COB calculation

How does AAPS calculate the COB value?

When carbs are entered by the user as part of a meal entry or carb correction, AAPS will add this calculation to the current carbs on board (COB). AAPS then calculates the user’s carbs’ absorption based on observed deviations to the user’s BG values. The rate of absorption depends on the carb’s sensitivity factor (’CSF”). This is not a feature within the user’s Profile but is calculated by AAPS according to ISF/I:C set up, and is determined by how many mg/dl 1g of carbs will raise the user’s BG.

Carb Sensitivity Factor

The formula adopted by AAPS is:

  • absorbed_carbs = deviation * ic / isf.

The effect on the user’s Profile will:

  • increase IC- by increasing the carbs absorbed every 5 minutes thus shorten total time of absorption;

  • increase ISF - by decreasing the carbs absorbed every 5 minutes thus prolong total time of absorption; and

  • change Profile Percentage - increase/decrease both values thus has no impact on carbs absorption time.

For example, if the user’s Profile ISF is 100 and the I:C is 5, the user’s Carb Sensitivity Factor would be 20. For every 20 mg/dl the user’ BG goes up and 1g of carbs will be calculated as absorbed by AAPS. Positive IOB also affects the COB calculation. So, if AAPS predicts the user’s BG to go down by 20 mg/dl because of IOB and it instead stayed flat, AAPS would also calculate 1g of carbs as absorbed.

Carbs will also be absorbed via the methods described below based on which sensitivity algorithm has been selected within the user’s AAPS.

Carbs Sensitivity - Oref1

Unabsorbed carbs are cut off after specified time:

Oref1

Screenshot 2024-10-05 161009

Carbs Sensitivity - WeightedAverage

Absorption is calculated to have COB = 0 after specified time:

AAPS, WheitedAverage

If minimal carbs absorption (min_5m_carbimpact) is used instead of value calculated from BG deviations, an orange dot appears on the COB graph.

Detection of wrong COB values

AAPS will warn the user if they are about to bolus with COB from a previous meal if the algorithm detects current COB calculation as incorrect. In this case it will give the user an additional hint on the confirmation screen after usage of bolus wizard.

How does AAPS detect wrong COB values?

Ordinarily AAPS detects carb absorption through BG deviations. Incase the user has entered carbs but AAPS cannot detect their estimated absorption through BG deviations, it will use the min_5m_carbimpact method to calculate the absorption instead (so called ‘fallback’). As this method calculates only the minimal carb absorption without considering BG deviations, it might lead to incorrect COB values.

Hint on wrong COB value

In the screenshot above, 41% of time the carb absorption was calculated by the min_5m_carbimpact instead of the value detected from deviations. This indicates that the user may have had less COB than calculated by the algorithm.

How to deal with this warning?

  • Consider cancelling the treatment - press ‘Cancel’ instead of OK.

  • Calculate your upcoming meal again with bolus wizard leaving COB unticked.

  • If you need a correction bolus, enter it manually.

  • Be careful not to overdose or insulin stacking!

Why does the algorithm not detect COB correctly?

This could be because:

  • Potentially the user overestimated carbs when entering them.

  • Activity / exercise after your previous meal.

  • I:C needs adjustment.

  • Value for min_5m_carbimpact is wrong (recommended is 8 with SMB, 3 with AMA).

Manual correction of carbs entered

If carbs are over or underestimated carbs this can be corrected through the Treatments tab and actions tab / menu as described here.

Carb correction - how to delete a Carb entry from Treatments

The ‘Treatments’ tab can be used to correct a faulty carb entry by deleting the entry in Treatments. This may be because the user over or underestimated the carb entry:

COB_Screenshot 2024-10-05 170124

  1. Check and remember actual COB and IOB on the AAPS” homescreen.

  2. Depending on the pump, the carbs in the Treatments tab might show together with insulin in one line or as a separate entry (i.e. with Dana RS).

  3. Remove the entry by firstly “ticking” the waste bin on the top right corner (see above photo, step 1). Then “tick” the faulty carb amount (see above photo, step 2). Then “tick” the ‘waste bin’ on the top right corner (step 1 again).

  4. Make sure carbs are removed successfully by checking COB on AAPS’ homescreen again.

  5. Do the same for IOB if there is just one line in the Treatment tab including carbs and insulin.

  6. If carbs are not removed as intended and additional carbs are added as explained in this section, the COB entry will be too high and this could lead to AAPS delivering too much insulin.

  7. Enter correct carbs amount through carbs button on AAPS’ homescreen and set the correct event time.

  8. If there is just one line in Treatment tab including carbs and insulin the user should add also the amount of insulin. Make sure to set the correct event time and check IOB on homescreen after confirming the new entry.