Monday, November 30, 2015

Dexcom's temperature compensation issues?

I wrote previously about the issues we had with warm baths and the predictive part of the Libre algorithm. You can have a look at what I wrote about warm baths in Some Libre peculiar behavior you should know about: temperature and I Love the Libre when it misbehaves. Temperature compensation is a bit tricky for CGMs. To some extent, the Dexcom G4 non AP algorithm's averaging behavior and the fact that packets were not received when the transmitter was submerged masked the effect with the G4. One had to dig into the details of the Dexcom patents to see it was also affected (as expected as it is a physico-chemical issue).

For some reason (I speculate, but don't have rock solid data yet), the G4 AP is clearly more sensitive to temperature changes than the non AP version. Here's a dump of some this last two week's traces. In all cases, after the drop of signal that indicates submersion, we observe false higher reported ISIGs (confirmed by BG tests). See for yourself.

At this point, I am simply reporting the fact and considering the hypothesis that it is indeed caused by the temperature increase. There could be another cause, for example water changing the conductivity between the two transmitter plots (resistance should be infinite).  We'll try to run an additional test to invalidate the temperature hypothesis, but that will not be a cold bath. I love experiments, but will not torture the kid.

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