Question about initial calibration on startup of new sensor

Just started a new sensor this evening and noticed something odd. I took my 2 initial BG values with my meter and they were 86 and 89. I entered these two values, and the Dexcom started off with a reading of 83.

I thought the Dexcom didn't really measure numbers, but evaluated trends and adjusted from the calibration values. If that's so, why didn't it average the initial two readings and start somewhere between them?

I've noticed in the past that whenever I input a calibration value, it tends to adjust the number to reflect the calibration reading. For example if Dexcom is reading at 85 and I finger stick a reading of 95 and input the calibration value, it tends to adjust upwards to around 90.

Is this everyone else's experience and why would it be different for the initial calibration?

Just curious,


BTW - I'm on week 6 of the Dexcom and I don't think I would ever want to be without it again. I love the constant feedback and it's made a tremendous difference in my amount of control. :-)

Yes, I see the same.

According to scuttlebut on new proposed sensor with one year lasting power, the sensors of the minimed/Dexcom are reading relative readings based on H2O2 equations - hydrogen peroxide and since no absolute oxygen sensor, to correct against residuals and get absolute reading; regular calibration entries are required to keep sensor on track and I assume has different sensitivities to each body out there.

Article attached!

2862-glucose_implant_sensor_glysyn.doc (117 KB)

I see the same. My guess at the explanation is because Dexcom is 'calculating' the SG with the lag time for the difference between Blood and tissue glucose levels.

So when it adjusted to 90, it was based on an 'assumption'/estimate that tissue glucose would be 95 in 20 min, and therefore 90 at the current time, based on the current 'rate of change'.

So if I check my Dexcom about 15-20 minutes after calibration, and didn't just eat, then usually Dexcom shows very close to the calibration number I entered.

In my simplistic language, the DexCom is measuring interstitial fluid rather than blood glucose and the chemical difference between blood and interstitial fluid as the previous replies addressed accounts for the difference. I consider the 83 pretty close. Remember, never dose based on the CGM. Always check with your meter. That's DexCom's warning not mine!


Yes. That's my experience as well. algorithm includes a lag statement. What you get is not an average of 2 values.