Dynamic ISF (DynISF)
Up until now, with AMA and SMB, ISF was defined in the Profile and was static for each defined period in the day. But in reality, a person’s ISF is not that static and varies depending on their BG level: when at a high BG level, the user will need more insulin to bring their BG down 50mg/dL / 3mmol/L than compared to a lower BG. Autosens was the first algorithm to try and address this issue, by adjusting ISF outside of mealtimes.
Dynamic ISF (also called DynISF) serves the same purpose but is more advanced as it can be used at all times. It is recommended only for advanced users that have a good handle on their AAPS’ controls and monitoring. Read the Things to consider when activating Dynamic ISF below before trying it out.
CAUTION - Automations or Profile Percentage change
Automations should always be used with care. This is particularly so with Dynamic ISF.
When using Dynamic ISF, disable any temporary Profile change as an Automation rule, because it would cause Dynamic ISF to be overly aggressive in correction bolusing and result in hypoglycemia. This is the exact purpose of Dynamic ISF and so there is no need for AAPS to be told to provide additional insulin by way of Automation in the event of high BGs.
To use Dynamic ISF, AAPS‘ database requires a minimum of 7 days of the user’s AAPS data.
What does Dynamic ISF do ?
Dynamic ISF adapts the insulin sensitivity factor (ISF) dynamically based on the user’s:
Total Daily Dose of insulin (TDD); and
current and predicted blood glucose values.
When using Dynamic ISF, the ISF values entered in the Profile are not used at all anymore, except as a fallback if there is not enough TDD data in AAPS database (i.e. fresh reinstallation of the app).
SMB/AMA - an example of a user’s Profile with static ISF as set by the user and utilised by SMB and AMA.
Dynamic ISF - an example of a user’s ISF subject to change as determined by Dynamic ISF.
The section circled in red shows: profile ISF -> ISF as calculated by DynISF.
Taping on this section shows a dialog with additional information, such as the ISF used for the calculator and carbs absorption (see Other usages of ISF below).
The DynISF value can also be shown in an additional graph, enabling “Variable sensitivity” data. It shows as a white line (see red arrow on the image above).
How is Dynamic ISF calculated ?
Dynamic ISF uses Chris Wilson’s model to determine ISF instead of the user’s static ISF value as set within the Profile. A detailed explanation can be found here: Chris Wilson on Insulin Sensitivity (Correction Factor) with Loop and Learn, 2/6/2022.
The Dynamic ISF equation implemented is: ISF = 1800 / ((TDD * DynISF Adjust Factor) * Ln (( current BG / insulin divisor) + 1 ))
The variables used in this equation are detailed below.
Note : Ln stands for natural logarithm, a mathematical function.
The implementation uses the above equation to calculate current ISF and in the oref1 predictions for IOB, ZT (zero-temping) and UAM. It is also used for COB and in the bolus wizard (see Other usages of ISF below).
TDD (Total Daily Dose)
TDD will use a combination of the following values:
7 day’s average TDD;
the previous day’s TDD; and
a weighted average of the last eight (8) hours of insulin use extrapolated out for 24 hours.
The TDD used in the above equation is weighted one third of each of the above values.
Dynamic ISF Adjustment Factor
This is set within the user’s Preferences and is used to make Dynamic ISF more or less aggressive. See the Preferences section below.
Insulin Divisor
The insulin divisor depends on the peak of the insulin used and is inversely proportional to the peak time. For Lyumjev this value is 75, for Fiasp, 65 and regular rapid insulin, 55.
ISF based on predicted BG for dosing decisions
Dynamic sensitivity is computed with the current BG value, and displayed as your current ISF in AAPS. But when doing dosing calculations, the oref1 algorithm computes and uses Future ISF instead.
This is done to prevent dosing too much insulin when BG is low or predicted to go low.
Future ISF uses the same formula as described above, except that it may use minimum predicted BG instead of current BG. Minimum predicted BG, as calculated in oref1, is the minimum value your BG is predicted to go during all the course of the predictions.
If the current BG is above target
and if BG levels are flat, within +/- 3 mg/dL:
BG is used in the formula as follows:average(minimum predicted BG, current BG).If eventual BG is above target and glucose levels are increasing,
or eventual BG is above current BG:
BG is used in the formula as follows:current BG.Otherwise:
BG is used in the formula as follows:minimum predicted BG.
