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Thread: Omnipod underbolusing for reverse corrections?????

  1. #11

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    Quote Originally Posted by swellman View Post
    I come up with +0.07U without rounding - +0.10 when rounded.

    EDIT: I don't think it's legit to round until all the math is over.
    I thought about this more ... and I was wrong about the rounding, I think. It's rounding to the nearest 0.05 which, in this case would be 0.05 (7 is closer to 5 than 10).

    EDIT: I think it's generally accepted that -0.18 would round to -0.20 mathematically. If it is rounding each step then that would explain the 0.00U, right?
    Stay at home Dad to son, 14.
    Diagnosed: 02/2006
    OmniPod: 09/2007, Novolog
    Dexcom Seven Plus: 02/2010 Dexcom G4: 01/2013

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  2. #12

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    You can always override the PDM if you don't like what it's come up with. It makes sense to me that it would round to give less insulin when under target. We rarely use the reverse correction, though. I'd be perfectly happy with a pre-meal 85.

  3. #13
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    I think that this isn't really a pod malfunction; instead it is a standard way that the pod works and your ISF needs to take it into account.

    Out of curiosity - in the 9 out of 10 times that the pod missed 1/500th of a unit, how did it affect her numbers afterwards? We are talking a reverse correction - so did she go really high? Or just not high enough?

    If her sensitivity factor is 1/200 - 1 unit of insulin will lower her 200 points (or 1 missed unit will raise her 200 points) - then I guess I am scratching my head why 1/500th of a unit keeps causing you so many problems? This should only affect your BS by 10 points max if your sensitivity is correct.
    I'm still here.
    DD - 15 - Lantus and MM Pump/Dex G4

  4. #14
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    Quote Originally Posted by Lee View Post
    I think that this isn't really a pod malfunction; instead it is a standard way that the pod works and your ISF needs to take it into account.
    I agree that it is not a pod malfunction -- but I need to understand the "standard way that the pod works." So far, I have not been able to, despite me and DH looking at the numbers, figure out what the standard way is. In most cases -- but not in all cases -- it appears that the reverse correction number is rounded up and the carb bolus number is rounded down. I have looked at the numbers with standard rounding, rounding all up, and rounding all down, and cannot find the "standard way that the pod works." I have a call into a clinical services manager for Insulet.

    Quote Originally Posted by Lee View Post
    Out of curiosity - in the 9 out of 10 times that the pod missed 1/500th of a unit, how did it affect her numbers afterwards? We are talking a reverse correction - so did she go really high? Or just not high enough?
    I had -- as a general matter -- noticed, particularly going in to dinner, that DD's numbers were more predictable when she went into dinner at or above her target and that it seemed that whenever we did a "reverse correction", we got much higher post-meal numbers and wound up doing nighttime corrections.

    Quote Originally Posted by Lee View Post
    If her sensitivity factor is 1/200 - 1 unit of insulin will lower her 200 points (or 1 missed unit will raise her 200 points) - then I guess I am scratching my head why 1/500th of a unit keeps causing you so many problems? This should only affect your BS by 10 points max if your sensitivity is correct.
    We started using the 1:200 ISF at pump start in the hospital. I agree that, ideally, .05 should only involve 10 points. But it does not seem to. And the reason that I particularly noticed it was that the PDM's suggested bolus for the 12 g. snack was zero. Based on my experience, obviously limited to my DD and only since last November, a 12 g. snack with no insulin would be a problem (we were doing uncovered school snacks on MDI).

    It would not surprise me at all if her ISF may need to change -- the 1:200 roughly correlates to some of the formulae out there and seems to work -- that may be because most of our big corrections are in the first half of the night when she seems to be a bit insulin resistant. We are continuing to work on DD's nighttime basals and I think once those are improved, we may well end up changing the ISF.

    I get that others may think that I'm making too much of this. That's fine. We are dealing with a multitude of variables. But how a suggested bolus is determined when a reverse correction is involved (and there is no IOB) should not be one of them -- it's math, not particularly complicated math.

    For the moment, I'll just recheck all the PDM's calculations and override the bolus as needed.
    Last edited by virgo39; 10-05-2010 at 11:25 AM.
    virgo39
    DD dx at 5 yrs, 3 months (11/09)
    Omnipod w/Novolog (7/10)

  5. #15
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    Quote Originally Posted by danismom79 View Post
    You can always override the PDM if you don't like what it's come up with. It makes sense to me that it would round to give less insulin when under target. We rarely use the reverse correction, though. I'd be perfectly happy with a pre-meal 85.
    I agree with the pre-meal 85, however, as DD's bedtime is 7:45, all other things being equal (and assuming this would even remotely be in my control ), I'd prefer a bit of reverse correction when the pre-meal 85 is at dinnertime.

