Cgm - BG diff calculation please

On some forum recently (I can’t recall the site but it was in a cgm discussion) someone gave me the formula for calculating a cgm value vs a BG value so you can determine whether the cgm is within the 20% range of the BG. If there is anyone here who can repeat those steps for me again I would appreciate it. Please include all steps. My memory just isn’t what it used to be. Sorry, and thanks. I tried searching on my three main sites but no luck.
Now if I can just remember where I posted this!

My cgm reads pretty accurately (the same as a finger stick), unless I have recently consumed carbs or exercized. What my cgm reads is what a finger stick will read about 10 to 15 minutes after that. It is a time thing, not a percentaget thing, for me. That is unless the calibration didn’t go well, and then I have to recalibrate. I don’t think that is what you meant, though. Can you clarify your question?

I think what you mean is:
meter reading+ (meter reading x 0.20) = high end
meter- (meter x 0.20)= low end

so, if your meter = 120
0.20 x 120 = 24
120 + 24 = 144
120 - 24 = 96
therefore, if your CGM is between 96 and 144, then it is within 20% of your meter reading


Are you thinking of the steps to take?

Wait till you are 3 hours post prandial or in the morning before breakfast.

Take blood test. Note time.

Wait 15 min. Read CGM. Meter test result less CGM reading divided by meter test result will be less than 20%.

That’s pretty circuitous, but it will do

TS, your calculation seems to do it. The other one I was given was quite different, but this one may be simpler. Thanks very much.

I had a one week test with a Dex and have been doing all kinds of things with the numbers. Playing! I found that the Dex was much more variable and less accurate when my meter numbers were below 120. Above that, they were very close most of the time. Am considering getting a cgm. Not sure whether to wait and see about the new MM one or go ahead with the Dex at the first of the year. I will be getting a new pump then too.

Thanks again. And thanks to Leo also. Good info.

PS. Leo2, that is a great suggestion to wait 15 minutes for the cgm reading. I should have thought of that but didn’t.

Thanks. See below for what I meant! You are fortunate to have such consistency!