Hour A: 90, 100, 120, 128, 119, 104, 98, 92, 88, 84, 79, 81
Hour B: 79, 81, 84, 88, 90, 92, 98, 100, 104, 119, 120, 128
All of us who've had hours like these know that there's a huge difference. With hour A, you don't know what to do- your blood sugar zoomed up 38 points in 20 minutes, then it dropped 49 points in the next half hour or so. With hour B, your blood sugar just kept going in the same direction, a 49 point rise, and you are either injecting to correct it or keeping your finger on the insulin waiting to see if it rises further- you know what you're watching.
But as far as Dexcom is concerned, hours like these- days like these- are the same. Your blood sugar average is the same, your standard deviation is the same, your high and your low and interquartile range and all these supposed measures of glycemic variability- they're the same.
It's no use complaining about these things if you can't suggest something better. Fortunately, I can. I suggested it a few posts back. I suggested looking at the average of the absolute value of the first derivative of blood sugar. This basically measures how far apart any two adjacent sensor readings are likely to be. First derivative, by the way, just means slope. Using the absolute value just means that a rise is counted the same as a fall- otherwise the average first derivative would be very close to zero for anybody with decent blood sugar control, where by decent I mean good enough to stay out of DKA. I suggest as a unit of measure change in mg/dl per five minutes, although you can certainly use any other measures of blood sugar and time that you want.
This probably sounds kind of difficult to compute, but actually it's not bad. You can do it in two ways.
The hard way: look at each data point, compute the difference between each two, add it up, and divide by the number of time intervals. For hour A above, that would mean the differences are 10, 20, 8, 9, 15, 6, 6 , 4 ,4, 5, 2, the sum is 89, there are 11 five minute intervals, so the score is 8.
09. For hour B above, that would mean the differences are 2, 3 4, 2, 2, 6, 2, 4, 15, 1, 8, the sum is 49, and dividing by 11 gives a score of 4.45. That is good; the hour with more variation had a higher score.
The easy way, something you can estimate with just a glance at your 24 hour screen, is to divide the screen into lines of up and down and just add up the differences between adjacent high points and low points. Looking at hour A, that'd mean breaking it into the rise of 90 to 128, the drop of 128 to 79, and the rise of 79 to 81; 38+ 49+2, which fortunately gives a sum of 89 again, to be divided by 11. Looking at hour B is even easier because it's all one line, from 79 to 128, which is a difference of 49, to be divided by 11.
Here is how you'd estimate it looking at a 24 hour screen (and this is much more doable with minimed which lets you scroll through data than it is with Dexcom which doesn't). Here is a 24 hour screen I took a picture of a while back:
I believe that this measure is a much better indicator of how difficult your blood sugar is to deal with than just standard deviation.