I need some help in understanding ISF. I know that it reflects how much my bs will drop using 1u of insulin. My pump is set at 50 which I’ve always thought has been high because I have frequent lows. So…I recalculated and came up with 73. My question is when I’ve been bolusing before meals I’ve been comparing the calculations…seeing what my bolus should be based on an ISF of 50 and then of 73. What has me confused is that when I use the ISF of 73 (using ezCarb) the bolus shown is higher than when the ISF is 50. The opposite was true when I did a correction bolus earlier today…which if my understanding of ISF is correct makes sense. Am I missing something? Thanks!

The ISF is really just for correction. So yes, when you correct with 73 vs 50 as your ISF, you will be doing less insulin.

Your EzCarb meal bolus is computed based on your I:C ratios, not your ISF. (How many carbs you can cover with one unit of insulin).

I thought that might be the case but then why is ISF displayed when I’m entering my bs when doing an ezCarb bolus? If it’s only based on I:C ratio why does the calc change when I change the ISF? Thx for the reply.

When you to an ezCarb bolus, it actually is doing multiple calculations at the same time.

First it uses the number of carbs you tell it your going to eat, divides it by your I:C ratio and gives you the amount of insulin needed to cover the carbs.

Then it takes your BG reading and compares it to your BG Target with the +/- amount. If your BG with within the range, no correction is calculated. If your BG is outside of your range the pump will subtract your BG Target from your reading, divide it by your ISF and give you the amount of insulin to correct BG. This amount will be positive if your BG is higher than your range or negative if it is lower. If the correction amount is positive, it will add that amount to the amount calculated for the carb intake to give you the recommended bolus. It will not subtract the IOB (Insulin On Board), if any, because it assumes you are still digesting a prior meal and the IOB will be used to cover that. However if the correction is negative, meaning your BG reading was below your range, the pump will subtract the correction from the carb coverage amount then subtract the IOB amount and that will be your recommend bolus.

As you can see, it’s a fairly complex process. It’s absolutely critical that you do the necessary testing, and periodically retest, to make sure all your pump settings are correct. That includes basal rate(s) first, I:C ratio(s), ISF, and one that most people forget about IOB Duration (sometimes referred to as DOA or Duration of Action) or how long insulin is active in your system after bolusing. Any one of these settings not being correct will throw off the entire set of calcuations.

Phil is of course, right. Sorry, if I led you astray by oversimplifying. I completely forgot when I responded that there is the option to “correct to target” at the same time you bolus. I almost never subtract insulin if I am “below target” aa I’ve found I’ll go high if I don’t bolus fully for the carbs. Sometimes you don’t know what somebody already knows when you respond to a question and so give an oversimplified (and sometimes obvious) answer!

That’s okay. Thanks for the help.