Friday, November 14, 2014

Another day - another Dexcom G4 (non AP) vs FreeStyle Libre chart

We are now in the third day of the comparison test. In theory, both sensors have now had the time to settle and should be performing without hiccups. Let's have a look. This time, I used a pure scatter plot to make the different sampling frequencies visible.

The night

We increased our Lantus by 1U in an attempt to master our dawn phenomenon. For some reason, it worked exceptionally well. Too well compared to the correction doses we normally have to use. There was actually less physical activity than usual during the previous day. This is the reason why I am a bit concerned to use a pump with increased insulin delivery to counter the dawn phenomenon (see previous posts).

The Dexcom continued to oscillate a bit (mild compressions?), fortunately without hitting the 55 mg/dL that triggers alarms, regardless of the settings. The Libre happily stayed in an excellent range, between 100 mg/dL and 80 mg/dL. I am sure the higher actual sample frequency (every 2 mins in theory) smoothed in one measure every 15 minutes helps with the oscillations.

The morning

Which one was correct? According to the clinical symptoms during the night and the meter test at 7:00, the Libre was correct. The morning calibration was clearly above the Dexcom data. But, as you can see, we have lost the period from 4:00 to 9:00 because Max forgot to pick up his third device and scan himself. Since the Libre only stores 8 hours of retrospective data and we could only scan Max when he came back from school at 17:10, we lost everything between 4:00 and 9:10. Win for the Dexcom here.

10:00 to  12:00 was PE class. At that point, the Dexcom went out of range. But as the Libre receiver was quietly resting on a shelf at the house, the sensor was still recording and we can clearly see the effect of the two sports drink Max took on his own to maintain his blood glucose. Win for the Libre.

The afternoon

We then have a small meal spike at 13:30 and a relatively stable, if low, afternoon. The Dexcom alarmed several times during that period, but Max did not take any drastic action as he wasn't feeling as low as the Dex suggested. I would again rate the afternoon as a win for the Libre.

Back at the house at 17:10, we scanned and decided to take a few grams of carbs (6-7grs) to raise the BG a bit. We know by experience that after 16:30 the Lantus has mostly run its course after 22 hours (note - this varies from person to person) and we correct very sparingly. The Libre picked up the additional carbs almost immediately and rose sooner and more sharply than the Dexcom. Another win for the Libre here.

A miracle

Then, at 19:10, a miracle happened: the Libre,  the Dexcom and the meter all agreed, within 2 mg/dL.
Max started eating and the Libre picked up almost immediately while, for some reason, the Dexcom continued to fall for a while.

Additional notes
  1. I have previously established that the Dexcom under-estimated our values relative to the BG meter, this seems to also be the opinion of the Libre.
  2. I am aware that this was a lowish day and that the dose of Lantus that keeps the dawn phenomenon under control is sub-optimal for the rest of the day. Compromises, compromises...

Stay tuned for the next episode, and have a good diabetes day, if there is such a thing.

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