Resistance on needle loading cartridge

I think i’ve read msgs about this but I’ll ask anyway. Occasionally, when filling the Tandem cartridge I encounter stiff resistance when pressing the plunger. I have to try a couple times to find a soft spot in the little white pad.

Today, it was MUCH stiffer, and I had to try nearly half a dozen times to make it go into the cartridge at all. I was very reluctant to just push hard for fear I’d either wreck the needle or the cartridge or both. And I certainly didn’t want to wreck the cartridge because I don’t have spares and Medicare is snotty about how many they’ll let me have every 90 days!

What did I do wrong? What should I do differently next time?


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Sometimes the pump valve mechanism is in the way.
So just try a slightly different angle until you hit it.
I find it’s easiest to install it into the pump first. Then the plastic pump valve is put into place and out of your way.

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Thx Timothy. I didn’t know it’s ok to use the order of the steps you mention.

Yes it’s actually better because I find the cartridges difficult to hold they are a weird shape.
People sometimes refill them and don’t take them off the pump at all.
The pump doesn’t k ow if the cartridge is in or out.
It measures insulin with a pressure sensor.

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From Tandem’s view, its not ok, so proceed at your own risk.
If it doesn’t work for you and you call support, they will likely instruct you on doing it based on their instructions. May give a reason why not to do otherwise.

I fill cartridge to max, and change when low, usually every 5 days, not every 3 days as instructed. Similar situation, Tandem would not support this.

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@Tom_in_SC it can also happen when you cork the needle. A simple needle change (needle, not entire syringe) usually solves that problem if it’s not a problem as described by @Timothy .

It’s called “coring”.

It’s called coring the needle. The trick is to lay the tip of the syringe needle down on the white cartridge rubber at a 45 degree angle with the tip opening facing up. Push the needle straight down, not into the white rubber, then start pushing the needle in as you tilt it up to go straight in to the cartridge at 90 degrees. You have to kind of roll it into the rubber…no coring when done this way.

As someone noted, if it does happen, just change the needle.

Smaller gauge needles may reduce the risk of coring but may make the cored piece more difficult to see should coring occur. Using blunt fill needles may also reduce the risk of coring (and needle stick injuries).


Well, after 38 years of drawing up insulin from vials into insulin pump cartridges and syringes, this is the first time I’ve read about coring. It makes sense. I remember thinking many times about how magical the performance of the vial’s rubber stopper was. I’ve not observed broken pieces of the stopper in my insulin vials. But I was never aware of this hazard, either.

As a non-nurse who has a ton of needle and syringe experience, it’s sometimes hard for to discern which nursing practices are essential for health and safety and which are simple niceties that are practices done out of an abundance of caution and rarely, if ever, lead to adverse outcomes.

Thanks for teaching me something new.

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Just a thought, If a piece of the stopper is injected into a cartridge, that could be a cause of a site occlusion, you think?

We use injectable ivomectin monthly to heart worm treat our dogs. We give it to them orally. The other day I had a needle occlusion, I could not draw air into the syringe at all. I wonder if I cored the needle. Interesting.

That is always a possibility, but I think I have had some go in before and nothing happened.

There was a period of time not too too long ago that I had problems with the needle getting stopped up very frequently. This of course ended up with me having to reuse needles since I didn’t have enough. I don’t know if it was a bad batch of needles or what.

In my experience it has been always been the cartridge rubber that gets stuck and not the vial rubber. There may have been one time when I was using humalog (which has similar white rubber) that the vial plugged it up. Fiasp has a different seal material, so I can rule that out. I have definitely never had the needle get blocked from those vials.

It is certainly very annoying when it happens. Occasionally you can pull the plunger all the way out and reinsert it and use the air pressure to push it out, but that has mixed results.

As a funny story when I was a kid and not on a pump yet I was playing around with one of my needles and sucked up some candle wax that obviously plugged up the needle. I did the whole pull plunger all the way out and put it back in thing because I thought I would be able to shoot the wax out, but instead the whole end of the needle shot off like a dart and went somewhere never to be found again. Needless to say my kid brain was like “WOAH COOL!!!” lol.