Calculating Bolus: why ignore Mesl IOB?

[cumbria-pumper]

The OmniPod ignores meal IOB when calculating a meal bolus. Not sure why.

E.g. I just hypo'd and ate jelly babies. The PDM said I had 2.5U IOB but when I calculated a meal IOB to eat 15g it suggested a full bolus for that ignoring IOB.

I assume my meal IOB after treating hypo will continue to take me low so I should eat 15g to "soak up" the IOB.

Can anyone explain if my logic is wrong and why the bolus calculator ignores meal IOB please?

Puzzled of Cumbria!!!

 

[GrantGam]

Tagging @himtoo and @noblehead for you

 

[iHs]

The Vibe does this as well when my bg levels have returned to my target within 2-3hrs which can be because I haven't carb counted too accurately or have come back to target because of basal needing to be adjusted or because of food having a delayed effect with digestion. I usually use trial and error if I'm not sure if the pumps bolus wizard is correct or not and if in doubt, I chance going low and learn from my mistakes and then deduct the IOB from the bolus calculation and use trial and error to see if I've done the right thing or not. Lots of bg testing or cgm or Libre looking. I've now worked it out that if my bg is back to my target within 3hrs, then the IOB will be ignored but if my bg is lower than my target, the IOB will be deducted.

 

[Deleted Account]

My experience of the Vibe is that it does not takes the IOB into consideration if I have bolused recently.
I have not worked out how recently. But it makes sense to me - if I am bolusing twice close together, it is likely that the IOB is for food which has not yet been digested so I can't use that IOB for my second meal/course/snack.

 

[catapillar]

Yes, the omnipod bolus calculator does not subtract insulin on board from the next meal bolus. It only subtracts insulin on board from correction boluses, not carb boluses. If blood sugar is below target range the omnipod bolus calculator will reduce the bolus, but if you are over target range and you have recently previously bolused for food and you bolusing for food again the omnipod bolus recommendation might be more than you need.

I'm sort of mainly hoping you aren't entering your hypo treatment carbs into the PDM?

@helensaramay animas pumps subtract IOB from carb boluses only when blood sugar is below the target range, according to John Walsh in pumping insulin.

 

[Crystalwand]

Thanks for that just thinking that myself. Always someone who knows so thanks again

 

[cumbria-pumper]

Yes, the omnipod bolus calculator does not subtract insulin on board from the next meal bolus. It only subtracts insulin on board from correction boluses, not carb boluses. If blood sugar is below target range the omnipod bolus calculator will reduce the bolus, but if you are over target range and you have recently previously bolused for food and you bolusing for food again the omnipod bolus recommendation might be more than you need.

I'm sort of mainly hoping you aren't entering your hypo treatment carbs into the PDM?

@helensaramay animas pumps subtract IOB from carb boluses only when blood sugar is below the target range, according to John Walsh in pumping insulin.

Be assured I didn't enter the hypo carbs!

This occurred 2-3 hours after food so I was assuming the food had been absorbed (not low GI). As it happened this time I didn't take any long lasting carbs and BG rise a bit further just on the jelly babies. The IOB insulin didn't take me low again.

Waiting for my copy of the Walsh book to arrive!

 

[cumbria-pumper]

To continue this discussion: just got a copy of Pumping Insulin 6th ed. A cursory read on p83 suggests my assumption is what Walsh recommends but no pumps do it. I.e. Pumps don't deduct IOB (BOB) from the carb bolus, but they should according to best practice.

 

[tim2000s]

Yes, one thing we've learned from the Artificial Pancreas work is that IOB calculation in pumps and bolus calculators is notoriously dodgy.

Technically what should happen is that for each bolus administered, the calculator should determine the IOB based on a decay curve (most pumps use the curves from Walsh's pumping insulin), and that should be offset against a standard Carb absorption curve (obviously GI differs, but by adjusting Carb Ratio, you effectively determine the amount of insulin supposedly needed to get glucose level back to start). This tool (Glucodyn) is great for playing with how all the factors interact. Then, if you have more insulin than you have carbs to cover it, it should remove that from the calculation for the next bolus, regardless of whether your current glucose is in our out of target. It should also consider, based your current glucose value relative to your target and on the glucose impact of the carbs and insulin on board, how much additional insulin you need.

Very few of the IOB calculations look like the above.

 

[cumbria-pumper]

And if necessary recommend additional carbs if IOB is greater than needed to get target BG (allowing for food still to be absorbed of course!)