Below are instructions for configuring Loop patches. To ensure the patches are more easily applied to future versions of Loop, all patch settings have been moved to the iOS settings app. Scroll down to Loop and you will see the settings there.

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Partial Bolus Application Factor

Automatic Bolus uses a fixed 40% application factor for safety. What this means is that if Loop calculates that it needs to deliver a 1u correction bolus, it will only bolus 40% of it and give a dose of 0.4. This adds the ability to adjust the application factor without rebuilding.

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Automatic Strategy Switching

This allows Loop to auto switch between Temp Basal and Automatic Bolus. This will force Loop to use Temp Basal anytime BG drops below the switching threshold. You can still use Temp Basal only by selecting that for the dosing strategy in Loop. This will only be enacted if enabled AND Automatic Bolus is selected for Loop’s dosing strategy.

The 2 primary issues this solves are:

  1. Better prevention of rebound lows.

    1. With AB, it can be overly aggressive with a quick rise from a low carb correction. You may take carbs such that you know BG will steady at 110, but Loop sees the fast rise and predicts 250 and decides to dose significantly more. The times this helps is when you see Loop change from a very high prediction down to a steady prediction over a few readings when you stop the fast rise.

  2. More stable BG overnight

    1. Without carbs on board, AB can cause more ups and downs overnight also. If this happens just before waking, you can also find issues with breakfast. If Loop had done an aggressive Automatic Bolus and then needed to cut basal off for a while, you can be hitting breakfast with the tail end of insulin activity rather than a steady state. That can result in a much harder rise when you start eating. Temp Basal is a better way to keep a gentle curve up and down within range as well as keep basal consistent.

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    Negative IOB Factor

    Thanks to Kenneth Stack for working out this code.

    To be clear, Negative IOB isn’t actually real. It is not possible to have less than 0 insulin on board. Instead, negative IOB is somewhat of a Loop oddity used in its calculations.

    When basal is at a steady state, it is approximately 2.1 x units/hour of IOB. If you have 1.0u per hour, you have 2.1 IOB as your baseline at all times, 24x7. This is obviously assuming no temp basals or suspended pump and not counting boluses.

    But Loop treats this steady state 2.1x IOB as 0 IOB. Confused yet? This is why you can use negative IOB to determine if your basal is too aggressive. If Loop reports negative IOB, it means you have less than your scheduled basal in your system which typically would mean your basal rate is set too high.

    That works fine for general purpose setting test times. But for the day to day dealing with sports, extra exercise, sickness, cycle, etc, the way that Loop deals with negative IOB can cause major issues. Loop assumes that your settings are correct and that you live in a perfect little bubble where everyday is exactly like every other day. I’m pretty sure that diabetes still wouldn’t behave the same way even if everyday was identical. So here’s an example we’ve all seen... For some random reason, you are running lower than normal and Loop starts withholding insulin but it doesn’t stop the drop and you just keep slowly going lower and lower. Over the hours you have one juice box, 2 juice boxes, candy, nothing brings it up and IOB just keeps getting more and more negative. When the mystery cause of the low finally breaks and BG starts to slowly climb, Loop sees negative 1.5 units of insulin and decides you need all that back or you’ll be 400 mg/dL in 30 minutes.

    This setting is to throttle Loop’s overly concerned feelings towards negative IOB. It simply adjusts the calculation by the percentage you select here whenever it uses negative iob to calculate a dose.