I'm having a brain dump and can't figure out the correct method for figuring out % difference/change.  In my case I'm trying to determine the correct percentage difference reading of some blood glucose measurements.  Comparing a CGM(continuous glucose monitor) sensor with a blood sample test.   The CGM is reading 75 while the blood strip is measuring 107.  The sensor has a 20% guarantee.   I have a phone app I use for figuring % calculations.   Do you do this calculation looking for a percentage change or a percentage difference?   Seems they are different numbers.

But in my case I need to know if the CGM sensor reading of 75 is above or below the 20% margin of error when compared to blood strip reading of 107.

If I use my phone app and enter "From value:" of 75   "To value:" of 107 I get 38.67.    However if I reverse those numbers I get -27.88

Both numbers you have are correct.  107 is 38.67% greater than 75 (because you are using 75 as your base measurement), and 75 is 27.88% less than 107, because you are using 107 as your basis, and a given difference will be a smaller percentage of a larger number (using a larger number will also result in a negative change, since the value is decreasing, though difference is often not considered to show direction of change, and will therefore be represented as a positive number no matter which measurement is larger).  Difference and change are the same, what changes the results are what you use as your starting measurement.  For an example which is both simple and obvious, let's say your two measurements were 100 and 200.  200 is 100% larger than 100, but 100 is NOT 100% smaller than 200.  Even more drastically, 0 is 100% less than ANY positive number, but no non-zero number can be measured as a percentage change FROM zero, because changing zero by any percent gives you zero.

There are two takeaways here:  First, neither way of measuring puts your continuous measurement within 20% of the strip.  Second, that does not necessarily mean that your monitor is not within 20% of the actual blood glucose level, unless you know the accuracy of the strip test.

Mike

Yes I get that but I tested using 2 different meters/strips to verify from same blood sample.   Both were exactly the same.  I actually have a 3rd I can test with.   ;)

When you say the CGM sensor has a guarantee of 20%, do you meant that the value reported by the sensor should be within 20% of the "true" glucose level?

If that's the case, 75 +/- 20% is a range of 60-90. That's still outside the range of 107 on the glucometer.

That feels like too wide of a range to make treatment decisions. A lot of diabetics start to feel low at 75 and might need to take a glucose tablet.

Next time you see your doctor, you ought to bring your glucometer and have them check the meter's result along with what your CGM is reporting at the time and then get a venous blood draw and a result on a validated lab analyzer.

Yes.    The CGM sensor is guaranteed to be within 20% of actual blood glucose.    The problem is the accuracy of the device your checking with.   Most measurement tools have an error variance.    Even precision ones used by the professionals.  Most home users do not have access to precision instruments.   When I test for accuracy I have 3 devices I can test with.   Most of the time they are usually only within a few points of each other from the same blood sample.    Not sure a home user can verify any better.
The other issue is that CGM sensor measures in the ISF(Interstitial Fluid) which offers a different value than the blood droplet you get from a finger stick.   Also, depending on specifics, the ISF readings may be lagging behind actual blood glucose by about 10 - 15 minutes.   So that has to be taken into consideration as well.  The CGM sensor value uses extrapolated data to present to the user.

I have found that Type 1s are not as anal about the readings as a Type 2 diabetic.   I'm a Type 2 so I am more anal about my readings.   lol