When I started my journey of Insanely Tracking Everything™ lifestyle, I naturally took to using a glucose meter frequently. I would capture mornings when I wake up and evenings when going to bed. But I likewise tested meals regularly as well.
Early on I happened to come across an article about the FDA insisting on tighter standards to be “+/-15% for home-use meters”. Wait… what??? A 15% margin up or down is considered the new, tighter range?
I got a taste of this myself a number of times when I had a reading that didn’t seem quite right. So I tried doing retesting for at least two more times whenever that happened, like so:
Okay, that’s swell. But what if the one meter was off on its retests too? I decided to do some doubling up and got both an Accuchek and a Precision Xtra for some redundancy. Whenever the two were distant from each other, I’d do a retest with both.
Unfortunately, this didn’t work all the time either, as you can see here:
… riiiiiight. So on the second test the gap actually widens. Sure, I’d like to just expand the testing further, but this is already getting expensive as it is.
Here’s a loose collection of a few pictures from that time period where I was using both glucometers frequently. As you can see, the error range is pretty obvious given all of these were taken from the same site on my finger.
As you can imagine, I tried not to put too much stock into it past that point but continued to be sure I took measurements in the morning and evening anyway with my one remaining glucometer — the Precision Xtra.
A New Divide
This week on Monday before my lab blood draw, I used CardioChek’s glucose strip alongside the Precision Xtra and was stunned to see the highest discrepancy yet:
Not only do I already have some studies provided by the PTS Diagnostics people on the higher accuracy of their glucose component of their device, I also had a pretty obvious context… I was, after all, 14 hours fasted. Which glucose reading makes more sense to you for someone on keto in a fasted state?
I then realized I actually had a nice bloc of data I hadn’t analyzed yet. All my previous CardioChek lipid tests included the glucose testing as well and I was careful to take the Precision Xtra glucose at the same time (and same site), but I was too focused on the lipids to see how that comparison was going. Well, today seemed like a good moment. So here it is:
Wow! Average difference was a whopping 16.36! And it was always higher, unlike the Accu-Chek from above. Again, remember this was from samples taken at the same time from the same site.
Naturally, I felt somewhat betrayed by my Precision Xtra in this moment. Not only did I trust the CardioChek numbers more, but the numbers themselves seemed to make more sense. Given 12-14 hours of fasting, wouldn’t it be much more likely my range would be in the late 70s to mid-80s?
Actually, this road gets quite a bit twistier…
So I decided to break out my spreadsheet and fill in the historical glucose readings I had in days where I also got a CMP (Comprehensive Metabolic Panel) in my blood draw. This was pretty much since the middle of 2016 up to now. Thus I had quite a few data points to compare now. Take a look:
|Side Note: The higher 100s fasting numbers from 4/13 to 4/26 were the Carb Swap experiment and its reproduction. Thus, if anything, these numbers were expected to be higher.|
Okay, so I had a solid five minutes of being genuinely awe struck. The 0.19 might as well be 0. Was the device really that accurate? Sure, the deviation of the Precision Xtra vs the Lab Results wasn’t perfect, but certainly much tighter than it was with the CardioChek. Or to put it another way, was the CardioChek that inaccurate?!?
… And then I remembered I was actually making a very different comparison between these two sets.
- The Precision Xtra vs CardioChek was performed in seconds between each test.
- The Precision Xtra vs Lab Results was performed with 2 to 3 hours between each, along with lots of water consumed as well.
The reason for this gap with #2 is that I always take my glucose first thing when I wake up (along with ketones, BP, etc). Since I try to time my fasting period around 14 hours each time I do a blood draw, that means I’ll likely have a gap in time until I can go to get the lab test. For example, I often eat at 6-6:30pm the night before, but wake up at 6am the next morning and thus wait until 8-8:30am before having my blood drawn.
Moreover, I always drink lots of water on the morning of the blood draw, typically 2-2.5 liters. (As an aside, this has potentially created a confounder with a few of the metrics, such as electrolytes. But I have to maintain this behavior until I can change it in isolation to see what the differences are specifically.)
So with this in mind, it is indeed likely my glucose fell in this span of time between my morning Precision Xtra test and my visit to the phlebotomist. But boy, wouldn’t it be coincidental if that margin of difference just happened to fit so well with the outcome of the lab results!
Naturally, this is where Scientist Me wants to have a lot of money to do all three tests at the same time from now on. Then I’d find out for sure who the odd man out was.