There’s a lot to your post, so I’ll try to home in on the key points.
First, the analogies you use about measuring time, temperature, and so on, all have something in common: there is no variability. The time “is what it is,” which is also the case with temperature, and so on. At least, within a small region of space. Here, your level of accuracy is determined by how closely it pairs with a standard measuring device within that same unit of space. that is, you can measure the time or temperature from a unit of space, and can rely on that measurement being standardized.
Glucose levels in the body are nothing like that. A single “sample” of biological fluid may have considerably different amounts of glucose from a different sample right next to it. What’s more, as short amounts of time pass, each sample may also be quite different.
All glucose measuring devices can do is measure the glucose they can detect from a given sample they’re given, but that’s it.
MARD is the difference between YOUR measuring device and another device that is deemed to be an accepted reference measuring device. But both rely on consistency between samples. If you were to use literally the same sample of fluid in each of the devices, you’d more likely get similar readings. That’s “accuracy.”
The problem with measuring glucose levels in humans is that glucose varies dramatically between individual samples, and glucose moves rapidly (in time) and dramatically (between samples). Compound that problem with the unequal distribution of glucose throughout different parts of the body, and you get the phenomenon that I described in the article.
MARD is a good way to compare measuring devices from sample fluids. Before CGMs, that’s the best you could do.
But CGMs blow up that measuring paradigm from a single measurement, such as a BGM, to a new paradigm, tracking systemic glucose movements throughout the body over time. Both of those (space and time) are like going from black and white photos to 3D images in virtual reality. The paradigm shift is enormous.
The best way to manage glucose levels is to track systemic glucose movements throughout the whole body, which is just not possible with reading individual samples. Even if each reading were 100% accurate, the volatility is so noisy, you just can’t make any sensible decisions, either in dosing or carb corrections. You have no choice but to use an algorithm that attempts to use those individual readings – which are NOT 100% accurate – along with prior trends and statistical probabilities to infer those trends.
Suffice to say, there will be a lot of errors in those estimates. And your case example is not dissimilar from my own, and everyone else’s.
But it’s the best we can get from these devices. I have found my personal management choices are more effective using the G6’s attempt at doing that than the G7’s.
