A tech support guy mentioned it on the phone, but I neglected to ask what it is. Can anyone explain? Thanks.UPDATE (April 2014): I found this explanation on the Dexcom web site. However, being a millimoler (Canadian, that is), I still don't quite understand it:
[you should calibrate] 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.