- Important warning: this comparison is based on one Freestyle Libre sensor and two Dexcom G4 sensors. IT IS NOT STATISTICALLY SIGNIFICANT. If we had started with a bad Libre sensor, and if we had been running the supposedly improved 'AP' algorithm, our first impressions could have been different.
- Disclosure: we are 100% self-funded both for the Dexcom and the Libre. While I did purchase and sell some Dexcom shares this year, I currently have no financial interest or link with either Dexcom or Abbott.
- Summary: the Libre outperformed the Dexcom G4 in almost every aspect. If we did not need night time autonomous alerts, we would have simply put the Dexcom G4 in a drawer.
Now that I have said that, more details.
We've collected a lot of data over the 14 days we ran both systems in parallel but giving objective numerical data is going to be a bit harder that I expected. I am still thinking on the best approach as there are a few issues that could bias the comparison in favor of one system or the other. The problems are mostly due to the different sampling frequencies of the devices. The Libre provides a new "spot-check" value every minute, the Dexcom every five minutes. However, the Libre "historical data" is smoothed on a 15 minutes basis. Comparing a meter test to an "historical" Libre data point is unfair to the Libre since, in practice, it reacts much faster to changes in glucose level when asked a spot check. In fact, it reacts so quickly that it is possible to see the changes induced by a meal or exercise in near real time. Looking at the last Dexcom trend value recorded prior the meter BG test is slightly unfair for the G4 as it is already a bit behind and this makes matters worse. Looking at the first value recorded post meter BG is also unfair if that value was used for calibration. I think I will proceed along the following lines: for every meter test, compare with the closest Libre spot check and the value that the Dexcom would have displayed at that exact moment. This is probably the most functional approach: when you want or need a meter test, you are likely to spot check and/or glance at the G4 receiver, possibly taking into account the slope of the curve.
The global view and general impression
This is a global overview of the last 13 days of the period. I dropped the first day since both sensors misbehaved to some point (the Libre stayed too low for about 10 hours, the G4 behaved somewhat erratically). The Libre historical data is in red, the G4 is in green and the meter checks are black dots. Be careful not to draw very specific conclusions based on this graph: the time scale is huge compared to the daily charts we normally use. I will show event oriented smaller charts in the future.
As you can see, both sensors saw roughly the same story: if you are interested in tracking your diabetes globally or reflect on the day's patterns, both systems are definitely up to the job and, on average, will not mislead you.
Each system has its character: the Libre is almost always faster than the G4 (this is by-the-way the reason why a pure accuracy comparison would give a very bad image of the Dexcom G4) . It is also, as already noted, more "trigger happy". Give the Libre a rising glucose level, see it shoot happily for the stars. We ended up liking that pessimistic mood as it was both an early warning and a motivation to control the situation. It's more emotional than rational but we felt happy to have been warned, happy when the meter confirmed the rise but also showed that it wasn't as bad as expected and, we felt a bit annoyed at the Dexcom that would have eventually warned us too late. Therefore, even if the Libre we had seems to have a tendency to overshoot highs a bit, we liked it better than the Dexcom that was slightly more accurate in its high values, but delayed.
The Dexcom felt sluggish compared to the Libre and had a tendency to linger in lows, long after they had been addressed and corrected. Again, we preferred the Libre: it picked up corrections faster and was less stressful than the G4.
The sensors weren't perfect: the Libre missed a single clinical, meter confirmed, low on the third day. It wasn't far off but still in the OK zone. The Dexcom had a several misses that we found extremely annoying. For example, not to be outdone by the Libre, the Dexcom missed a bad low on day 12. There were more incidents, not visible on this chart as the calibration and interpolation between pre-calibration and post calibration data makes the Dexcom look better than it was. Since a single unattended error can mean trouble, we wouldn't run a totally automated artificial pancreas that aims for our level of control on either sensor alone. Of course, the Dexcom had its usual share of "compression events" leading to annoying false lows. The Libre had no visible compression event in this run (it is not however totally immune to them as I have noticed running a Libre sensor on myself). .
