For the most part, my Dex 3 has been right on with cgm and meter reads...but occasionally i get some sync issues.
I got a tip yesterday from Dexcom technical support...regarding Dex saying 190 and meter saying 120. Tech Support said and claims to have had some success with the following 'fix'
When this is discovered...upload your meter read to Dex....15 minutes later take another meter read and upload again. Within 30 minutes the Dex algorithm should get back in sync and be more in line with meter reads.
Issue - depending on how far off Dex is---you know the occasional runaway reading--the kind where the trend starts on a rapid pace up and then just keeps going and going and going...Dex says 367 and you are actually 180...uploading meter reads at this point will cause and OUT OF CALABRATION X--in those cases, the suggestion was to separate yourself from Dex for 30 minutes to 1 hour...move the receiver to a different room so there is no transmission - no 'Y'. After 1 hour, meet back up with Dex and get your first read---many times you dont even need a finger stick - apparantly the algorithm 'fixes' itself. But if you are still way off, but not so much to cause a giant X out of calabration, then you can do the double read mentioned above with a 15 minute pause in between.
This doesnt happen that much to me...but when it does it can be very frustrating...especially when i am on my way out the door and dont want to drag my meter along.