Understanding the determine-basal logic

The core, lowest level logic behind any oref0 implementation of OpenAPS can be found in oref0/lib/determine-basal/determine-basal.js. That code pulls together the required inputs (namely, recent CGM readings; current pump settings,including insulin on board and carbohydrates consumed; and your profile settings) and performs the calculations to make the recommeneded changes in temp basal rates that OpenAPS could/will enact.

Short of reading the actual code, one way to start to understand the key determine-basal logic is to understand the inputs passed into the script/program and how to interpret the outputs from the script/program.

Summary of inputs

The determine-basal algorithm requires 4 input files:

  • iob.json
  • currenttemp.json
  • glucose.json
  • profile.json

In addition, the algorithm can accept 2 optional input files:

  • meal.json
  • autosens.json

When running oref0-determine-basal.js, the first thing you’ll see is a summary of all the inputs, which might look something like this:

  • meal.json = {"carbs":0,"boluses":0}

    • carbs = # of carbs consumed
    • boluses = amount of bolus insulin delivered

    Those data come from what you entered into your pump or Nightscout web app. If provided, allows determine-basal to decide when it is appropriate to enable Meal Assist.

  • glucose.json = {"delta":5,"glucose":161,"short_avgdelta":4.5,"long_avgdelta":3.92}

    • delta = change in BG between glucose (most recent BG) and an average of BG value from between 2.5 and 7.5 minutes ago (usually just a single BG value from 5 minutes ago)
    • glucose = most recent BG
    • short_avgdelta = change in BG between glucose (most recent BG) and an average of BG values from between 2.5 and 17.5 minutes ago (that average represents what BG levels were approximately 10 minutes ago)
    • long_avgdelta = change in BG between glucose (most recent BG) and an average of BG values from between 17.5 and 42.5 minutes ago (that average represents what BG levels were approximately 30 minutes ago)

    Those data come from your connected CGM or from your Nightscout web app.

  • temp_basal.json = {"duration":0,"rate":0,"temp":"absolute"}

    • duration = Length of time temp basal will run. A duration of 0 indicates none is running.
    • rate = Units/hr basal rate is set to
    • temp = Type of temporary basal rate in use. OpenAPS uses absolute basal rates only.

    Those data come from your pump.

  • iob.json = {"iob":0,"activity":0,"bolussnooze":0,"basaliob":0,"netbasalinsulin":0,"hightempinsulin":0,"time":"2017-03-17T00:34:51.000Z"}

    • iob = Units of Insulin on Board (IOB), net of your pre-programmed basal rates. Net IOB takes all pre-programmed basal, OpenAPS temp basal, and bolus insulin into account. Note: iob can be negative when OpenAPS temp basal rate is below your pre-programmed basal rate (referred to as “low-temping”).
    • activity = Units of insulin active in the previous minute. Approximately equal to (net IOB, 1 minute ago) - (net IOB, now).
    • bolussnooze = Units of bolus IOB, if duration of insulin activity (dia) was half what you specified in your pump settings. (dia_bolussnooze_divisor in profile.json is set by default to equal 2, but you may adjust this if you’d like OpenAPS to activate a low-temp sooner or later after bolusing.) bolussnooze is used in oref0-determine-basal.js to determine how long to avoid low-temping after a bolus while waiting for carbs to kick in.
    • basaliob = Units of net basal Insulin on Board (iob). This value does not include the IOB effects of boluses; just the difference between OpenAPS temp basal rates and your pre-programmed basal rates. As such, this value can be negative when OpenAPS has set a low-temp basal rate. Note: max_iob (described below) provides a constraint on how high this value can be. The determine-basal logic will not recommend a temp basal rate that will result in basaliob being greater than max_iob.
    • netbasalinsulin = this variable isn’t used in OpenAPS logic anymore, but hasn’t been removed from iob.json yet.
    • hightempinsulin = this variable isn’t used in OpenAPS logic anymore, but hasn’t been removed from iob.json yet.
    • time = current time

    Those data are calculated based on information received from your pump.

