Saturday, February 7, 2015

Dexcom G4: bad initial calibration and wound "oscillations"

Post Insertion Issues

I suspect most Dexcom users have faced situations like the one shown below. Let's look at it in details

A fresh sensor was inserted a little bit after 7:00PM and calibrated around 9:30PM. Unfortunately, the initial double meter calibration turn up to be a double 54 (actually a triple, we rechecked). Based on our experience and a previous analysis on additional user submitted data, I positively hate low calibrations, especially initial ones. We hate them so much that we call them the "low calibration lottery". I'll get back to that issue in a future post.

We correct the 54 with 2 or 3 dextrose tablets, aiming for 100 mg/dl. We are never very aggressive in our corrections and, in that case, the Novorapid injection is 3 hours old and shouldn't have a drastic impact anymore. For some reason, the Dex decides to start at 45 mg/dL and seems to correct itself after 30 minutes. We appear to reach the expected 100 mg/dL but then start dropping quickly. As I am about to check on and correct the alert (no obvious compression event), the Dex shoots back up. At that point, we still have a choice: either a compression event that I did not spot or the post-insertion chaotic oscillation we have often noticed during the night that follows a fresh insertion. (btw, I do not have any idea about the root cause of that oscillation - there's some micro edema and healing going on, for sure, but why does it oscillate?). Twenty minutes later, we are in insanity territory at 180 mg/dL. A blood check is obviously required as the extremely quick raise doesn't make sense. The meter gives us a mid 60ish value (consistent with a conservative correction and the tail of the insulin activity). I decide to recalibrate the Dex and it almost immediately goes in the dreaded "???" mode. Max takes another 3 dextrose tablet "better safe than sorry" correction and I double check on the meter at 1:00 AM - at this point, I do calibrate the Dex again, even if it is still in "???" mode. I know BG is not wildly fluctuating, I know Max often trends up slowly over the night. About 2 hours 30 later, the Dex picks displays a single point (according to raw data, the oscillations are decreasing in amplitude) on the way down, then goes "???" again as the raw data doesn't correlate with the trend. Finally, while it keeps oscillating, raw data stops being in profound disagreement with the trend and the sensor starts behaving as it should.

How was the following week? Quite good actually as you can see from the chart below. Days 3 to 7 were essentially perfect until, of course, the sensor was ripped off during a basket-ball game at the school... (yes, I felt a hint off despair that day)

As the weekly Clarke grid shows, the only really problematic value came from the initial oscillation.


Errors we made

  • calibrating in low range. We know it is a lottery, we should NEVER do it.
  • playing basket-ball :-)

Errors we haven't made

  • over correction in the 60sh range.
  • acting on senseless CGM data
  • removing a sensor that seemed to be misbehaving badly


The Dexcom here is still running the "old" algorithm, whose calibration behavior I have analyzed extensively based on user submitted data. It probably does not apply to the new 505 algorithms.

As far as the calibration analysis is concerned, I am aware that I haven't released the full results in a single place. The reason is as follows: I examined the data, came to some (non ground breaking) conclusions and detected a few peculiar behaviors that I wanted to test experimentally with Max. When those tests were completed, as I was about to summarize them, Dexcom released the 505 updates to its US customers. The analysis I had done was potentially obsolete and I decided not to release a final report.

I have now learned that the 505 algorithm will never be released in Europe and that Dexcom users are likely to remain stuck with the old algorithm until the new Dexcom products (probably the Dexcom+Share and the Apple integration) are released. Because of that delay, I will probably post some kind of final report in the coming weeks.

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