is, they’ve taken the feedback and switched back, but who knows how long it will the warehouses to get down to the new supplies.

In the meantime, try entering both the insulin vial and the cartridge at an angle, with the hole in the needle pointed up. It’s supposed to reduce coring.

With enough force, you can actually shoot that core out, but it can possibly cause occlusions in the tubing later. I don’t think it’s possible for the core to actually make it through the infusion set and into us, though. I try to save a spare needle or two for just in case.


I have this happen SO OFTEN (viral coring; TIL) that I run low on needles before anything else. It’s very frustrating and I have resorted to reusing them because of this! Glad to know it’s an issue for others, as well.

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Good to know, never heard of that. Thaks!

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@Robyn_H - As always, you are wonderful! Thank you for explaining.

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I never put that much in but maybe you filled it up. It only holds 300 units and it’s difficult to gauge from the syringe.
I usually put 150 in mine that’s more than 3 days worth.
Everyone needs different amounts. But are you using 100 units per day? Or are you keeping it on for longer?

If I go past day 3 my site gets irritated

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I fill it up to slightly under 300 units and keep it on for longer. I don’t see the sensitivity become less effective after the 3rd day so I’m able to do this but I note others have complained about the insulin going bad.

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Worse than the site getting irritated, the Tandem pump just does not deliver the last 50 units in the cartridge well. You can hang it up with 50 units left. I can only put 200 units in to begin with. With my old 530 I could go a week while changing the site in between and my A1cs were under 6. I do not have as good numbers with the Tandem. I too wish I could see the inside of the Tandem cartridge.

I change infusion site every 3-4 days, but change reservoir (insulin) every 5-6days, when it is < 40 units. Insulin is too expensive to waste!!

Me too. People also don’t realize that you can draw leftover insulin out of the cartridge even if it says zero units left. I can get a good amount back out, which I use in the next fill-up. (I don’t do this indefinitely - when I get a new bottle out I start completely fresh.)

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I also thought about this but wonder whether its worth contaminating my fresh batch of insulin I’m about to fill the cartridge with. I read things like the plastic inside the cartridge starts deteriorating the insulin. So in theory if the insulin is less effective by day 3, could mixing it with a new batch would make the new batch not as effective?

It hasn’t been a problem for me over the past two years. If anything it’s a very tiny amount so it’s not likely going to destroy the viability of the next cartridge. I’ve heard other people say insulin degrades in the plastic but don’t know how accurate that actually is either.

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I agree, never an issue. Insulin is a pretty durable beast.


My humalog says it’s only good for 2 days in my tslim.
I keep it for 3 though and I don’t have any issues.

Where did you get this idea?? Tandem?–I doubt it. FB?–Maybe. Given the world-wide number of needles puncturing vials everyday, your idea seems unlikely.

I’m a retired medical researcher, and my background is in molecular and cellular biology. This is rudimentary information in the medical field… precisely because it is so common. Vial coring is taught to nearly every nursing/med school student, lab tech, and other paraprofessionals. If you type “vial coring” into a search engine, you will find countless results from medical journals.

I will admit, though, that I didn’t read the OP as thoroughly and I should have. I missed the comment that he was trying to fill cartridges to the max capacity when he met the resistance. The coring problem is VERY COMMON with the large gauge needles Tandem was using for a while, which results in a syringe full of insulin that you can’t push out… But I admittedly jumped the gun, and it might not have been the answer to this particular inquiry. I suspect this particular case has more to do with the cartridge capacity.

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Actually I don’t think it was at capacity when the resistance happened. The cartridge holds 300 units and I wasn’t there yet. Furthermore when it happened, I drew out some insulin from the cartridge using the same blocked needle and then pushed the insulin into the cartridge again and everything went in the second time, so I do suspect it’s probably due to the coring you are talking about. When I drew insulin back into the syringe, it must have dislodged the blockage and coincidently didn’t block up a second time. The fact it filled the second time confirms to me that the cartridge capacity isn’t full yet.


Being of engineering bent, I took a used cartridge apart to see how it worked.

The insulin fills a bladder that runs the length of the cartridge and is attached to the clear plastic mechanism at the top where the cannula is attached.

The bottom of the cartridge has one opening into the chamber where the bladder resides. I would hazard a guess that there is air pressure on the bladder ensuring that insulin is supplied to the clear plastic mechanism.

The clear plastic mechanism appears to be a metering device controlled by an actuator coming into the side facing the pump mechanism. I can not see this mechanism working due to the black plastic hood. I’ve been tempted to remove parts of this black plastic hood on a used up cartridge and see what goes on inside the clear plastic mechanism but have not gotten around to that as of yet.

When the pump is supplying insulin, there are two primary sounds you can hear. I’d guess one is an air compressor that puts pressure on the insulin bladder and the other sound is most likely the side actuator that allows the insulin to flow through the cannula to the infusion set.

I’m quite sure this is more than you wanted to know…


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Tandem told me that humalog can only be used for 2 days in their cartridges.