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Questions about Dexcom g5 sensor inaccuracy during day 1

Discussion in 'Blood Glucose Monitoring' started by prakash68, Feb 10, 2019.

  1. prakash68

    prakash68 Type 1 · Member

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    Hi All,

    I am trying to understand if the sensor behavior during day 1 is expected. Day before, I changed my sensor around 8 pm, I inserted the sensor in my right arm. CGM reading was spotty as expected but around 6 am next day I got up because of urgent low alarm. CGM reading was 55 but was 110 in blood. Later in the morning cgm reading was 140 but 110 in blood. This was almost 12 hrs after insertion. Following is the graph

    WhatsApp Image 2019-02-10 at 6.36.14 AM.jpeg


    I dont see this level of inaccuracy and this late(12 hrs in this case) often, but, at times, i do see sensor readings way off than what i normally observe for more than a day after which the accuracy improves.

    Questions:
    1. I understand cgm readings can be inaccurate during the first day, but is this level of inaccuracy expected? Also is it normal for inaccuracy to persist for more than 12 hrs since insertion. I have read about 'soaking the sensor' for ~12 hrs to reduce cgm inaccuracy during day1, would like to know your experiences if this helps.
    2. I know that other factors that can contribute to cgm inaccuracy are sensor insertion closer to muscle, dehydration, I was wondering if cgm inaccuracy can be caused by rolling over the sensor during sleep as a result applying pressure on the sensor?
    3. How do we differentiate a sensor that starts way off but eventually becomes accurate after a day or two vs bad insertion site or bad sensor?
    Thanks
    Prakash
     
  2. Scott-C

    Scott-C Type 1 · Well-Known Member

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    Hi, @prakash68 , I don't use dex, am on libre/miaomiao/xDrip+ so it's broadly similar.

    On soaking the sensor, yes, I think it makes a difference for day 1. I usually apply a new sensor about 24 hrs before starting it. There's an article at this link, A Tale of Two Compartments, which says that when a sensor is first applied the slight puncture wound from the insertion needle causes repair mechanisms to spring into action, and also the foreign body response defence mechanism gets to work as well because the sensor filament is a foreign body. Both of those mechanisms are fuelled by glucose, so if they're busy eating it up as fuel for their activities, there's less of it be measured in that area, so it'll report as low. Waiting a while before starting gives a higher chance of those mechanisms settling down so they don't influence things so much.

    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2903977/

    It's an interesting read for people using cgm.

    That's definitely looking like noise at 6am. xDrip+ has a function which tells me when there's sensor noise going on, so I can be more wary of what it's telling me.

    It's one of these things which just happens from time to time. I recall reading somewhere that sometimes the filament can press up against some cells which limits the surface area exposed to interstitial fluid so less glucose gets in to be measured.

    Rolling onto it during sleep can affect it too. I suppose it's a bit like when you pinch skin and it goes whiter because the blood flow is getting restricted. The pressure from lying on it is likely to push interstitial fluid away from it so there's less glucose to be measured. Can't find the link now, but I recall reading a paper about an artificial pancreas, the authors said they were planning on using two cgms with some sort of way of figuring out which one was being slept on and switching between the two - a bit optimistic given the darned price of these things!

    One book I found useful when I started with cgm was William Lee Dubois' Beyond Fingersticks (not sure if it's still in print, but it's on kindle). It's quite old now, about 10 yrs, and the technology has moved on a lot since then, but he makes the still valid point that one of the skills in using cgm is to know when to trust it but also when not to trust it. I love my cgm but I know that it can still throw wobblies now and then. Been using it for about 3 yrs now, so it's become easier to say, yup, don't trust that one, get a strip out.

    Another thing I pay a lot of attention to is when I calibrate it. I made the mistake in the early days of calibrating too often and at the wrong times, and that just makes it worse. Now, I'll only calibrate when I've been level for an extended period, morning when I wake is normally the best as there is a fair chance then that bg and ifg will be about the same.
     
    • Agree Agree x 2
  3. prakash68

    prakash68 Type 1 · Member

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    Hi Scott,

    Thanks a lot for the detailed response! The link you provided was very informative, will try soaking the sensor for 24hrs next time. I will also checkout 'Beyond Fingersticks' book you suggested. I sincerely appreciate for taking the time and effort to answer my questions. Thanks again.

    Regards
    Prakash
     
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