This Time Self-Hosted
dark mode light mode Search

A FreeStyle Libre Retrospective, Nearly Seven Years Later

I have diabetes, not that it is a secret. As many other people with diabetes, I have been trained to prick my finger multiple times a day to test my blood sugar (glycemia) since I left the hospital. The finger-pricking routine involves drawing just enough capillary blood to load into a test strip (or cartridge) for it to give you an instantaneous reading. It’s a time-tested method but also one that has a lot more drawbacks than you may think.

First there is the obvious problem of pain: pricking the tip of your finger is painful — even more so if you work with your hands (including typing) and you end up using the tip of your finger all the time as well. There’s also the risk of permanent nerve damage, which is how long-term diabetics tend to not care about that particular pain anymore. To add insult to (literal) injury, diabetes affects platelets, making fingers bleed longer if you prick them too deep. To deal with these parameters, glucometers tend to distinguish themselves based on the amount of blood necessary for their test strip (less blood means you don’t need to prick very deeply), as well as based on the spring-loaded pricking needle device they come with (in the hospital I was an in-patient at, they used painful and deep spring-loaded single use devices that left my fingers literally livid after just a couple of days.)

If you never teste your blood sugar with this method, you may wonder why is it that critical to get the right amount of blood. The reason for it is that the test strips and cartridges usually only allow you a single attempt to put enough blood — and if you fail, an error is given, and the strip consumed. The strips themselves are way more expensive than the glucometer (in a razor and blades business model), and even in countries with a national healthcare service exists (I have been in care of SSN, HSE, and NHS in Italy, Ireland, and UK respectively) the amount of test strips you’re given is not unlimited — as an insulin-treated diabetic I think my allowance is around three or four strips a day. So another differentiation on glucometers become how fast you need to put the blood into the strip, and whether you’re allowed to have “a second go.”

If you wonder why the expectation is to take just a few datapoints during the day, one of the many reasons is that you’re looking at ballparks, not precise details. This may sound counterintuitive, but since I have been buying and reverse engineering glucometers as a matter of hobby for the past few years, I can tell you that taking measurements even from the same finger across multiple meters (even with the same strips, even with the same model of meter) can give you a ±10% result with no doubts! And since you’re not meant to repeatedly prick the same finger… the numbers are unlikely to be directly comparable between readings anyway.

I detailed all of this to explain why CGMs (Continuous Glucose Monitors) and the similar (but not quite the same) Flash Monitors such as FreeStyle Libre are such a change of pace from what existed before. And yes, I mixed these two concepts up when I originally got the FreeStyle Libre — there’s a very fundamental distinction. Both of these types of devices are generally composed of a sensor (that replaces the whole strip-and-blood dance) and a “recorder” (the meter itself), with many different options for the way the two communicate. For both types, the recorder may be a smartphone, talking either Bluetooth or NFC. But while CGMs will “push” their readings to the recorder continuously, flash meters require you to scan the sensor to download some amount of data. In the particular case of the FreeStyle Libre, the sensor reads the blood sugar (equivalent) every 15 minutes, and the reader or smartphone will download up to eight hours worth of samples.

The first generation FreeStyle Libre – which I bought in Ireland “smuggling” (not really) it across the border from the UK – was a simple flash monitor — it did absolutely nothing if you didn’t scan the sensor. The Libre 2 I’m using right now still is a flash monitor, but it includes a Bluetooth LE alert “return path” (but only for one smartphone and optionally one reader device) to alert if your blood sugar falls below or climbs higher than a set threshold. An even newer version called Libre 3 exists, and Daniel wrote about it, but it is not available in the UK at the time of writing — this is an actual CGM system, and I’m looking forward to try it.

Before digging into my personal experience, I also want to explain that “equivalent” I put there. Most glucometers are not actually taking a reading of your blood sugar. That is because what interests doctors is usually the equivalent of what a drawn blood test would report as your blood sugar. Which means the meters apply some type of scaling factor to give you a value that can be compared with your in-patient blood tests. Since most meters still deal with blood, this tends to be ignored much — but obviously all of the CGMs I know of, and the FreeStyle Libre, are not actually drawing blood and are not installed within veins or anything. Instead what happens is that these apply some calibration curves to the raw reading of their sensors to calculate a matching value.

This “virtual value” has quite the results. I have noticed two camps of people, among those I discussed this topic with: those who swear by the Libre’s accuracy and those who swear by Dexcom’s. I have yet to find anyone who liked both. In my particular case, Libre appears to match my experience a lot better than Dexcom, so I’m very happy with it, but I also learnt not to judge if others end up with different experience than mine.

