Tuesday, May 10, 2016

CGM: post insertion wound impact - an interesting paper/patent.

We all know (well almost) that the first 24 hours after sensor insertion can be a bit tricky in terms of accuracy. The Libre and Dexcom typically display very different behaviors: the Libre tends to read too low for a while. The Dexcom tends to oscillate and be a bit (or a lot) noisy...

During our fist Dexcom year, I tried various mathematical techniques to analyze and possibly improve the first day results. Once you have collected a bit of data, it is relatively easy to retrofit a solution and find out how interstitial glucose evolved. This is unfortunately not that useful in practice: smoothed retro-active data is nice, but hardly helpful in the moment.

Tapping in the "Internet wisdom" isn't very helpful either.
  • a lot of Libre users have a few hours where the sensor reads too low, but some don't.
  • a lot of Dexcom users see a noisy post insertion period, some see oscillations, some don't.
  • for some people, bleeders are keepers. For others, bleeding means sensor failure.
  • etc...
Our strategy for dealing with them has been to feed more calibrations during the first 24 hours, with the goal of reaching a close approximation as quickly as possible, thanks to error averaging. That approach worked well, but isn't easily explainable to users who aren't comfortable with maths and can't analyze the situation they are in. If the sensor values oscillates for example, you need to recognize that and feed the calibrations along the baseline. But what worked with the G4 non 505 will not necessarily work with the G4 505 or the G5 as the "blackbox" algorithm basically does as it pleases with the calibration data you provide...

The bottom line is that we'll simply have to wait for manufacturers to improve the way they deal with the first day.

As far as the root causes of that first day inaccuracies are concerned, the medical literature usually does not go into details. It is obvious that a less traumatic insertion will lead to better results (blood is not the sign of trauma, just the sign that you went through a small vessel). It is well known that the foreign body gets encapsulated over time. Individual responses in terms of trauma and encapsulation run the usual biological variability gamut...

If you are interested in the detailed mechanisms behind those phenomenons, I suggest you read this Analyte Sensor patent. It's a patent, but it reads like a very nice accessible science paper from location [150]. And it is a fascinating read! I knew of some, but by no means not all, the source of interference it describes.

Please keep in mind however that this is a relatively old patent which means that while all the potential issues it describes are valid, some of them have been addressed in new sensor designs.

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