Missing insulin when filling

I know that the t:slim only shows a +240 vol when it is totally filled...however it feels like I am not getting all the insulin loaded. I fill it with 3cc and I know it takes about 20-25cc to fill....so at the MOST I should have a fill of 275cc....It seems like it only loads 250cc or less....anyone else not getting the full 300u (or 275 after loading) or is it just me?

When I go to my T: connect program and see avg fill amy of 260...where did the rest of my insulin go?

Ideas of what I might be doing wrong?


My average was 259. I notice when I fill it that the last bit seems to have some pressure to it - as in, after trying to push 300u in, it tries to fill back into the syringe. I think while the bag in the cartridge can "hold" 300u, it doesn't have the ability to deliver all of it. Kinda lame to advertise 300u though, when we're getting 40u under that.

I actually ended up calling tandem because I couldn't sit with the idea of all the insulin wasted after i started calculating what was getting thrown away. Upon calling tandem, they told me that:
They allow for approximately 20 units as a cushion which is considered "unusable", as well as small amount for filling tubing. My fill tubing takes approximately 16-17 units. So the variation should be approximately 36-40 units depending on the length of tubing. Hopefully that helps.

This is why I chose T-slim: the 300 unit capacity. Since I was using about 92 units a day, it would give me 3 days of use. Lo and behold, I was only getting 2.5 days!! Called Tandem and was advzd: fill with 300 units (done) but it ony displays 240+. This is normal. Plus, there are units used in filling infusion set tubing ( started on CLIO, hated them as they hurt to insert, failed to adhere to skin after a minute or two And they use almost 20 units to fill tube and cannula) I now use Inset Animas and love them us it only takes 15 units to fill tubing, no pain to insert and no ugly blotches under the patch when removed. I have lowered my intake to 84 units a day due to the almost human delivery of the insulin.

This is starting to drive me crazy. I love my T:slim but miss my Ping when it comes to never knowing how much insulin I'm actually going to get to use. I have called my doctor twice already to increase my insulin prescription and I think I'm going to have to call again. My pump says I average about 55 units a day and my last prescription was for 60 units a day and that has turned out to be very difficult to make last the full time. I always fill my cartridge with the same amount of insulin but the actual amount shown on my pump varies by about 20 units. It feels so wasteful! If I fill the syringe with 220 units, why do I only get to use 175 or so? I've gotten into the habit of using a syringe to remove any leftover insulin from the cartridge before I change it out. Lol I'm sure that's not recommended but I hate throwing it away. Also, I learned the hard way that you can't load the pump with less than 50 units. I filled my cartridge with what I had left in my vial because I wasn't allowed to fill my prescription until the next day and got an error that I loaded less than 50 units. The result? You just lost all of your insulin! I had to draw it all back out into a syringe and use MDI until I could refill my prescription. I can't wait until I get this all figured out!

Hi, Cassie. I've also been having issues related to this with my T:Slim. It was explained to me during training that the dead-space (the hard, clear plastic at the top of each reservoir cartridge) traps around 25 units of insulin that cannot be delivered by the pump. Add to this the approximately 18 units give-or-take that is used to prime the tubing and you've got around 40 units of wasted insulin each time you change cartridges. This is the "dead space":

I have also been regularly removing the leftovers from the “empty” cartridges and returning it to the vial of insulin. I’m not 100% sure that this is the cause of my recent BG problems but I’ve been running ridiculously high every second day after changing my site/reservoir. I just got a brand new bottle of Novolog from my pharmacy and changed everything out this morning, returning the wasted insulin to the OLD bottle just in case there is some kind of contamination going on from the inside of those baggie-things. I’m not sure if anyone else has experienced inexplicable highs while on the T:Slim.

Thanks for “listening” and good luck!

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I agree that the display is misleading and that unlike the cozmo that would allow you to actually have 300 units after filling tubing.. But the insulin is not wasted
you can use the syringe with out the needle to pull the insulin from the tubing and with the needle to pull the insulin from the cartridge ...

When you say insulin was missing after loaded. Are you referring to the amount in the cartridge. Is the cartridge pre-filled? I like the ideal of a pre-filled cartridge, so you can just drop in and get on with the insulin show, but will Medicare allow you to have other types of insulin around to use in case of emergencies. Also, maybe you can't answer this, but since T-Slim doesn't offer a trial am I stuck with it if I don't like it for whatever the reason because of Medicare guidelines. I am thinking the pump to replace the inulin pens and i-port and the expense to me. I thought I would let Medicare pay for my insulin therapy and not me.


Sorry, I was thinking about the Snap insulin pump, which is the one that uses the prefilled cartridge.

From reading past questions like this I have come up with an answer. Many other T;Slim users have called the company asking about this problem. I usually load 200 units/ml w/e. it rounds to 20u to fill the tubing and then the system uses and 20 units as a sort of buffer (maybe if the pump fails or LVidal stated, gets trapped in the system). I've just calculated 40 units off the total I need. So really you will never get the full 300 units you've filled with.

I major bummer the reps never tell you about before purchase.

Ive had the same thing happen before when I load 300 and it only show 250 or less, but recently with my 3-4 year pump I will load aprox 250units and it will register maybe 100-120 units. That seems like way too much of a loss for unusable insulin and tube fills. maybe firmware problems?

My experience with the Tandem proprietary cartridges in both the tFlex (480 unit cartridge) as well as tSlim (300 unit cartridge) was that I would “lose” a significant amount of insulin with each cartridge change (even accounting for tubing priming etc, typically more than 40+ units were “lost”), Tandem’s answer is that that is typical for their pumps. I went back to my Medtronic Paradigm 723 (300 unit cartridge) with no loss other than priming. I used the Tandem pumps for almost 2 years, but still find day to day use easier with the Medtronic. Tandem looks slick and cool with touchscreen, etc, but with Medtronic it’s easier to do routine functions with less button pushes. I’m hoping my out of warranty 723 keeps on ticking until I can upgrade to one of of the new generation Medtronic pumps.

How did you pull insulin from the tubing with syringe without needle? The syringe (without needle) that come with the tslim supplies doesn’t fit the end of the tubing.

4 years ago, highroller.

the old cartridges used luer lock - which was same as syringes

Welcome back @Doug7. Nice to hear from you again.

I use a slightly different method to save a little insulin in my “new” t:flex cartridge: I use the last little bit in my old cartridge to fill the cannula of my Inset infusion set. That only saves 0.7 units, but every little bit helps.

My biggest insulin waster is the amount still showing on the screen but which will run out while I’m sleeping. I don’t like to be awakened with the “you’re below 20 units” alarm, followed just under three hours later by the “under 10 units” alarm. I sleep bad enough without those interruptions. As a result, I tend to change my setup if it is/will be <40 units showing at bedtime.

This is made worse since I’m now using FIASP and I’ve found that I get uncorrectable occlusions if I load >300 units into the cartridge. So I lose ~40 units three times with a vial. And the t:flex has a minimum first alarm amount of 20 units.

I may have to change my sleeping habits and just let it alarm.