So now that the very long preamble of how all of this works is out of the way, what’s my experience so far with the FreeStyle Libre (and the Libre 2)? Absolutely fantastic. Not just for the accuracy, which as I said matches my experience with normal blood-based glucometers, but in particular for the observability (I mean, it is the current buzzword) of the blood sugar trends.

The first takeaway for me was understanding just how different metabolism can be between different people. With the exception of recipes that remove all and every source of carbohydrate, I always found myself struggling when trying the most common solutions for diabetic-friendly meals, and “low glucose” recipes. I had generally a better luck with “low glycaemic index” recipes, but even that was hit and miss: any type of fruits-based recipe tended to make me spike a lot more than what should have been a higher amount of carbohydrates from pasta. Well, after I started monitoring more than just a couple of readings a day, I could confirm that indeed, I don’t take well to most of those. And I decided to stop worrying about it — I can have my pasta just fine, in moderation, and if I avoid nut and most rice based recipes, I don’t spike my blood sugar at all.

The clearest example was in the Google Dublin café. At some point in 2015 they were serving two types of mini bread rolls: a wholemeal one that was supposed to be good for you (according to the signs and marking), and some white ones with either sesame or poppy seeds. A single wholemeal roll (weighing less than 50g in my estimate) would see my sugar spike 5→11 within half an hour; I could have four of the white rolls with cheese (at about the same weight) and it would maybe take me 5→10 in a couple of hours. Could I have had a salad instead? Probably, but I wouldn’t have enjoyed it, and that would likely have gotten me to snack more the rest of the day, so I was pretty okay with the bread-and-cheese with occasional meat — when they wouldn’t decide to make the most boozy sauce they could think of.

Being able to fine-tune my choice of food is to this day a great thing: I choose to eat things that are enjoyable and are known to make my blood sugar behave, rather than having to feel like I’m sacrificing myself with food I don’t enjoy because it’s said to be good for me, and then still suffer for that choice later, as it didn’t work well for me. As an aside, I think I remember seeing on 23andme a link to a study that suggested that I would be more likely to tolerate a Mediterranean diet — I can’t find that anymore. But as it turns out, that appears to be the case: having my carbs come from pasta rather than potatoes or rice appear to help a lot with my blood sugar control!

A second takeaway is to be found in math. Because with the Libre I’m no longer looking at a single point in time, but rather at a curve, I can judge the derivatives of it — that is, whether it going up or down, and whether it is going fast or slow. When deciding when and what to eat, this actually prepares me a lot better than just knowing that my blood sugar is, for instance 5.8 mmol/L — is that 5.8 going down from lunch, and likely going to bring me close to low blood sugar before we hit the restaurant, or is that 5.8 going up from the mid-afternoon snack that I had because I already hit the low blood sugar, and it’s likely that it’ll hit 8 before by the time we order dinner?

It is true that if you learn to listen to your body you can guess whether the sugar is going up or down and whether it is close to hypo or hyper thresholds. And it is recommended for you to keep listening to your body that way so that you don’t end up waking up in the middle of the night with a hypoglycaemic event and no sugars at home (it happened to me – thankfully only once – when I was about to move out of Dublin.) But being able to learn those feelings by comparing the sensations with actual readings is quite useful in my opinion.

In a similar note, with COVID among us, having a way to check my blood sugar turned out very useful to quickly tell that something was off with me. Indeed even with the most recent “simple” flu I experienced, I could tell when I was about to have a bad day of fever by noting that my blood sugar remained in the ~12 range for the whole day, no matter whether I ate or not. I have experience the opposite as well, when I ended up eating something that my stomach didn’t enjoy and my sugar hovered around ~4.5 for hours even after having sweets and sugary drinks.

Now, it does not mean that this is an absolute slam-dunk. I already noted that this does not work for everybody, and I know a number of people that would much rather use the Dexcom — I hear y’all, you don’t have to tell me. The only thing I’m mildly surprised about is that there doesn’t seem to be a whole lot of research papers looking into the reason why this feels so polarized.

In addition to this, there’s an optimization risk. I know of people who got so fixated with having an optimal number that they lost track of what the number is meant to represent. As I said, the main way I make use of the Libre data is to know what I can eat that makes me the happiest, while maintaining healthy. This does not mean death to all carbs, or optimize to always be “in the zone.” But that does mean there’s a psychological attitude to using solutions like these, which I’m ill-placed to discuss.

