Tuesday, January 20, 2015

Some Libre peculiar behaviour patterns

The standard wisdom about the Libre is to consider that it takes a measure every minute (spot measures) and then averages them every 15 minutes in values that are stored in a 32 entries circular table (8 hours) as I previously described here. Some progress has been made connecting the raw values with actual data, by Vicktor and friends for example.

It should be noted, however, than a straightforward interpretation will not always work as the sensor varies or encounters one or more of its many possible error conditions.

In addition to, even when the sensor is working smoothly, peculiar behaviors can be observed.

Consider the first example below

Around 17:20, we did both a blood check and a Libre spot check. The Libre indicated 154 mg/dL and the meter 137 mg/dL a still acceptable discrepancy, if a bit on the high side compared to how the sensor has run until now. Twenty minutes later, the Libre spot check was at 175 mg/dL. We skipped the blood test as Max was about to eat and we were going to inject the pre-meal insulin anyway.

Now, if we look at the average historical values, there is really no way we can end up averaging higher values into lower values, unless we also had an equivalent number of spot checks well below the average, in the 60-70 range. But that never happens... While we did not monitor both the raw and the official values every minute (there's more to life than living by the ticks and tocks of your CGM), we have seen that pattern quite a few times already. Interesting facts here
  • it is hard to reconcile the high spot value with the historical average
  • the high spot value is almost perfectly what would have been predicted by linear extrapolation of the three previous historical data points.
This begs the question: how raw is the displayed spot check value. Is what we are seeing simply the reflections of jumps in the raw measurements or the result of a clever algorithm predicting the future and hopefully displaying what the meter will show. The repeated uncanny accuracy of the Libre vs observed meter values is still a bit surprising as far as I am concerned. Interstitial Glucose Values should, to some extent, trail Blood Glucose Values (the latest research I have seen estimates a 6 minutes delay). I have the impression that the Libre does better than that in many cases... (but of course analyzing 3 sensors on a single individual isn't enough)

And there is more: the historical value, which we would have expected at 18:53 is missing. The sensor is operating normally and delivering accurate and consistent spot checks at 19:03

178055    2015/01/20 18:38    0    84                                                           
178056    2015/01/20 19:02    1        85                                                       
178128    2015/01/20 19:03    1        85

Does that mean that something malfunctioned? Not at all. The historical data just appears later than a naive interpretation of the process would have expected.

178056    2015/01/20 19:02    1        85                                                       
178128    2015/01/20 19:03    1        85                                                       
178178    2015/01/20 18:53    0    87                                                           
178179    2015/01/20 19:08    0    71                                                           
178180    2015/01/20 19:30    1        59

and we have now an explanation for the relatively strange record numbering scheme Abbott uses and the answer to some related questions about raw data and its processing.

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