Questions about insulin sensitivity factor

OK, I have 2 questions about ISF. (Which I just mistakenly called ‘ICF’ in a response to someone else’s question…)

  1. I know about the 1500/1800 rule, but it seems a bit “one size fits all” to be very accurate. Does anyone ever actually just take, say, 1/2 a unit without eating (and without IOB other than basal) and sit around watching the numbers? Obviously one would only do this while safely at home, with juice handy.

  2. Something puzzles me. My current settings, which seem to be working well, are an ISF of 80 and an I:C ratio of 1:10 or 1:12 depending on the time of day. So, allegedly, a unit of insulin drops me 80 points, and covers 10-12 carbs. But how can that be? Shouldn’t it follow that 10-12 carbs will spike me 80 points? But they won’t.

(Or do I just have misleading data, because the only times I take 10-12 carbs without bolusing is when I’m low, and perhaps have IOB “trying” to drag me lower, so really some of the spike is cancelled out?)

I personally don’t find the various formulas too useful because they are “one size fits all”. What I found best both to figure I:C ratio and ISF is trial and error. Each time you do a correction (being conservative to start of course and taking into account IOB), keep track of how much you come down. I always wrote this number at the end of the line and circled it. Gradually you start to see a pattern, like 1 unit consistently brings you down 75-85 points (I have always seen a bit of variability). Then after awhile you can start to compute using that number as your ISF. The same thing with I:C, and many of us have 3 different ratios for the three meals. Many people start at 1:15 and then work up or down from there depending on results. For me, for the first year I would use a tentative I:C for each meal and then at the end of a page of my log (3-4 weeks) see what percentage of the time it kept me in target range. So if my dinner is 1:18 and I see that 1/2 the time I’m too high I would try lowering it to 1:17 or 1:16 and see how that works.

I have never compared my ISF to my I:C and don’t see that they have any relation though maybe I’ve just not thought about it enough…lol. It does seem like apples and oranges to me. If, by definition (and experience) 1 unit covers 10-12 carbs for you than that means you don’t spike 80 points. I think you’re on the right track when you say the food changes the equation, but perhaps someone can explain it better. I just figure out what works and don’t worry too much over why.

If you are recently diagnosed, you may have a lot of remaining insulin production, in which case the “rule of 1500” probably isn’t so great.

Some other rules of thumb (e.g. about half of daily dose is basal and half is bolus) don’t work so well in those with a good chunk of remaining insulin production either. That’s because the remaining insulin production is not always well split.

These rules of thumb work remarkably well for me today.

The ratios are not laws, they’re ‘rules of thumb’ and only serve as a starting point for therapy. to that extend they are one size fits all, but they aren’t written in stone. They need to be adjusted as therapy progresses on an individual basis.

People do actually take 1/2 unit and sit around watching the numbers. That’s how they do basal testing for pump therapy. It’s one of the ways to test whether the rule of thumb works for you and how it needs to be adjusted.


I don’t know about the rule, but for me the ISF and I:C ratio do work out. One unit will lower me by 2 mmol/L, and I currently have an I:C ratio of 1:10. If I ate 10g of carbs without a bolus it would make my blood sugar go up by about 2 mmol/L if not more.

My ratios are similar. I use a correction ratio of 1:75 and an insulin to carb ratio of between 1:10-1:12 and it works pretty well. I think of 1 carb raising my blood sugar about 5 points so there is a mismatch but it isn’t that much. I dose for net carbs so my ratios are slightly lower than for someone who doesn’t back out fiber.

I was taught that if you want to test your correction ratio you should start when you’re hight (160+) with no insulin on board. I never bothered. The ratio works and I adjust it by “ear” quite often when I correct.


My ratios coincide reasonably well. I use a correction factor of 90, 1 unit lowers my BG 90mg/dl, and a Insulin to Carb ration of 1:18, when correcting for lows I reckon 1gm carb raises my BG 5mg/dl. That seems to work. My TDD is about 20, depending on how much I eat & if my BG is high.

In an ideal world, the relationship you state will work. But lots of things affect the relationship and while you think 10-12 g carbs may not spike you that much, your remaining insulin production probably blunts that rise. It is quite reasonable to set your ISF by doing the tests, just like you noted. About 4 hours after your last bolus and meal, when you have an “elevated” blood sugar, inject a correction dose and see what happens.