Finally, there’s the biggest drawback of these solutions ever: they are expensive, at least by European expectations (remember that most people here don’t pay for insulin.) Since I’m diabetic I don’t pay VAT on the sensors, but even then they set me off more than £100 a month — each sensor lasts two weeks and they come down to around £50 each. In a couple of cases I even “thrown away” a day or two of the sensor’s lifetime by changing it early before a trip, to avoid having to take two “new in box” sensors on a trip. Why two? Because at least a few times this year the sensor applicator (which is effectively a spring-loaded needle to pierce your skin and land the sensing strip under it) failed to pierce my skin, leading to a wasted sensor.

Thankfully, Abbott does replace those failed sensors free of charge, and the same goes for the couple of sensors that failed in the middle of their lifespan — although that has not happened at all in the past couple of years. Similarly, I have not “lost” a sensor due to my own mistakes in a long time: earlier on I struggled finding a good placement for the sensor, with once or twice bumping it off while taking a shower, hitting the side of a door (yes), or during physical activities.

In the UK, NHS covers the cost of the sensors for only a few people, particularly those who are unable to keep their sugars under control easily, and those who couldn’t afford it. Since I’m in the privileged position of both being able to afford the sensors, and also have enough control of my blood sugar even without the sensors, I don’t qualify for those, but I think it is okay.

The biggest drawback for me personally? Probably the anxiety at not having the information available for a long time. I’m not at the point where I would get anxious about going out without tracking for a few hours (like in the article Daniel sent me when I started using the Libre), though I do tend to plan my going out around when the sensor needs to be changed, leaving an extra one hour buffer around just in case the sensor was to not initialize correctly.

Most recently, due to distractions between work and other things happening around me, I had forgotten to order the new sensors in time, and came down to change my sensor with a single box to spare. My fear was that it wouldn’t initialize and I would have found myself having to go back to five fingerpricks a day. It was all in my head anyway, the sensor initialized fine, and having ordered the sensors early on Friday morning, I had them before lunch on Saturday (actually, before getting out of bed, but both me and my wife were feeling under the weather so we stayed sleeping in.)

So at the end of the day, I have to say that I’m not going back to fingerpricks any time soon, and I believe that, putting aside the whole accuracy problem, this type of technology will eventually be considered the baseline of care for diabetes — and will literally save lives.

Final disclaimer: this post describe my personal experience and is obviously not medical advice. Consult your doctor before making treatment and medical decision. I also have a minimal financial incentive in Abbott’s as I have bought some of their stock years ago in awe of their achievement with the Libre.

Comments 8
  1. Regarding the failing FreeStyle Libre 2 applicators, after inserting the top half of it into the bottom half: separate them and then always pull on the blue part to make sure it has come out all the way. If you can pull it out further, do it, and then carefully insert the top part back into the lower part of the applicator and separate them again. It’s a very common problem with the applicators. (Sorry, this will only make sense to FreeStye Libre 2 users.)

      1. My failure rate has been around one in three since late 2019 or thereabout. This “hack” works around the issue, and it also works around the problem where you accidentally stab yourself with the needle before activating the applicator (because the blue ring is supposed to protect you from the needle). Let me know if it works for you!

  2. My arch nemesis are door frames and door handles. Heavy doors and door frames rip of my sensor, and my insulin pump tubing snags on- and gets janked out by door handles. *sigh* The troubles we cyborgs have to deal with in every day life.

    I regularly rip off my sensor when passing through unfamiliar door frames. I started in a new job last month, and I lost a sensor to one of the office’s heavier doors on the second day. I have to “get to know” a door before, especially heavy ones, before I can safely navigate them. (Yes, I do realize I sound nuts.)

    1. I actually get it quite a bit. I think I lost one to the new flat when we moved.

      I since started wearing the sensor closer to the inside, seems to work better for me, but YMMV.

      1. I have very little … surface area to work with, so the sensor has to go where it has to go to maintain a good site rotation.

  3. In Italy the Libre 3 is available since July and I have been playing with its internals with my open sourced-project:

    Two years ago you suggested me the article and I discovered a totally new world thanks to you, even though my reverse engineering capabilities are quite limited.

    I succeeded in dumping 8 KB of the Libre 2 firmware where the keys to decrypt the BLE/Data are stored, so I decided to publish them and the same did all the other Libre apps after me.

    Unfortunately the Libre 3 adopts totally new different BLE protocols and ECDH ephemeral keys: I think it would be a really intriguing new work for you… 😉

    1. Would love to see what you published/wrote up about the firmware dump. I’ve always ben intrigued as to the inner workings of the sensor…

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.