Dear Dr. Edelman,
I love my CGM, but sometimes it’s considerably off from my finger stick number and I was wondering if there’s an acceptable variance between the two? How accurate should CGM values be compared to values from blood glucose meters?
Dr. Edelman: A lot of us folks with diabetes who use CGM devices wonder how accurate they are, and also how accurate they are required to be. There are several schools of thought, but most feel the following national standards are reasonable:
- For glucose values less than 100 mg/dL, the CGM value should be within 15 mg/dL of the blood glucose meter number. For example, if the blood glucose meter number is 90 mg/dL, then the CGM value should be between 75 and 105 mg/dL.
- For glucose values greater than 100 mg/dL, the CGM value should be within 15% of the meter value. For example, if the blood glucose meter value is 180 mg/dL, then the CGM value should be between 153 and 207 mg/dL.
There are still lots of other variables that come into play. Were your hands clean and dry before testing with your meter? Were the glucose strips exposed to air for an extended period of time, or were they expired? Was it the first day of a new sensor? What brand of CGM are you using?
Remember that CGM devices continuously auto-calibrate, and because you get a new value every five minutes along with a trend, having a CGM number that’s not exactly perfect isn’t as detrimental as a single finger stick that is way off.
Finally, remember to test your glucose using a blood glucose meter when your symptoms do not match the CGM value.
Fifteen percent tolerance isn’t good enough! If my son’s Dexcom were only accurate to within that standard, we’ve have never bothered to buy it. In our experience (after the first day of a news sensor) the G6 is usually the same as a finger stick within about 5 mg/dl, for just about any reading from about 60 to over 200. I think the +/- 15% standard is too lax, too outdated. Since today’s CGM’s are typically much more accurate than the standard, the standard should be updated to reflect reality.
Wife freestyle 1 is now, after 3 days try #2, 30 points low compared from the finger. the freestyle 2 failed on launch, they sending us another. Dexcom was 10-20 points off the whole 10 days. now that the freestyle 1 sensors (x 2) are acting the exact same, I believe it just that way. But one finger stick compared to 6 or more fingers sticks a day, are worth just going along with the addition.