4 ms·

You are on HN, and likely reasonably technically inclined. You would probably pay attention to instructions that indicate that it might not be accurate during t

by lambda 9y ago

You are on HN, and likely reasonably technically inclined. You would probably pay attention to instructions that indicate that it might not be accurate during the first 12 hours of use. You would know to be a bit skeptical of readings that show hypoglycemia during that time, and might either do a fingerstick to check or just ignore it. There are many users who would not. They might make a rash decision based on that data. If the data is in Zone B a larger percentage of the time in testing, in much wider general use there will likely be cases that weren't caught in clinical trials in which it could land in Zone C. During this 12 hour calibration window, you can still use fingersticks, which you have to do for calibration purposes with other CGMs anyhow. And it likely would be a fairly minor change for them to support doubling up on sensors, so that you could overlap them in order to avoid the 12 hour window; but that change would require further testing and approval, so for now the 12 hour calibration window is left in. It was only fairly recently approved with this 12 hour calibration window, so I wouldn't be surprised if the company is working on support for pairing two sensors to one device to support doubling up like this.

ng12 9y ago
> During this 12 hour calibration window, you can still use fingersticks, which you have to do for calibration purposes with other CGMs anyhow. Those aren't comparable situations. With my Dexcom I test in the early afternoon and before bed and that gives me 24 hours worth of extremely reliable information. Here's a hypothetical: let's say you test and your blood sugar is 150 mg/dl. What does that tell you? Almost nothing. Maybe your sugar was 200mg/dl 15 minutes ago and you're dropping like a rock or it was 100mg/dl 15 minutes ago and you're spiking. Maybe you've been at 150mg/dl for the last two hours. You can't really tell, so you test again in 15 minutes. Let's say it's 100mg/dl -- looks like it is dropping, unless your last reading was on the high side and this one on the low meaning nothing's really changed. So you test again in another 15m and maybe you can start putting together a picture of what's actually happening. This is a terrible process and only works if you're sitting at home doing nothing. What if you're stuck in traffic or on a crowded subway car? In a meeting at work? At the gym? On a date? In a movie theater? It can become overwhelming quickly and that's when mistakes happen. And remember this is all compounded by your body's biology: you have to think about the food you ate, how much insulin you took and when you took it, when the last time you exercised was, how much stress you're under, etc, etc, etc. And keep in mind fingersticks don't perform that much better than CGMs. They're still susceptible to error (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317395/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317395/) and much more susceptible to human error. Failing to wash your hands properly, not drying your hands properly, not letting the alcohol dry completely after a swab, or leaving your strips in a hot car can all cause your readings to be wildly incorrect. Another thing to note is even if your Libre is off by a huge amount (say 50mg/dl) the trend will still be accurate. Meaning if your blood sugar is constant over an hour the Libre will give you a reading that's also constant. This is extremely important because I'm much more likely to take action on trends (e.g. rising/falling blood sugars) than on individual data points. You said this in the other thread: > I would think that 12 hours of having to use a fingerstick during calibration would be preferable to twice a day fingersticks. I will tell you it is definitively not. Currently Dexcom takes two hours to calibrate and those are the longest two hours of my week. The ability to schedule those two fingersticks is much preferable to 12 hours of radio silence. > You are on HN, and likely reasonably technically inclined. Sure. I'm a "good diabetic" but that's the nature of the beast for a disease who's only treatment is constant vigilance. There's a reason why I've never gone into a hypoglycemic coma yet know people my age who are already struggling with neuropathy. The lowest common denominator here is extremely low, and if an error margin of 30mg/dl or a single false reading is going to cause you serious problems you have much greater issues with your diabetes management than your CGM.
- tomca32 9y ago
  Agreed. 20% margin of error isn't really that relevant to me either. I don't really care that much if I'm 70mg/dl or 100mg/dl if the line is horizontal. Both values are, roughly, in the "OK Zone" that I just don't care about better precision. Even if I'm going low, the difference between 50 or 70 isn't nearly as relevant as the rate of change. If I'm going down, but the slope is gentle enough, I know I probably don't need much to get back to normal, while if it's steep going down, I probably need to eat something regardless if I'm 50 or 70, or 80.
  - daveguy 9y ago
    It could also be that the issue isn't the magnitude of the absolute error. It may be that the device is more highly variable in the first hours giving the impression of spikes or drops where there are none. It's hard to tell without the raw data.
- lambda 9y ago
  Fair enough. You obviously have a lot more skin in this game than I do, I have just done a bit of quick Google Scholar research to see why the FDA might have made the decision that it did, and I'm less familiar with how much the trend vs. the point accuracy is important. But what it comes down to is, there was something about the accuracy during the first 12 hours that gave the FDA cause for concern. Whether the FDA is overly cautious or not is an open question; I do know that in the US we get worse health outcomes for more money than most other developed countries. How much of that is due to our terrible system for paying for health care, which is privatized but with enough regulatory capture and inefficiency to drive prices way up, and how much of that is due to higher costs and lower access to medication and devices not being available due to excessively stringent regulations from the FDA would be something interesting to study. I think that the FDA could probably loosen up its regulatory process just a little bit and provide a net benefit. Not sure with how much there is wrong with health policy in the US, and partisan politics around it, that's very likely, however. It would be interesting to see if there have been any studies done on this.

