CGMS Accuracy Oddities

I have noticed something truly bizarre lately with respect to my CGMS accuracy.

For the last couple of months (I’ve been using sensors continuously during this pregnancy), my first 2-3 days with a sensor are crap in terms of accuracy.  Not completely useless crap, but pretty unreliable crap.

Then, when Day 4 hits, and I restart my sensor, it’s almost like it magically becomes uber-reliable and fantastically accurate.  For the next 3 days I’m in sensor heaven!

Then Day 7 hits and my transmitter needs a recharge (otherwise I get all kinds of Lost Sensor errors), so I recharge and restart….and still accurate!

The girl who used to have to remove sensors by Day 4 because they weren’t working anymore is now reluctantly removing (almost) perfectly functioning sensors on Day 7 or 8 because of skin irritation.  (At that point I’m starting to get nervous about infection if there’s any sort of itchiness or redness.)

What is going on?!  Is this a new pregnancy thing?  Is this to do with the fact that I pinch up my skin on insertion now?  Is this something new with the sensors?  I’m totally lost for an explanation.

Now I just have to figure out how to improve on the accuracy during those first 1-3 days!

The funny thing is, Medtronic recommends only using the sensors for 3 days and then tossing them.  Hmmm…..


9 responses to this post.

  1. Happens to me too. On Day 1 the numbers are pretty useless. (Bad starting calibration? I’m not sure…) It lasts me to Day 6, more and more accurate each day, then lately I abruptly get a “Lost Sensor” on Day 7. I figured the sensor was done, because the “Sensor Batt” on the pump screen still says “Good”.

    I think the insertion of the sensor means everything in terms of how long it will last. That and luck — because I’m sure that all Medtronic sensors are not created equal.


  2. Posted by Sarah on February 27, 2012 at 4:21 pm


    You aren’t alone in this. Whether it is pregnancy or something else, I am not totally sure. I am on Dexcom and pregnant, and I see the same thing with junk readings for the first couple of days of having it on. I have also noticed that the rise rate is really off too (I will get an alarm with double arrows up, and testing lets me know that it just isn’t true). I do get really long wear out of my sensors after the initial craziness though. I can wear for at least 8 days with carefully placed Tega, and I only lose it due to adhesive and not due to poor readings. It stinks nonetheless though!


    • Interesting to hear that Dexcom has the same issue. I sometimes wonder how Medtronic CGMS and Dexcom compare. Dexcom isn’t available to me in Canada. 😦


  3. Posted by Sarah on February 28, 2012 at 4:18 pm

    That really stinks. I have tried both, and went with Dex. I’m more comfortable with the Dex adhesive, since I tend to break out with the Minimed type. Prior to pregnancy I was able to get 10, sometimes 14 days out of my sensors with a restart in between. That was nice. Keeping the adhesive stuck was the only pain. The numbers stayed right on, but I wouldn’t recommend “experimenting” like that while pregnant. Do let us know if you find the “silver bullet” for this issue of accuracy. I really hate it being out of commission for those days.


    • How do you keep it in for 10-14 days without irritation/infection?! Wow!


      • Posted by Sarah on February 28, 2012 at 9:30 pm

        I have never had that issue with the Dex CGM sensor for some reason. I often had to pull the MM 3-4 days after insertion due to those issues though. I never even considered it until my doctor asked me to wear it continuously for 2 weeks to try to avoid the gap. I was at first reluctant and nervous, but gave it a try anyhow. I cover each site completely with Tega cut to fit Dex, so maybe that helps? Anything beats a huge chunk of downtime when you need real results. I decided to let them go a day more each time for a few weeks to see if I could really extend wear without consequence (I monitored site as closely as possible, and it helps that my husband is a doctor and did the same religiously). It worked for the most part- this is pre-preg of course. They don’t all last that long, but some have. I can never wear a pump site any longer than about 2.5-3 days- insulin left or not without absorption being off or irritation/infection/knot, so it is only a CGM thing for me. Even sometimes helps save a bit of money though.

  4. Posted by Constantin on February 29, 2012 at 9:41 am

    wow…in the german dexcom community this is common knowledge 😦
    they all restart their sensors after the first day…
    that could explain alot of your frustrations with the dex ^^


    • I’ve never really thought to just restart after Day 1. I always assumed it needed the full 2-3 days to calibrate or “wet” or something. I’ll have to try that next time. Thanks for the tip!


  5. Posted by Sylvie on March 15, 2012 at 1:23 pm

    I often just let the sensor get “wet” for the first 24 hours and then turn the sensor function on (on the pump) after that. My last sensor lasted 9 days. Although I don’t have issues with skin irritation, especially if it is really well secured and water proof, I still fear scar tissue from leaving it in that long…but the cost of each sensor plays into my decision to leave it in or not 😦 Just my two cents 😉


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