I know that mysterious algorithms are at work, but if I restart my G4 sensor, and then calibrate with fingerstick tests of 5.1 and 5.2, why does the Dexcom tell me I’m 4.9?
I’m not worried about it. I’m just curious why it’s a (slight) mismatch from the get-go.
Let me throw out my conspiracy theory. There is a time lag of about 10 minutes between glucose in interstitial fluid and glucose in blood. In other words: When you calibrate the Dexcom, you input a value that the Dexcom should display 10 minutes later. Dexcom knows the current rate of change. It must have reasoned: Glucose levels are currently rising at about 0.2 per 10 minutes. Let’s display 4.9 now so that in 10 minutes the user input value is being displayed.
That can’t be it. I have pondered the exact same question as the OP. I have tried inputting my two calibrations with the higher reading first or second - it doesn’t make any difference. If the two readings are fairly close (e.g. 6.2 and 6.5) it will always come back with a CGM number slightly below the lower of the two calibrations, irrespective of the order in which you enter them.
I also wondered if the CGM is working in the background (even on a newly started sensor) and decides on the basis of whether it thinks that levels are rising or falling. Again. we can rule this out. It doesn’t seem to matter what the subsequent trend is (rising or falling), the first CGM point is always slightly lower than the calibrations).
Occasionally it also seems to do stuff that is completely counter-intuitive. For example CGM readings were sitting around 7.0 and flattish. I enter calibration of 7.2 and the next sensor point is 7.9 - go figure???