While this often works, it can swing the other way too. That is, sometimes the CGM (a dexcom G6) can be more reliable, not because its readings are more accurate, but that it’s a better representation of what systemic glucose levels are–and especially its trajectory.
Read that carefully: I did not say the CGM is “more accurate,” I said “more reliable.”
Dosing should never be done on glucose values alone, but the rate of change leading up to that value (and the actions that were taken that might account for that rate of change).
If you see 200, whether it’s a BGM or CGM, what really matters is where it was just prior to that. If it was 250 coming down at 5mg/dL per minute, that’s entirely different than if was 100 rising at the rate.
You might say, “yeah, well, of course…” Fine, but now let’s get into that:
Now that you take into account the rate of change—which a BGM can’t show you, but a CGM can, accuracy notwithstanding—you then need to consider what you did beforehand that might have caused that rate of change. Did you eat beforehand? Or dose beforehand? Or exercise? Or even more? Chances are that there isn’t just one thing, but multiple things.
This is where things get murky, particularly for an algorithm. I already know I don’t need to convince you, Dr. John, about that. You already know the CGM’s value (or even rate of change) is insufficient for an algorithm to make “good” dosing decisions, but your description of using a CGM for dosing decisions alone is similarly error-prone. There are a lot of factors that have to be taken into consideration that only you the human can know, and the weighting to give each of those factors is what most T1Ds eventually intuit from years of experience: when I do this, that happens. But when I do that, this happens.
Obviously, that doesn’t mean it’s easy to fully take into account all the factors, or how much weighting to give to each of them. This all accounts for why glucose is so hard to manage. Duh. But relying on an automated system to know this–or to see patterns that will reliably repeat themselves–is a pipe dream.
But I always have to emphasize, AID systems can do better than a T1D that never looks at a CGM at all, and/or who makes really poor dosing decisions. And sadly, that’s a huge number of people (mostly children and adolescents).
But let’s get back to your point, Dr. John: Dosing based on BGM values alone can sometimes work, but it can also fail, not just for the reasons I described above, but because of the mistaken belief that either meter represents the body’s “systemic glucose levels.” That is, the total amount of glucose in the body. That’s not the case, and can lead to just as many bad decisions as good ones.
When glucose values are volatile–which is more often that not if you look at your daily chart—a BGM test may be useful to do a finger-in-the-wind sanity check on whether your CGM is in the ballpark, but it should never be the true guiding light.
A full discussion of this, with citations to medical literature and graphic illustrations of glucose levels and CGM vs. BGM readings, can be seen in my article, “Continuous Glucose Monitors: Does Better Accuracy Mean Better Glycemic Control?”