Thursday, November 13, 2014

Dexcom G4 (non AP) vs Freestyle Libre vs Glucose Meter

The test has begun



Our small test has started. A couple of days ago, Maxime volunteered to have two sensors installed. His usual Dexcom G4 and our newly received Abbott Freestyle Libre. I would have liked to put both sensors at the back of the left arm since the back of the arm is the only recommended location for the Abbott Freestyle Libre. Unfortunately, as you can see in the picture below, the previous Dexcom sensor had left Max with a small irritation that I did not want to cover before it was fully healed. The Freestyle went where it was supposed to go. The Dex went in a location we had used in the past with great success (BG meter relative MARD of 12.9% for the two weeks the sensor ran there).


The Libre insertion was totally painless as other users have already reported. Abbott has definitely nailed (hmmm) the insertion process. Once once you have inserted the sensor in the applicator, a spring is primed. You simply place the applicator on the site and press. At some pressure point, the spring is freed and the sensor is both inserted and glued to the skin. I was a bit unsure about how hard I should push and did not want to hurt Max so I fiddled a bit. The trigger caught both of us by surprise, and that was it: sensor in and glued. This compares very favorably with the Dexcom insertion process where you have to manipulate a device that looks a bit like a torture instrument that proudly displays a very long visible needle that you should stick carefully in place before counter-intuitively holding and pushing at the same time before pulling and holding to withdraw the needle. The Dexcom insertion process was however smooth, very little pain and no bleeding. 

I should however mention that during next day's tennis session, Max complained that he was feeling the sensor in his arm. That had never happened with the Dexcom sensor. 

Start-up Sequence and day 1

Note: you may want to download the full chart below and keep it open if you want to look at the first day results in details. It's a big chart because there are a lot of things I wanted to track and also because I like big charts... ;-)


The Freestyle Libre becomes available an hour after it has been inserted. The reader displays a nice precise countdown and tell you how many minutes are left before you can take your first reading (another plus compared to the vague Dexcom green pie). That's with some trepidation that we did our first scan. It was instantaneous and reported a slightly worrying 60 md/dL. (orange dots are spot readings taken with the Freestyle Libre) We immediately double checked with a meter blood test (white stars) and the result was a very good, and clinically expected, high 80s mg/dL reading. There was a small downtrend to 50 mg/dL until 21:00, which we measured to be in the 70 mg/dL range. The Libre was reporting values about 20/25 mg/dL lower than the blood meter and did so consistently until 7:00.

We calibrated the Dex around 90 mg/dL, in a stable situation and it started humming along nicely. Then, something that we had never seen before happened: the Dex started climbing at full speed, reaching 150 mg/dL a bit before midnight. I had previously seen the Dex fall quickly because of a compression event. I had seen it drift slowly away from the real value. But I had never seen hit shoot for the sky after an insertion. Midnight triggered a recalibration (red dot + white star). We were still in the 90s and the Libre was still too low by about 25 mg/dL. Then, after less than an hour, the Dex's horn started to sound the alert for a severe LOW. That, of course, wasn't the case but triggered an additional blood check and calibration. 

During the next 5 hours, the Dex started to oscillate around a slowly rising line. The Libre was also rising and oscillating, not necessarily in sync with the Dex... I can't imagine how a new CGM user would have reacted...

I assumed this was reflecting the typical slow increase we have been fighting for a while and decided to ignore any fast rise or fast fall alerts the Dex could give. Knowing your current patterns and the clinical symptoms of lows is, in my opinion, essential. Don't fall into the "blind faith in technology" trap! 

5:00 led to our now traditional early morning correction dose (2U Novorapid), yet another calibration and finally, at 7:00, Alleluja! The Meter, the Dex and the Libre (sounds a bit like the title of a Sergio Leone movie) finally agreed. 

Max went off to school where, at 10:00, he preemptively over-corrected a possible low, went a bit high pre-meal at noon without adjusting his insulin dose. He came back home at 13:00 at which point we had the following surprise (the meter is still on summer time, that has been corrected for the chart).


Now, I have to confess something: whenever I see someone who posts a set of matching numbers and starts raving about how perfect everything is, I usually exhale deeply and starts thinking about the widespread innumeracy that plagues this world. But even rationally knowing that this does not meant much in itself, I was floored and thought "woaw!". The Dex was trailing, but it can be argued that it would have caught up within 15 minutes or so. Keeping in mind that delay, I would not have taken a wrong decision if I had relied on it and that is what matters.

We corrected immediately with 6U of Novorapid and a 30 minutes wait before the meal. A few interesting things happened, but they need further investigation.
  • the Libre spot measures continued to climb aggressively. Looking at the averaged data in details it is borderline strange but too long to discuss here.
  • the Libre went AWOL during the steep fall - it kept telling us "No glucose data available, check again in ten minutes" The Dex kept tracking the fall.
  • when the Libre recovered, the Dex went into "???" mood for about 25 minutes around 14:00
Around 15:00 we began our carb loading for the tennis practice that started at 16:00 and lasted to 17:00. The Libre tracked the rise before the Dex did. But the Dex tracked the fall during practice before the Libre. 

Between 17:00 and 18:00, we saw our usual post training increase (could be caused by either a disconnect between the adrenalin+cortisol release triggered by the exercise but irrelevant because the exercise had ceased, or excessive "re-carbing"(doubtful in this case) or also the lowest Lantus on board point of the day. 

The evening calibration ended up closer to the Libre than the Dex and, from that point, both the Dex and the Libre essentially agreed 100%

We had a mild symptomatic low between 21:00 and 22:00. We had added 1U to our usual evening meal dose because of the high pre-meal glucose level and we probably overshot because the post-exercise increase was a "fake" increase, caused by the mobilization of existing glucose stores rather than by an excessive input. Finally, we saw an early rise that we corrected around 3:00 AM when it reached 150 mg/dL.

That was indeed a busy first day! Stay tuned. 

PS: typos, non intelligible sentences, other errors, do not hesitate to shoot. 

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