by lambda 9y ago

ng12 9y ago

> During this 12 hour calibration window, you can still use fingersticks, which you have to do for calibration purposes with other CGMs anyhow. Those aren't comparable situations. With my Dexcom I test in the early afternoon and before bed and that gives me 24 hours worth of extremely reliable information. Here's a hypothetical: let's say you test and your blood sugar is 150 mg/dl. What does that tell you? Almost nothing. Maybe your sugar was 200mg/dl 15 minutes ago and you're dropping like a rock or it was 100mg/dl 15 minutes ago and you're spiking. Maybe you've been at 150mg/dl for the last two hours. You can't really tell, so you test again in 15 minutes. Let's say it's 100mg/dl -- looks like it is dropping, unless your last reading was on the high side and this one on the low meaning nothing's really changed. So you test again in another 15m and maybe you can start putting together a picture of what's actually happening. This is a terrible process and only works if you're sitting at home doing nothing. What if you're stuck in traffic or on a crowded subway car? In a meeting at work? At the gym? On a date? In a movie theater? It can become overwhelming quickly and that's when mistakes happen. And remember this is all compounded by your body's biology: you have to think about the food you ate, how much insulin you took and when you took it, when the last time you exercised was, how much stress you're under, etc, etc, etc. And keep in mind fingersticks don't perform that much better than CGMs. They're still susceptible to error (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317395/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317395/) and much more susceptible to human error. Failing to wash your hands properly, not drying your hands properly, not letting the alcohol dry completely after a swab, or leaving your strips in a hot car can all cause your readings to be wildly incorrect. Another thing to note is even if your Libre is off by a huge amount (say 50mg/dl) the trend will still be accurate. Meaning if your blood sugar is constant over an hour the Libre will give you a reading that's also constant. This is extremely important because I'm much more likely to take action on trends (e.g. rising/falling blood sugars) than on individual data points. You said this in the other thread: > I would think that 12 hours of having to use a fingerstick during calibration would be preferable to twice a day fingersticks. I will tell you it is definitively not. Currently Dexcom takes two hours to calibrate and those are the longest two hours of my week. The ability to schedule those two fingersticks is much preferable to 12 hours of radio silence. > You are on HN, and likely reasonably technically inclined. Sure. I'm a "good diabetic" but that's the nature of the beast for a disease who's only treatment is constant vigilance. There's a reason why I've never gone into a hypoglycemic coma yet know people my age who are already struggling with neuropathy. The lowest common denominator here is extremely low, and if an error margin of 30mg/dl or a single false reading is going to cause you serious problems you have much greater issues with your diabetes management than your CGM.

tomca32 9y ago
Agreed. 20% margin of error isn't really that relevant to me either. I don't really care that much if I'm 70mg/dl or 100mg/dl if the line is horizontal. Both values are, roughly, in the "OK Zone" that I just don't care about better precision. Even if I'm going low, the difference between 50 or 70 isn't nearly as relevant as the rate of change. If I'm going down, but the slope is gentle enough, I know I probably don't need much to get back to normal, while if it's steep going down, I probably need to eat something regardless if I'm 50 or 70, or 80.
- daveguy 9y ago
  It could also be that the issue isn't the magnitude of the absolute error. It may be that the device is more highly variable in the first hours giving the impression of spikes or drops where there are none. It's hard to tell without the raw data.
lambda 9y ago
Fair enough. You obviously have a lot more skin in this game than I do, I have just done a bit of quick Google Scholar research to see why the FDA might have made the decision that it did, and I'm less familiar with how much the trend vs. the point accuracy is important. But what it comes down to is, there was something about the accuracy during the first 12 hours that gave the FDA cause for concern. Whether the FDA is overly cautious or not is an open question; I do know that in the US we get worse health outcomes for more money than most other developed countries. How much of that is due to our terrible system for paying for health care, which is privatized but with enough regulatory capture and inefficiency to drive prices way up, and how much of that is due to higher costs and lower access to medication and devices not being available due to excessively stringent regulations from the FDA would be something interesting to study. I think that the FDA could probably loosen up its regulatory process just a little bit and provide a net benefit. Not sure with how much there is wrong with health policy in the US, and partisan politics around it, that's very likely, however. It would be interesting to see if there have been any studies done on this.