It may seem like overkill because I already wear a Dexcom, but I would be happy to pay for the FL out of pocket and still continue to use my G4. I always love having access to any additional data, and the features of both devices are very attractive to me.
I’m totally over pricking my fingers (as someone who plays string instruments my fingers are already calloused enough), so the benefits of the Libre completely replacing that are great. Yet I would still want the high and low alarms of the Dexcom.
Has anyone found one system better than the other (the cost aside)? Or is there anyone out there who is crazy like me and would wear/ are wearing both sensors simultaneously?
I’m confused. In the US the FDA advisory panel recently recommended allowing the Dexcom to be used for dosing decisions. The final FDA decision has not been made, but it is anticipated that they will approve the nearly unanimous advice. Given that, in the US Dexcom users should be able to shift to only checking blood sugars twice a day and have that be an approved use of the Dexcom. I am not sure that using a Libre for calibration has even been studied. What I would recommend is that you talk with your doctor about trying to reduce the number of finger sticks you make during the day and depending more on your Dexcom.
I know that the Dexcom G5 in Europe is documented to permit users to make treatment decisions on the Dex CGM only. They also included a short two-statement caveat in their UK User Guide:
Your symptoms may not always match your sensor glucose readings. In this situation, use your BG meter, not your Dexcom G5 Mobile CGM System to make your treatment decisions.
Some sort of instruction like this will likely be a part of the Dexcom instruction to users for some time to come. This will encourage users to consider any physical symptoms before dosing on the Dex alone. At this time I would also be a bit more skeptical of Dex readings in the first 24 hours or so.
I still poke my finger about 10x/day but I know I’m definitely in a minority. I do this out of an abundance of caution and perhaps I should relax some. I would wear the Libre since I view it as truly an independent second source to keep my CGM data in check.
In any case, whenever the the treatment decision involves adding more than a 1/2 unit of insulin, I would feel better getting “a second opinion.” If the CGM number suggested adding a significant amount of corrective carbs, I would also want to do a double check.
Technologically, the Libre is no different than the Dexcom. Both place a proprietary chemical compound on the end of a wire in the near-surface interstitial fluid of the skin.
Absent some data suggesting the Libre is more accurate/reliable, I see no reason to rely on Libre information as more accurate than a properly calibrated Dexcom sensor.
Dave, while I agree that they both place a proprietary compound on the end of the wire to measure glucose in the near-surface interstitial fluid, they actually are quite different in operation. The Libre is a second generation sensor, while the Dex and MDT are first generation sensors. The result is that the second generation generally is more linear through the entire physiologic range, while first generations usually are only linear through the first 1/2 to 2/3 of the range. For dosing between 80-150 mg/dl they both are probably equally safe, but at high glucose levels, the second generation tools should outperform first generation hands down. Now until there is some clinical data to support this I would be wary, but I am somewhat conservative by nature.
The important difference between a flash glucose monitor (FGM), like the Libre, and a CGM is that the FGM does not push any notifications out into the world. It must be interrogated to see current BG. It shares the CGM’s ability to store data. I think it can store 8 hours of data. The ability of the CGM to provide an alert to a trending high or low is a distinction that makes a difference to me.
You are, of course, correct in the operational aspect of the current systems on the market. However, as the user interfaces mature, the ability to have an accurate sensor will become more important as more decisions are expected of the sensor data.This is when things will get interesting, and hopefully more money will be put into sensor development. Right now, Gen 1 is late 80’s technology and Gen 2 is early - mid 90’s technology.
One other advantage (for swimmers, at least) is that the FGM stores data on the transmitter until it’s scanned. Therefore, one could actaully see how something like swimming affects BG, which isn’t possible with the CGM systems. I wish that Dexcom could store a couple of hours of data on its transmitter for times like this or times it goes out of range.