  • preferences.json = {"carbs_hr":28,"max_iob":1,"dia":3,"type":"current","current_basal":1.1,"max_daily_basal":1.3,"max_basal":3,"max_bg":120,"min_bg":115,"carbratio":10,"sens":40}

    • Contains all of the user’s relevant pump settings
    • max_iob = maximum amount of net IOB that OpenAPS will ever allow when setting a high-temp basal rate. This is an important safety measure and integral part of the OpenAPS design. You should set this value based on your current basal rates and insulin sensitivy factor (ISF, or sens in the OpenAPS code) and after studying how the OpenAPS algorithm performs in low-glucose suspend mode for (at least) several days.

    Those data are set during the openAPS setup script (or modified by you directly) and based on information received from your pump.

Summary of outputs

After displaying the summary of all input data, oref0-determine-basal outputs a recommended temp basal JSON (stored in suggested.json), which includes an explanation of why it’s recommending that. It might look something like this:

{"temp":"absolute","bg":110,"tick":-2,"eventualBG":95,"snoozeBG":95,"mealAssist":"Off: Carbs: 0 Boluses: 0 Target: 117.5 Deviation: -15 BGI: 0","reason":"Eventual BG 95<115, setting -1.15U/hr","duration":30,"rate":0}

In this case, BG is 110 mg/dL (6.1 mmol/L), and falling slowly. With zero IOB, you would expect BG to be flat, so the falling BG generates a “deviation” from what’s expected. In this case, because avgdelta is -2.5 mg/dL/5m (-0.1 mmol/L/5m), vs. BGI of 0, that avgdelta is extrapolated out for the next 30 minutes, resulting in a deviation of -15 mg/dL (-0.8 mmol/L).

deviation = avgdelta * 6 (or every 5 minutes for the next 30 minutes) = -15 mg/dL (-0.8 mmol/L).

The deviation is then applied to the current BG to get an eventualBG of 95 mg/dL (5.3 mmol/L). There is no bolussnooze IOB, so snoozeBG is also 95 mg/dL (5.3 mmol/L), and because (among other things) avgdelta is negative, mealAssist remains off. To correct from 95 mg/dL (5.3 mmol/L) up to 115 mg/dL (6.4 mmol/L) would require a -1.15U/hr temp for 30m, and since that is impossibly low, determine-basal recommends setting a temp basal to zero and stopping all insulin delivery for now.

Full definition of suggested.json:

  • temp = type of temporary basal - always “absolute”
  • bg = current blood glucose
  • tick = change since last blood glucose
  • eventualBG = predicted value of blood glucose (based on openaps logic) after full effect of IOB
  • snoozeBG = predicted value of blood glucose adjusted for bolussnooze IOB
  • predBGs = predicted blood sugars over next N many minutes based on openAPS logic, in 5 minute increments
  • IOB = net insulin on board
  • reason = summary of why the decision was made
  • duration = time for recommended temp basal
  • rate = rate of recommended temp basal in units/hour

Exploring further

For each different situation, the determine-basal output will be slightly different, but it should always provide a reasonable recommendation and list any temp basal that would be needed to start bringing BG back to target. If you are unclear on why it is making a particular recommendation, you can explore further by searching lib/determine-basal/determine-basal.js (the library with the core decision tree logic) for the keywords in the reason field (for example, “setting” in this case would find a line (rT.reason += ", setting " + rate + "U/hr";) matching the output above, and from there you could read up and see what if clauses resulted in making that decision. In this case, it was because (working backwards) if (snoozeBG > profile.min_bg) was false (so we took the else), but if (eventualBG < profile.min_bg) was true (with the explanatory comment to tell you that means “if eventual BG is below target”).

If after reading through the code you are still unclear as to why determine-basal made a given decision (or think it may be the wrong decision for the situation), please join the #intend-to-bolus channel on Gitter, paste your output and any other context, and we’ll be happy to discuss with you what it was doing and why, and whether that’s the best thing to do in that and similar situations.

Note about Square Boluses, Dual Wave Boluses, and Basal Pump Settings of Zero

Due to the way the Medtronic Pumps operate, it should be known that temp basals can only be set when there is no bolus running, including extended (square) / dual wave boluses.

Thus it should be noted that if you use an extended bolus for carb heavy meals (e.g. Pizza), which may still be the optimal approach for you, OpenAPS will not be able to provide temp basals during the extended bolus.

If you have periods in the day where your pump normally has basal settings of zero - your loop will not work! You can resolve this by setting the lowest possible basal setting your pump will permit. OpenAPS will then issue temp basals of zero, as needed.