Aug 17

The Game of Glucose

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?

Early Results

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?

Sleuthing Time

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?!?

Oh Yeah

… And then I remembered I was actually making a very different comparison between these two sets.

  1. The Precision Xtra vs CardioChek was performed in seconds between each test.
  2. 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.

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35 Comments on "The Game of Glucose"

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Both fascinating and frustrating! Thanks for sharing this info so that people realize these monitors can vary greatly.

Bill Robinson

2.5 liters in the morning???? I can barely
drink 2 liters over the course of a full day.


i aways take BG when I’m at the lab because many things can change it on the way to lab (45 minutes away). Next time you have a blood draw, take your two or three meters to the lab and test there, using fingerstick not the blood they draw from a vein. Do three tests with each meter.

Check to make sure the lab doesn’t let the blood sit around before testing as this can lower the reading.

Mike Kramer
Dr. Richard K.Bernstein isn’t up on ketogenic diets or LCHF, though what he in fact recommends is both a ketogenic and LCHF diet. He is a T1 diabetic, and his patients are both T1s and T2s. He recommends that diabetics keep very tight control of their blood sugars, in order to mimic the BG dynamics of healthy (not “normal”) people He recognized long ago that the FDA standards for glucometers were woefully inadequate, so he keeps abreast of changes to meters and strips and constantly. He was an engineer before later going to medical school. He tests virtually all the… Read more »
Am I the only one with a deranged system? I’ve been low carb for almost 4 years, with a lot of intermittent fasting, and I’m slowly transitioning to an all meat diet (when I don’t fast, that is). Here are my results yesterday (ketones on left, BS on right). This is fasting all day (only 1 cup coffee, black, in the morning): Wednesday 6:03 AM 0.3 101 7:37 AM 105 8:40 AM 108 9:41 AM 105 10:53 AM 104 12:54 PM 97 2:58 PM 0.4 88 5:09 PM 80 7:19 PM 0.8 88 And that’s FASTING since the night before.… Read more »
George Henderson (@puddleg)

Wait till you get two different lipid testing devices!
Seriously, you can test lipids with a hand-held device? That’s unreal.


Yet another Pandora’s box you’ve opened. Those of us with our docs itching to prescribe statins are oh so thankful for your work and now … those of us with our docs itching to prescribe metformin and insulin because of Fasting Blood Sugars in the diabetes range, though prediabetic HbA1c, have new relief. My last 2 FBS: 144 and 140 mg/d. I’ve only now started with an Accu-chek Performa and already hit 147 after an apple, cheese and pecans, 152 after a chicken frankfurter, red peppers, onions and butter, etc. Many thanks, again, for so much work and stamina.

Michael Babcock

Here are 3 results taken at 7:28 am, 7:29 am & 7:31 am on a OneTouch VerioIQ: 152, 134, 113.


As demonstrated in your post, it is very hard to capture consistent blood readings as it varies every few seconds and the mechanics of blood sugar control in our bodies is highly sophisticated. On top of it inaccuracy of blood meters makes things even worse. The best thing to read blood glucose so far is dexcom CGM even that only measures every 5 minutes or so and will miss spikes during this time, it also needs calibration regularly. I would not be surprised to see fasting BG of over 100 in someone who has been low carb long term.


Hey Dave, Ever had experience with any CGM device? I was looking to get one but seems like they need a doctors prescription.

Kerry Ireland

You are amazing!

Several years ago I tested somewhere between 30 and 50 strips on my One Touch Ultra glucose meter. I used a calibrated glucose solution. It’s low cost and avoids other influences. As I recall the results were within about 5% of the average. My result refers to repeatability not to accuracy for which some standard has to be defined. However, on the few occasions that I checked my meter against a blood draw it has never differed by more than 3%. I conclude that my meter results are more than adequate for the blood tests I do. In the event… Read more »

It is frustrating the inaccuracy of these monitors. I am just tracking my sugars. I cannot imagine what patients have to go through that use a sliding scale of insulin depending on results. I have the Precision Ultra and two OneTouch. The same drop of blood gave three different numbers. Not off by a little but off by a lot. I have a dog that is a type one diabetic. I think his monitor is more reliable than mine.


I’m curious whether the Keto Mojo kit promoted by Jimmy Moore on the Keto Talk discussion on his FB site has been tested for accuracy.

I myself use the Precision Xtra…not sure if I want to invest or recommend a “new device” even though the cost (for now) may be more affordable.


Have you reached out to Jimmy and discussed your research findings?

Chuck Tannenbaum

With these increases in Fasting BG that are occurring in some on a low carb diet, that I assume are Physiologic Insulin Resistance, and the great variability in Glucose Monitor readings, how does a low-carber with increasing Fasting BG know that he is managing his insulin sensitivity/resistance well. Is a Fasting Insulin the best test?

larry r

In Canada our devices are within 10 percent and I have found bayer canada contour next to be very accurate for what its worth…..but for a t2d its the trend that’s important….


[…] different glucometers could be from each other and how this compared to lab results themselves in The Game of Glucose. At that time I wasn’t frequently taking glucose measurements within minutes of the actual […]

Janet Kaufman
I have a few questions for you Dave. I have been on the ketogenic diet for 4 months, taking my fasting glucose every morning. My glucose reading is usually between 85 – 95, except a couple of times after sugar binges at parties the night before. Well, I just had” formal” lab work done, and not only was my glucose 104, my total cholesterol shot up to 400, LDL 297. Now I did stop taking statins a few months ago, but even 2 years ago when I wasn’t on statins my total cholesterol was maybe at 240, LDL 146 I… Read more »
Siobhan Huggins
Hi Janet! A few things… Going by your high HDL and high LDL and relatively low trigs, I suspect you might be a Lean Mass Hyper-responder – someone who is lean and/or active *and* powered by fat. If you were already relatively lean before starting keto, then that plus a small bit of weight loss could explain the spike in LDL. In order to understand why, you may find it helpful to watch Dave’s talk from Breckenridge 2017 where he explains it, if you haven’t seen it already. Essentially, you’re using VLDL to traffic fat-based energy to your cells, which… Read more »