For a simplified explanation, refer to the screenshot below, which illustrates the above situation. Orange dots use predicted BG, purple dots use average(predicted BG, current BG), and blue dots use current BG.
Other usages of ISF
ISF and COB absorption
As described in the COB Calculation page, usually, the absorption of COB is calculated with this formula :
absorbed_carbs = deviation * ic / isf
When using Dynamic ISF, the ISF used here is the average of past 24h Dynamic ISF values.
ISF in Bolus Wizard
When using the Bolus wizard, ISF is used if BG is above target to add a correction.
When using Dynamic ISF, the ISF used here is the average of past 24h Dynamic ISF values.
Nustatymai
Check Enable dynamic sensitivity in Preferences > OpenAPS SMB to activate. New settings become available once selected.
Dynamic ISF Adjustment Factor
Dynamic ISF works based on a single rule which is supposed to apply to everyone, implying that people having the same TDD would have the same sensitivity. As each user has their own personal sensitivity, the Adjustment Factor allows the user to define whether they are more or less sensitive to insulin than the „standard“ person.
The Adjustment Factor is a value between 1% and 300%. This acts as a multiplier on the TDD value.
Increasing this value above 100 % makes DynISF more aggressive: the ISF values become smaller (i.e. more insulin required to decrease BG levels a small amount)
Lowering this value under 100% makes DynISF less aggressive: the ISF values become larger (i.e. less insulin required to decrease BG levels a small amount).
The Adjustment Factor is also altered when activating a Profile Switch with percentage. A lower Profile Percentage will lower the Adjustment Factor, and vice versa in respect of higher Profile Percentage.
For example, if your Adjustment Factor is 80%, and Profile Switch to 80% is actioned , the resulting Adjustment Factor will be 0.8*0.8=0.64.
This means that, when using DynISF, you can use Profile Percentage to temporarily fine tune your sensitivity manually. This can be useful for physical activity (lower percentage), illness (higher percentage), etc.
BG level below which low glucose suspend occurs
BG value below which insulin is suspended. Default value uses the standard target model. A user can set this value between 60mg/dl (3.3mmol/l) and 100mg/dl(5.5mmol/l). Values below 65/3.6 result in use of the default model.
Enable TDD based sensitivity ratio for basal and glucose target modification
This setting replaces Autosens, and uses the last 24h TDD/7D TDD as the basis for increasing and decreasing basal rate, in the same way that standard Autosens does. This calculated value is also used to adjust target, if the options to adjust target with sensitivity are enabled. Unlike Autosens, this option does not adjust ISF values.
Things to consider when activating Dynamic ISF
Dynamic ISF is recommended only for advanced users that have a good handle on their AAPS‘ controls and monitoring. Users should ideally have attained good control with SMB before moving onto Dynamic ISF.
As mentioned above, turn off all Automations which activate a Profile Percentage in relation to BG because it will be too aggressive and may over deliver in insulin! This is already part of the Dynamic ISF algorithm.
Profile Percentage is taken into account for the Dynamic ISF calculation (see Dynamic ISF Adjustment Factor above). It is bad practice to use a Profile Percentage other than 100% for a long time. If you determine that your Profile has changed, create a new Profile with your revised values in order to replicate the Profile with a specific percentage.
Dynamic ISF may not work for everyone. Specifically, you may see unexpected results if one of these situations apply to you:
Variable lifestyle (inconsistent eating or physical activity patterns)
Inconsistent TDD or sensitivity from day to day.
There is no precise guide to set the initial value of the Adjustment Factor. However, as a starting point: assuming your Profile values are correct, when you are in range and BG levels are flat, the DynISF value should be about the same as the one you had in your Profile before.
If you see that Dynamic ISF is too aggressive, lower the Adjustment Factor, and vice-versa.Even though DynISF does not use Profile ISF at all, if you notice that your sensitivity is very different from what was previously stored in your Profile, you should consider keeping it up-to-date. This may be useful in case you loose your AAPS data (i.e. new phone, new AAPS version…), as your Profile ISF will be used as fallback for the next 7 days.