G6- what if the Dexcom number disagrees with meter

The G 6 number usually agrees with my Contour Next One meter number. Occasionally, the numbers disagree. For example, this morning, G6 number was 70. Contour was 90. Do you calibrate the G6?

It is my understanding that G6 requires no calibration and we can dose our insulin using G6 number. Am I under the incorrect impression?

I don’t calibrate regardless of the error. I find that my G6 and my Contour Next One pretty much agree after the first 24 hours when my blood glucose is in the range 100-150mg/dl. Outside that range, particularly below 80 or about 180 on the G6, and everything is off.

I use the readings to control correction boluses but not so much for correction carbs; in the latter case I more rely on my body to tell me to eat something. The G6-Contour slope seems to be steeper than 45 degrees; the G6 reads low for low BG and high for high BG. Consequently my correction boluses are probably too high but that doesn’t really matter; I can correct for a consistently high reading from the G6 by using a lower mg/dl/IU ratio and I probably have ended up doing that. I certainly don’t overcorrect unless I start stacking corrections.


I also wouldn’t calibrate then and generally don’t calibrate. If it’s extremely off, I might to see if it gets back in line, but then I’d suspect a bad sensor, especially if after the first 24 hours.

With the G5 you aren’t supposed to calibrate unless there is a 20% difference but I’m not sure if the G6 has those same types of guidelines. When were switch to the G6 I’ll probably calibrate if it’s off by 20% or more.

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Unless your bg is perfectly flat and stable and has been for quite some time and the g6 is still off quite a bit, it seems to do more harm than good

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There’s no harm checking your BG with a fingerstick if you are suspicious of the CGM reading. For myself, if I find the CGM is off I don’t calibrate, I just check occasionally to verify that it is staying in the ballpark.

As far as insulin dosing, I feel comfortable dosing from the CGM number. If an error causes me to give too little insulin my BG will rise until the CGM alerts and I take more. It really doesn’t matter whether it’s because of an error in carb counting or an error in the BG meter or CGM. Similarly, if an error causes me to take too much insulin my BG will fall, and when it gets too low the CGM will alert me so I can take some fast carbs. I have my alerts set pretty narrow, 85-130, because if my BG goes outside that range I want to look at the graph and see whether I want to do anything about it. With those alert thresholds, even a 30 point error in the CGM isn’t going to cause me actual harm, I would still have plenty of time to fix it. So I don’t calibrate and I don’t worry about it.

But then I’m also not worried about generating a Clarity AGP report that would “make me look bad” to the doctor, because I can explain the “severe lows” as falsely being indicated because of the CGM reading low during the first 12 hours of a new sensor.

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Here is the 20-20 rule regarding when to validate accuracy. In lower range it is 20 points, higher range 20 %.

If you’d like, you can calculate the 20 rule on your own. The Dexcom G6 reading must be within:

  • 20% of the meter value when the meter value is 80 mg/dL or higher
  • 20 mg/dL of the meter value when the meter value is under 80 mg/dL

Please note: the information listed here is applicable to Dexcom CGM users within the US only.

Link to full info on dexcom website:

Dexcom 20-20 rule