Wednesday, February 18, 2015

Another view of the Freestyle Libre vs Dexcom comparison

Here is another view of the Freestyle Libre vs Dexcom G4 non AP comparison.

The first graph shows the 8000 min comparison run previously described.

The second graph shows shows the ideal time shift determination by optimal correlation (see here) that corroborates the impression that the Libre is faster than the Dexcom non AP algorithm and puts a value of 9 to 10 minutes on that reaction speed difference.

The third chart shows the Bland - Altman plot (see here and here) for that period, with the Libre as S1 and the Dexcom G4 as S2, time shifted by 9 mins for optimal correlation. I used Bland Altman plots in me Dexcom real-life calibration analysis project before but have not posted any of them as they are possibly a bit harder to immediately understand than glucose as a function of time charts, numbers such as MARD and SD or Clarke plots.

The general idea behind a Bland Altman plot is to compare the results of two unreliable measurement techniques by plotting their differences as a function of the average of the measured values. In this case, that plot confirms the subjective impressions which were.

  • The Libre is more "trigger happy" than the Dexcom, especially when IG is rising quickly, a bit less so when falling. A least-squares regression line clearly shows the different characters of the sensors. The Dexcom G4 non AP and the Libre algorithm clearly implement a different philosophy: the Libre relies more often on its raw data and is more aggressive in extrapolating from it.
  • Most of the results fall in the +/- 1 SD band, which confirms that the differences between their long term views weren't clinically significant. The short term usefulness is another story: catching highs or lows earlier definitely has benefits.
  • Interestingly, the mean of differences for that period is about 11 mg/dL (Libre: 106.68 - Dexcom 95.49) which is typically what we observe, on average, when comparing with BG meter tests. Our latest HbA1c (hospital lab blood test) turned out to be 5.2%, a value that correlates well with the 106.68 mg/dL Libre result whereas the Dexcom 95.49 mg/dL should have resulted in a value of 4.9 to 5.0. 
This being said, let's not read too much in those values as the differences are clinically very minor and within the range of acceptable laboratory errors.

One last comment: whenever I publish a post that can be interpreted as saying that the Libre is better and faster, I receive at least one comment implying that I am an Abbott shill. Of course, whenever I am negative about Abbott on the spyware issue, some think I must be a Dexcom stooge.

Let me re-state that I am totally independent and self funded, not even covered by social security. We pay for our sensors and haven't received any gifts or review units.

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