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Situation Spotlight: ISF Matters
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If your ISF settings have any significant drops (e.g., at bedtime), there is a potentially dangerous scenario you should be aware of.
To illustrate this, suppose ISF is reduced by half at 8 pm. If carbs are entered before 8 pm and insulin goes in after 8 pm, the effective CR will be half of what it was before 8 pm and the amount of insulin given will be twice what you may have expected. (Note that the CR after 8 pm doesn't matter here.)
If you want to quantify how this could affect you, apply these formulas at times where your ISF changes: - Insulin Multiplier = ISF before / ISF after
- Effective CR = CR before / Insulin Multiplier
There are a couple of common ways this scenario can happen: - BG low enough that no insulin was given up front
- Carbs retroactively entered after the ISF drop, set to an earlier time
The following example comes from a recent post in the Loop and Learn Facebook group, and had been happening every 6 months or so. - Insulin was initially suspended during a low
- 75g carb was entered before 10:30pm and a 45g carb entry was added later (but with a time before 10:30)
- In both cases, the carbs are before 10:30 and all of the insulin is after
Because the ISF setting was 120 before 10:30 and 45 after, the result was 120/45 = 2.7 times the amount of insulin that would have been given using the CR from before 10:30. The Looper was okay, but spent a rough night fighting lows.
Why does this happen?
Loop first makes an estimate of how much the carbs alone would raise BG based on the CR and ISF. [If this seems unintuitive, remember that CR relates insulin and carbs. It also relates the amount that insulin lowers BG (ISF) to the amount that carbs raise BG.] Then it recommends insulin needed to lower the BG back into range using the ISF. If the ISF is very different before and after the drop, it makes sense that Loop would recommend very different amounts of insulin to counteract the predicted rise from the carbs.
How can this be prevented?
If you must have a large drop in ISF, make sure it's at a time where you won't be entering carbs. If you do enter carbs before the time of an ISF drop, be sure you understand the risk involved in the scenarios above.
[Thank you to Bryan Wilson for his elegant analysis of this situation.]
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