Basically how I understand it is this. My glucometer said I had a reading of 165, and my CGM said I was 216. So the difference of those two is 51. So you take 51/216, and it’s 23% which is more than 20%, which is inaccurate. Is that correct in how I figured that out ?
And also <img src="//cdck-file-uploads-global.s3.dualstack.us-west-2.amazonaws.com/tudiabetes/original/3X/4/0/400cb6a47e81793431a62fc30b392c16561efe97.PNG" width=“281” height=“499”
Is that not super confusing?
Not quite.
Your meter read 165, the CGM said 216, the meter value is theoretically the correct one, so the percentage difference is determined from the meter value, not the CGM value.
Divide the CGM value by the meter value, in this case 216/165 and you get 1.309, which is 30.9%. Alternatively, you could take the difference of 51 and divide by 165 (the meter value) and get .309, or 30.9%
Oh really? Okay. I think that makes sense. Ugh. This is all so confusing to me! And the explanatjon on the dexcom website is even more confusing.
and whenever the sensor is inaccurate. Inaccuracy is defined as when the difference between your sensor glucose reading and blood glucose value is greater than 20% of the blood glucose reading and blood glucose value is greater than 20% of the blood glucose value for sensor readings > 80mg/dL or greater than 20 points for sensor readings < 80 mg/dL.
That seems very confusing how the dexcom website explains it. [quote=“YogaO, post:2, topic:59031, full:true”]
Not quite.
Your meter read 165, the CGM said 216, the meter value is theoretically the correct one, so the percentage difference is determined from the meter value, not the CGM value.
Divide the CGM value by the meter value, in this case 216/165 and you get 1.309, which is 30.9%. Alternatively, you could take the difference of 51 and divide by 165 (the meter value) and get .309, or 30.9%
[/quote]
I apply the rule “trust no one.” I don’t trust my meter, I often get flaky readings. If I go to calibrate and I get a meter reading that is off by 20% from my CGM, I’ll carefully wash my hands and take another meter reading. If I find my first meter reading was way off, but the second “more controlled” was closer I’ll use the second reading. If both readings support each other I’ll enter the “average” reading.
We use exactly the same MO as @Brian_BSC Brian, and have pretty good results.
When we do an extra good hand wash for the 2nd reading, we also leave hands for a good minute under warm water to make sure there is plenty of fresh warm blood, and dry up thoroughly with clean paper towels. We also swing arms violently a few times right before taking the reading, and keep the hand as low as possible when poking.
In general, after this procedure we get a huge drop of bood that seems to give very accurate results, based on follow-up spot-check averages.
Healthy skepticism is good!
I will re-test when I get a meter reading that seems out of whack, but I have done enough of the poor preparations for fingersticks to now have a reliable, repeatable protocol. Additionally, I always do a fingerstick test immediately before an A1C blood draw as a method to validate my meter readings. So far, my current meter has been pretty spot on.
Nice. What meter do you use?
Accu-chek Aviva Combo.
Cool!
And for good measure:
CALIBRATING YOUR DEXCOM G5 (or G-whatever)
There are “good” and “not-so-good” ways to calibrate. The following is what has worked well for my daughter:
1. After starting a new sensor (or re-starting one that’s still working at the 7-day mark), wait at least 10 minutes between the first two calibrations. Calibrate at least two hours after a meal or after treating a low BG.
2. Although it can be very tempting, do not calibrate more than twice daily and only when requested to do so. *Exception: I throw in one to two extra calibrations during the first 12 hours of a new sensor if the numbers are a little wonky.
**Sometimes I get a little calibration-happy when my daughter’s sensor is getting a little “ripe”, i.e. over 15 plus days old, if the Dexcom readings seem significantly wonky (at which point, she’d be better off if I just bit the bullet and put in a new sensor.)
3. CALIBRATE ONLY WHEN THE DIRECTIONAL ARROW IS HORIZONTAL. I repeat: CALIBRATE ONLY WHEN THE DIRECTIONAL ARROW IS HORIZONTAL. Trust me on this! Your sensor continues to function after the big red blood drop screams at you to calibrate right now. Tell it to hold its horses and you’ll calibrate when you’re good and ready (when the directional arrow mellows out and becomes horizontal.) No ones Dexcom has stopped working or imploded, creating a black hole that engulfed the universe, because they didn’t calibrate the instant the Red Drop of Dexcom appeared.
4. Try to avoid calibrating when BG is below 80 or over 200. Correct and get your BG within this range (80 - 200) and then calibrate ONCE THE DIRECTIONAL ARROW IS HORIZONTAL.
5. Learn to question the accuracy of your BG meter as much as, if not more than, your Dexcom. Personally, I trust the Dexcom more often than I trust the BG meter. But my daughter’s Type 1 vibes seem to resonate exceptionally well with those of her Dexcom. I often think that my daughter and her Dexcom are as one.
6. Stay well hydrated. I believe this helps produce more interstitial fluid that more accurately reflects BG levels.
7. Learn to accept the fact that the lag time after treating a low BG is going to
be longer than you’d like. Keep this in mind so you don’t over-treat lows.
Good luck! In short order, you will wonder how you lived without this miracle of diabetes technology.
Question for you. Why would you need to wait 10 minutes beteeen the first two calibrations? And also, why would you need to calibrate no lower than 80 or higher than 200? Just curious. 
I’ll calibrate more than twice a day if I notice that it’s off by more than 20%.
[quote=“music_rox84120, post:12, topic:59031, full:true”]
Question for you. Why would you need to wait 10 minutes beteeen the first two calibrations?[/quote]
My take on a this is waiting 10 to 15 minutes provides the Dexcom with a bit more useful information right out of the gate as there is likely to be more variation between the two readings with those extra few minutes.
Dexcom gives the most accurate and up-to-date readings when BG is not on the lower end of the scale or towards the higher end of the scale. Calibrations are more useful the more accurate your meter BG reading.
You will not be supplying your Dexcom with more useful information if you calibrate at times when your rate of change is rapid, i.e. when the directional arrow is NOT horizontal, regardless of the “percentage off”.
Calibrating when above 200 and below 80 are areas where the Dex sensor is less likely to be accurate (especially on the low end, in my experience).
Note that early on in my Dex experience I calibrated many, many, many times when my BG was in the 70-80 range, but the Dex was convinced that I was under 55 and giving me lots of grief for it. Again, in my experience I just wasted a bunch of test strips, because the Dex just won’t respond.
I guess the bottom line is the Dex calibration works best when your BG is stable (or at least relatively so) and in the 80 - 200 range.
As for the 10-minute dealio, I think that just helps to establish that your BG is stable when doing the initial sensor start up, but I would love to hear what @rgcainmd says. I have had a Dex tech support person suggest waiting 15 minutes between the two start up calibrations.
I find this an excellent strategy!
After the first 12 hours or so (and sometimes right from the get-go), I trust my daughter’s Dexcom readings to be more accurate than her pretty-accurate Freestyle meter, as demonstrated by repeat meter checks taken when the first meter value is way off the Dexcom reading. It’s almost invariably the meter that is off during times when her rate of change is not high.