Insulin Stacking

Hi,
I'm always frightened of injecting for 1 course of food then injecting a second time if I then decide to eat more.

Please can someone explain exactly what insulin stacking is?

Insulin stacking is taking further QA, when the previous QA taken on board is not yet all used up. It makes it a little more difficult to predict where your BGs will end up when all the QA injected has finished acting. It isn't really something to be scared of, but just be aware that the margin for error is a bit higher as a result of multiple injections within the action period of the QA.

If you test several times afterwards, around the peak action period of the QA (usually about 90 minutes after the injection), and every 20 minutes after that until the QA runs out, then you should be able to catch any major swings up or down, and take corrective action.

I sometimes have say my breakfast and two hours later want to eat more. I just match the cp with the qa regardless of my BG reading.

If you know your breakfast ratio is right and that the carbs you counted were definitely what you ate, then just matching CP with QA is the spot on way to do it if eating less than 4 hours after you last ate. If however you have a high post-meal rise and you aren't sure if your ratio is right or not, or if you guessed the carbs and think you may have guessed less than you actually ate, then you can also correct. Some blood test machines will give you an indication of how much insulin from the previous injection is still in your system (Freestyle Insulinx or Accu-chek Avivva Expert and probably others too) but if yours doesn't do this you can use some maths if you want to:
30mins after injection you'll have around 90% of the dose still unused
1 hour after, about 70% remains
1 1/2 hours after, about 50% remains
2 hours after, about 35% remains
2 1/2 hours after, about 20% remains
3 hours after, about 5% remains
4 hours after, about 0% remains
Although this will vary for individuals it's a good guestimation to go on.

So an example of avoiding 'insulin stacking' this morning for me; I woke at 18.0mmol and dosed up 3.4units. Two hours after I was 17.4mmol, and my machine predicted I had 1.2units left in my system. I knew the 1.2units left wouldn't get me down as low as I wanted to go but also knew re-dosing the original 3.4units again would be far too much as even though it didn't look like it was working, it still had time to do so. So the maths based on my correction ratio meant that the remaining 1.2units in my system would probably get me down to 12.6mmol and so I needed an additional top up of 1.2units to get me down to the 7s. If I had just taken that 17.4mmol score on its own without the history and dosed up for purely that score I'd end up close to hypo based on what was already in my system, because I dosed up based on roughly what I thought was in my system I should be almost normal now. (I'm on a pump so can get the exact dose values, but you still use the same maths for pens just round up or down to the nearest whole number). Injecting each time you eat doesn't risk stacking as the additional QA should be matching the additional CPs and so will happily be used on what it was intended for - correcting each time you eat does however risk stacking as your previous correction may not have had chance to do its job and then the additional correction will be 'correcting' (or 'mis-correcting'!) something that given time doesn't need correcting.

That's really useful Gemsa, thanks!

I tend to eat little and often and worry about how much insulin is still in my system when I test my BG so this will come in very handy