Hello,
We have been using the Dexcom G-6 for about a month now. I notice it is consistently about 20- 30 units off from the glucometer. I know they are measuring two different things, but at what point should I be worried about accuracy? I know there was talk about dosing of off CGM numbers but I haven’t ben comfortable with doing it.
@Restless_Daddy hi again Chad. Your finger stick glucose meter is accurate to +/- 20% meaning if you test and get a 185mg/dl your real blood sugar Is +/- 37 mg/dl (185 X 0.20). Or something between 148 and 222 mg/dl so if your CGM is between 148 and 222 it is considered SPOT ON”. Cheers!
Hi Chad @Restless_Daddy, I feel very comfortable dosing from readings of my G5 - and in a few months when I upgrade I’m looking forward to the higher accuracy of the G6.
As @joe said, your glycose meter is not necessarily accurate - even if it is a “newer” model that is supposed to by within 15% accuracy. As a side-note, I will confirm my CGM reading before I make a correction dose if “my feeling” differs from the CGM value - I have over 60 years recognizing my BG levels.
One little bit of advantage for you is your word “consistently”; if the values are truly at that differing rate, you may want to keep that difference in mind when you do insulin calculations.
I’ve been using the g6 for a little over a month and I’ve been pleasantly surprised with the consistent accuracy–I’d say I trust it more than my glucometer at this point because it tends to match my symptoms better when I’m out of range. Of course you can always calibrate if you feel it’s necessary. 
I actually just talked to my daughter’s nurse about the accuracy range this morning! She’s had the G6 for all of 20 hours now, and her readings seem to be about 30 under her blood glucose readings. I don’t have an issue with +/- 30 and her office requires finger pricks any time she will be given insulin anyway.
She mentioned that range was acceptable to dexcom but that once she gets lower the range tightens up, so it’s more accurate the lower the number is.
One thing I find disturbing about my G4, is that the majority of times that I calibrate, the receiver immediately reports back roughly 10 BG lower than what I have entered and almost always shows a steady trend afterwards.
I suppose this is a great precaution to help avoid lows, but most electronics I calibrate will come back to read what I have told it to be TRUTH.
I was told to never use the dexcom to dose by. It is more of a warning statement than a dosing one. They are working on one to dose by. I think it is put out by freestyle. I received a free sample card in the mail
10 mg/dl is noise. the thing you’re calibrating with is - at best: +/- 15% accurate, many are +/- 20%. if your cal is at a blood sugar higher than 75 mg/dl, then within 10 is a miracle.
anyway if it is consistent, and it bugs you, subtract 10. happy calibrating!
As a reminder, glucometers available on the market are supposed to be within 20% of a serum lab glucose taken at the same time as a finger stick. As glucose levels rise, the margin grows.
All CGM devices, whether it’s a Freestyle Libre or any of the Dexcom models, are measuring interstitial glucose (that the fluid between your body cells), and will be slightly different. If you’re getting readings on your CGM that are trending up or down quickly, it’s likely to be further off from a finger stick.
All that said, my son’s Dexcom G6 has been accurate to within 10 mg/dL on the occasions when I’ve had him check, and as such, we don’t worry about doing finger stick checks unless he’s having symptoms that don’t seem consistent with his readings, and it works seamlessly with his tSlim x2 pump.
As always with T1D, it never hurts to check, just be careful about chasing your tail. It takes about 40 minutes to see changes in his CGM trend after dosing or eating.