Two sensors are definitely better than one: with two sensors running simultaneously and a bit of attention (the clinical lows missed by one of the sensors), we really could have stopped pricking. In fact, during the second week, we took most of our decisions, including insulin shots, on the basis of the sensors alone and, to tell the truth, mostly on the Libre. We kept the two mandatory/recommended G4 calibration finger pricks and that was it. If we did a few additional checks, it was more out of curiosity than out of need. Unfortunately, running two different sensors simultaneously on a self funded basis wouldn't be possible for most diabetics and insurance or social security will find the idea hard to swallow... There was, however, an unexpected downside to running two sensors: we had learned to live with the Dexcom 15 minute delay and we mentally compute where we will soon be, based on the value displayed and the look of the curve. But having two devices show significant differences because of timing issues are definitely disturbing at first (for example post-meal, where the Dexcom would show 80 mg/dL stable and the Libre would show 125 mg/dL rising because it already had picked up the post-prandial increase). By the second week, we had adapted and usually considered what the Libre showed us to be what the future held for the Dexcom.
Overall, we trusted the Libre more than the G4: the ability to see a trend that generally agreed with the Dexcom, while at the same time being ahead and spot-checking generally very close to the meter was a confidence booster. I plan to look at certain occurrences in detail and give some numerical data later, as soon as I have a bit of time.
An elephant in the room? No, there are more!
There is, of course, an elephant in the room: while the Libre continuously monitors your glucose, it doesn't actively tell you about it. No alarms on lows you might be unaware of, no alarms on fast post-prandial rises (or for that matter, clogged infusion sets). The lack of alerts will be a deal-breaker for those of us who rely extensively on that functionality.
But, and this is where the situation becomes extremely frustrating, there are other elephants.
The Dexcom data lives in the short term past. If it is all you know, you get used to it, adapt and interpret. But once you have tasted the Libre, this becomes very hard to accept. It's just like having two PCs side by side, one running Windows XP with 2GB RAM and a decent hard drive and one running Windows 7 with 16GB RAM and a SSD... In theory, the AP Dexcom firmware improves things a bit. I will of course test it as soon as it becomes officially available in Europe (note: a big public "Thank You!!!" to the two readers who offered to ship me an upgraded Dexcom receiver from the US so I could test it - there are amazing people in the diabetes community)
And then, the practical issues
Both sensors had a difficult start, for about 12 hours. The second Dexcom sensor was a bit better in that respect, not spectacularly accurate during startup, but not misbehaved either. The Libre told us once that it could not deliver a reading. The Dexcom went '???' a few times. The Libre insertion was painless, but the small trauma of the injection could be felt for about a day. The Dexcom's insertion was slightly more painful (we've had totally pain free ones) but couldn't be felt afterwards. The wound of the Libre was much cleaner and neater after 14 days than the Dexcom's after 9 days. I had hoped to run the Dexcom for 14 days - despite several adjustment and adhesive/tegaderm repair sessions, it proved impossible: the sensor fell off during the Saturday tennis practice. What became obvious is that living with a Dexcom isn't always easy: you've got to make it stick; it has to be dried now and then; after a few days, it is adhesion check and repair time; it needs tegaderm, hypafix, mastisol, skintac (which causes allergies with my son); it may require to undress carefully; you may need small scissors for repairs; you have to be careful with calibrations if you want to get the most out of it; the receiver has to be with the user at all times, etc...
Living with a Dexcom G4 is, of course, much better than living with 10+ finger pricks per day. But it is always a bit a matter of comparison...
On the other hand, the Libre had zero practical issues. While we put a tegaderm on it after a week because there was some slight peeling on the edges, it was excessive caution: we had to pull hard to take it off on day 14. You load the sensor, "fire" it, and then forget about it for 14 days. That scenario is, by the way, repeating itself with the 10 days old Libre sensor I am wearing as I write this. Max inserted it into my arm in about two seconds, I forgot it was there after 12-16 hours and, it has since been running with a MARD of 7-8% relative to a BG meter.
Let me finish on two totally subjective impressions:
Max (my son, nearly 14yo, T1D) told me this: "It's really strange, I found the Dexcom wonderful when it arrived, now it looks a bit lame and old."
As far as I am concerned (51 yo, not T1D, possibly at the edge of T2D, on the correct side for now), wearing the Libre is like having glucose sensing as a sixth sense. I could very well see myself wearing this permanently, just as some wear a fitbit. The info it provides on the impact of my eating habits is tremendously interesting and useful. Correction: if we didn't have to go through some hoops to get the sensors and kits (the Libre is not yet available in Belgium but fortunately we are a mixed Belgian-French family), I WOULD wear one permanently.
I have never felt the urge or even desire to wear a Dexcom...