Both of these sensors using Glucose Oxidase (GOx) enzyme as the foundation for detecting glucose. In both cases they are stabilizing the GOx in the sensor. I am not sure about the Dexcom, but in the Abbott product the enzyme is contained within a cross-linked polymer matrix. The differences between Gen 1 and Gen 2, is that in Gen 1 sensors after the GOx has reacted with the glucose, you wait for oxygen to perform a second reaction and create a byproduct which is Hydrogen peroxide. The Gen 1 sensor then detects the hydrogen peroxide that is produced. The Gen 2 sensors contain a metal in the polymer that performs the oxygen function which is why it is more linear through the physiologic range (i.e. oxygen can’t be depleted in the sensor). Both of these sensors will also react to Acetaminophen and ascorbic acid, amongst other things that will add current to the measured values.
Chris, thanks for all the great information regarding sensor chemistry!
Just to be sure, are you certain current Dexcom sensors are gen1? They haven’t been changed since the G4 came out about 3 years ago, but I seem to recall that the sensor technology was new (and better) than the previous product, the Seven and Seven Plus systems.
I’m going from vague, years-ago memory from when the G4 Platinum system came out, so I could be completely wrong on this.
Or is this newer chemistry that you describe above what’s in the G6, and is part of what is responsible for the single daily calibration rumored for the G6?
I think the big performance jump with Dex was the 505 software upgrade. I’m not certain but the form factor of the current sensor goes back to the 7+ days.
Dave, I have seen a Dexcom document that points to the G4 as a Gen 1 sensor. I have tried to enter into conversations with them on the phone, but they have been quite hush-hush with me, even though my son is using their product to good effect currently. I doubt they have moved on to Gen 2, simply because that would take quite a technical leap, but I am hoping they do at some point in the future. The Gen 2 sensor is calibrated in the lab, and doesn’t need to be calibrated after that. So the creation of a single daily calibration leads me to believe that they are still optimizing Gen 1 sensors which is more susceptible to electrical drift than Gen 2. While clinically the generation doesn’t really matter, as a scientist I would like to see the technology pushed so the artificial pancreas can come to full fruition. With that said, Dexcom has done a great job with their sensor coatings and other improvements that they have made to make a great product, but I would like to see further improvements so the accuracy is improved further. I have seen too many times that the sensor has been 30 points off around 90mg/dl to be completely trusting.
Let me share what I remember from reading the book “In Vivo Glucose Sensing”. Generation 1,2,3 are not to be understood like iPhone 5,6,7. Better terms would be type 1,2,3. This means that a generation 3 sensor is not necessarily more accurate than a generation 1 sensor. Generation 3 sensors measure glucose directly and as far as I know no commercial generation 3 sensors exist. As @Chris72 pointed out, the difference between generation 1 and 2 sensors is that generation 1 sensors rely on oxygen in the ISF. If there is little glucose and a lot of oxygen then the hydrogen peroxide output is proportional to glucose. If there is a lot of glucose and little oxygen then the hydrogen peroxide output is proportional to oxygen. In order to measure glucose, excess oxygen is needed. Unfortunately, ISF has lots of glucose and little oxygen. This is where Dexcom’s membrane technology comes in. The membrane reduces the glucose so that the sensor sees little glucose and excess oxygen.
Helmut, absolutely correct, the issue is that at high glucose concentrations in the tissue, even with good membranes the systems don’t work quite so well. Hence the higher level of uncertainty with glucose levels above 100 mg/dl for Dexcom. Clearly, Dexcom has excelled at creating good coatings since their product works so well. Generation or type 3 sensors have yet to be perfected, but would be really interesting. With that said, the problem with Gen 3 is that they don’t last very long in testing, but offer some interesting ways to avoid the effect of things other than glucose being measured.
Yesterday I saw I message on Apple support group about a device which could send data from FreeStyle Libre sensor to mobile app every 5 minutes. I guess a company Ambrosiasys.com is launching that product at the end of January, 17. If that is real we will have best of both systems.
