As a follow-up to my latest OmniPod report, I have done some homework on the Insulin on Board (IOB) issue. Of course, I couldn't let this go :)
It turns out I opened a big can of worms, 'cause this feature is highly complex.
First I queried Insulet Corp. on why they made their IOB (aka BOB for "bolus on board") function work the way it does: it currently shows only insulin delivered as a correction -- and not remaining insulin from a meal bolus -- therefore it can prompt the user to take a post-meal correction too soon after eating if the BG reading is too high. When I posted this, some folks shouted that this approach was "disconcerting" and potentially "a big safety issue."
Hmm, representatives of Insulet tell me that during product development, they surveyed a number of doctors, and the consensus was split fifty-fifty on how to handle IOB. Some pumps, like the Paradigm, apparently treat IOB exactly the same way. But many others do consider remaining meal bolus, and based on this, block the user from taking another meal bolus while a previous one remains in the system. In other words, if you wanted to eat an additional apple after a meal, the pump wouldn't let you bolus for that food while it still sees an active bolus dose.
Insulet apparently wanted to give users the freedom to bolus again without hesitation. But of course we have to be careful. This can lead to insulin stacking and lows.
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However, the Insulet folks did say that considering customer feedback on their IOB function is "on the top of our priority list for future development." I guess I'm not the only one tripped up by it.
Meanwhile, I also queried a couple of pumping experts on the whole IOB/BOB function, and their thoughts on safety and efficacy:
* Virginia Valentine wrote back to me, saying simply, "I would agree that using only the correction for the IOB does not make sense."
* John Walsh, author of Pumping Insulin, had A LOT more to say --
"I have had a few Paradigm patients and talked with several others who are in relatively good control except that they have "unexplained" hypoglycemia that is sometimes frequent. Unexplained, that is, until I show them that their pump is not subtracting any BOB when it is in excess of their correction need from the carbs they are eating. The recommended boluses in these two pumps as well as in the Animas (when the BG is above target) will be excessive whenever the BOB is greater than the correction need.
From my unpublished data, this situation arises in 1 out of every 9 boluses (or about every two and a half days for the average pump user), although I would estimate that in only 1 out of every 20 boluses would the dose be sufficient to cause a significant low. Well over two-thirds of boluses are given with BOB present.
This danger is offset to some extent by the relatively high average glucose levels for most pumpers, but I cannot explain the logic for why these pumps were set up to handle BOB in this way. The engineers appeared to lack direction from a competent medical board. The only rationale that I have been offered after much questioning is that "All carbs need to be covered by a bolus." Well, excess BOB is exactly that, bolus insulin. Why any excess bolus insulin taken earlier in the day should not count as much as a current bolus baffles me.
Two other things need to be kept foremost in mind for an accurate BOB calculation. The first is whether the duration of insulin action has been correctly set. Many pumpers set their DIA too short. When they do this they hide the true activity of their boluses.
The second is that BOB is only calculated after a BG has been entered
into the pump. The pump cannot accurately account for BOB unless it has
a BG to make a calculation for both carb and correction requirements.
Unfortunately, only about 1 out of 7 boluses has both a carb intake and glucose value entered at the same time. Most pumpers are doing blind bolusing."
Eeew, that sounds scary. Since I've actually had relatively few lows since starting on the OmniPod in February, I guess I'm flying pretty well "blind" over here.
In any case, if you want to learn more about how the D-industry handles BOB, see John's latest presentation on the issue, especially slide 19. This man knows his stuff.