Your simplified “bottom line” is backwards.
Accuracy is how close to something being described the description is. Precision is how detailed the description is. For making decisions - the fundamental action of management, accuracy of the information used is paramount, precision is secondary. Accuracy determines direction, precision can sometimes help to refine that - to a degree.
Two examples of how accuracy and precision relate to making decisions:
1:A digital display that says the time is 9:01:01 all the time is more precise than an analog clock with hands that point toward hour and minute marks and has no second hand.
If I have to be somewhere at noon or suffer consequences, the digital display is wrong 99.997685185…% of the time . It’s precision is useless as a time indicator.
But If I know that the analog clock indicates that it is approximately 12 when the sun appears to be directly overhead, I can use its measurement and leave early enough to assure that I make my appointment.
2.In the middle of the night, if I can smell and see smoke, I know that there is a fire. Knowing that there is a fire is more important than knowing exactly where the combustion is, more important in deciding to wake the family, to pick the evacuation route, and to call the fire department.
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Accuracy, precision and effects on PWD behavior - aka “glucose management”
Comparing the two CGMS, the precision of the G6 and G7 are the same - one mg/dL in the FDA-approved versions. MARD does not measure precision; it measures statistical accuracy. A MARD of 10% means that there is a 10% chance of any one reading being wrong by more than the allowable accuracy range that is the FDA minimum requirement. That is a measure of permitted INaccuracy.
Unless you are a statistician studying group effects, the difference between a MARD of 10% and one of 8% isn’t relevant to management decisions.
Gylcemic management. A CGM is a measuring tool. Tools don’t manage, people do.
The reliability/consistency of any tool is more important than its accuracy. If a tool isn’t reliable it isn’t trusted.
No CGM made today is reliable enough or accurate enough to bet my life on its readings alone. I can’t rely on a CGM sensor -even for trending information - until after I have confirmed its behavior with a BGM. I have had enough troubie with wonky and slow starting sites to ever trust a CGM sensor during the first 6 hours after insertion.
My distrust originated with the CGM manual. It says to use a BGM when symptoms “aren’t consistent with the CGMs reading”. But I can be far above target range with no symptoms, while I have to be below 75mg/dL to have any. How trustworthy is a measuring device that is inaccurate by up to 20 mg/dL low, if it triggers a low alarm at “70 mg/dL” when I’m at 90 or 50?? That is why I switched from using Dexcom’s and Tandem’s phone apps to Xdrip+, which lets me set a series of level alert tones. They keep me trend-aware without looking. I can evaluate or independently measure BG to determine what’s happening.
CGMs have limits, but knowing the G6’s limitations I can use them confidently. The basis of engineering is applying imperfect knowledge and known behaviors within limits to predictably perform needed functions.
A measuring tool has less to do with the results of a user’s decisions, than the user’s knowledge and ability to use that information. Confidence that the measurement is trustworthy determines how confident they are in using the measurement to do something
The most important thing to determining glycemic control is what the person does with the measurements they have. That is based on the person’s education about diabetes , their experience of how their life is affected by glucose level, diet and exercise, their understanding of the relationships between them, and their willingness to modify their behavior.
The pandemic and my retirement gave me an opportunity to study and experiment which few working adults have. Having used MDI for decades, with very limited hypo, gave me the confidence to try to do things that an endo today wouldn’t try with an old man. I decided that I should adopt using a pump before I needed one to survive. When started, I was given an A1C target of below of 7.5 - higher than I was getting with MDI. The initial pump setting were determined by the endo,. Within a day I was having problems - during a weekend. Within two days I was far above range.
I’m a slow learner, have average intelligence and a mediocre formal education. Over a long time, l became an avid reader, a competent writer and an adequate engineer. So the simple tech and programs used in CGMs and pumps today doesn’t intimidate me. I waited for 10 years for the tech to almost catch up with what I can do without it. So I understood the three settings on the pump and within a day had gotten back in range.
I spent a few months tuning my diet and starting to do measurements, before I found that the book Think like a Pancreas had a simpler way of doing it. Since then I have been dealing with a chronic issue - declining good infusion/sensor sites because of weight loss and old scarred injection bands. But the problems I’ve had have given me a limited expertise in using CGMS and the hours of reading research about them has given me a better understanding of their limitations, “care and feeding”, adn their utility.
My most recent C6 sensor took 24 hours and 2 calibrations before it stabilized and became accurate enough to use. It was immediately obvious that the new sensor was reading far low (50 vs 120mg/dL )I disabled Control IQ and the CGM low alarms. I slept confidently with my pump using my basal profile and made a small correction around 3AM. After the sensor started giving steady readings that were plausibly consistent with my BGM, I did a calibration and re-enable Control-IQ. My TIR declined for a day due to its completely erroneous low readings, but my control didn’t waver. readings, but my control didn’t waver.
This isn’t “rocket science”. Aside from packaging, less reliabilty, time delays, and indirect measurements, CGMS use the same kind of chemical reaction as BGMs. The primitive programs, “algorithms”, of CGMs and insulin pumps don’t do anything we weren’t told how to do with BGMs and syringes - and side from doing it more frequently, they don’t do it as well as we could.
We don’t need to do a calculation to see that a dropping line, if continued, will drop down to a level. It’s the same tracking ability we use watching a ball fly. If we aren’t prepared and willing to use what we know and what we can do, results we get from using tools will be limited by the tools limitations.
You see this in every clinical trial of CGMs and insulin pumps. Before and at the conclusion of clinical trials, those who have A1Cs below 7 are the minority (under 20%) those below 6 are outliers. 80% are above 8.0 and overall they drop by 0.5%. Outcomes haven’t changed in the past 20 years despite newer insulins and easier to use tech. What has changed is the cost per person for the same results has dramatically risen.
Bottom line: You can give people better tech, but you can’t make them think about what they don’t know or don’t value. Better tech won’t make them use it more effectively.