    Once I understand how the PDM is actually doing the calculation, I'll be more comfortable overriding the suggested bolus (we do that now to address IOB for a meal/snack bolus), for now, I'm just redoing the math myself.
    virgo39
    DD dx at 5 yrs, 3 months (11/09)
    Omnipod w/Novolog (7/10)

  6. #16
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    Quote Originally Posted by swellman View Post
    I thought about this more ... and I was wrong about the rounding, I think. It's rounding to the nearest 0.05 which, in this case would be 0.05 (7 is closer to 5 than 10).

    EDIT: I think it's generally accepted that -0.18 would round to -0.20 mathematically. If it is rounding each step then that would explain the 0.00U, right?
    Well, the carb bolus would have been .24 and the reverse correction would have been -0.18. So it looks like it rounded the reverse correction up to -0.20 and the carb bolus down to .20. Of the 10 or so examples I found in the last couple of weeks, that approach accounts for all but one of them (where reverse correction would have been -0.205 and carb bolus 0.400 and suggested bolus was 0.20, so it looks like the reverse correction was not rounded).

    I'm thinking that there must be some fairly standard rule that the PDM is programmed to apply -- it's frustrating to be unable to figure it out (I've not found one single approach to rounding that explains all of the examples).
    virgo39
    DD dx at 5 yrs, 3 months (11/09)
    Omnipod w/Novolog (7/10)

  7. #17

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    We simply don't use the reverse correction feature on the MM, because I don't like how it calculates it. I shave off a few carbs when he goes into a meal low, and do not enter a BG.
    Mom to J., age 10
    Dx 2007 @ age 3
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  8. #18

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    Quote Originally Posted by hawkeyegirl View Post
    We simply don't use the reverse correction feature on the MM, because I don't like how it calculates it. I shave off a few carbs when he goes into a meal low, and do not enter a BG.
    We did not use reverse correction on our old cozmo and do not use it now with Omnipod either. I know it should work in theory, but it just doesn't for us. I can give ds a couple fruit snacks if he's lower than I want him to be or just shave off a few carbs like Hawkeyegirl said.

  9. #19

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    Quote Originally Posted by hawkeyegirl View Post
    We simply don't use the reverse correction feature on the MM, because I don't like how it calculates it. I shave off a few carbs when he goes into a meal low, and do not enter a BG.
    That's what I do, too. I know, in theory, the reverse correction is supposed to work, but that's not the case for us.

  10. #20
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    I spoke with an Omnipod representative today. She confirmed what one of the PP's surmised -- that the "suggested bolus" is determined by "rounding" the calculated carb bolus down and any correction down minus any IOB (based on the pump's linear formula, which is in the manual), so that the "suggested bolus" will be the lower, more conservative amount of insulin.

    So if the math results in a carb bolus of .24 u, the PDM will round that down to .20 u. Similarly a reverse correction of -.16 u, will be rounded to -.20 u. Those two numbers will be added, and assuming no IOB, the "suggested bolus" will be 0.0 u. Of course, if you simply added .24 u and -.16 u, you would get .08, which you might round up to .10 or down to .05.

    I'm glad I understand this now (in reviewing the spreadsheet that I created to look at some of the numbers, I realized that some of my formulae were using numbers carried to three, rather than two, decimal places, which is why I couldn't see the pattern.).

    When we started pumping, I did look at the individual calculations, but quickly started routinely relying on the "suggested bolus".

    I think this rounding may be contributing to some of the wonky numbers that seem to follow going into dinner a bit below Target. I haven't looked at the numbers (not sure I need to), but we typically pre-bolus 20-25 carbs at meals, and then generally do at least one more bolus after, but sometimes, particularly at dinner, it can be more. I had been thinking that those "mini-boluses" were better, but I can see how they could add up in a way that results in DD getting a less insulin.

    Now that I'm aware of this, I will be more mindful of the calculations and be more ready to make changes to the suggested bolus.
    virgo39
    DD dx at 5 yrs, 3 months (11/09)
    Omnipod w/Novolog (7/10)

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