When you enter a calibration value into your Dexcom receiving device, your Dexcom data does not automatically line up on the new value.
it is easy to understand why. The measurements from a manual glucose monitor are only specified to be accurate with +/- 20% – an awfully poor expectation, but seemingly the only one that allows a reasonable cost for the strips used to read a BG value. On the other hand, your Dexcom, over time, averages out many calibrations (normally about two per day): after a while, it may actually have averaged out the measurement noise. In fact, some Dexcom users consider their Dexcom readings more trustworthy than a manual glucose meter reading.
Since the Dexcom algorithm averages out noise, it makes sense that it would not automatically line up your next reading to the last calibration.
Calibration higher than CGM
Experimentation shows that, if your calibration is higher than your Dexcom reading, in most cases the Dexcom algorithm will more or less average out its calculated new reading (taking into account your most recent BG change) with the new calibration value (modified by your recent BG change since calibration).
Calibration lower than CGM value
On the other hand, if the calibration value is lower than the last Dexcom reading, the Dexcom drift may potentially be more dangerous to you, since it now possibly overstates your BG and puts you at risk of a low. Experimentation shows that, when your calibration value is lower than your Dexcom reading, the Dexcom algorithm largely lines up on your last calibration (modified by your BG change since calibration).
What is the number Dexcom displays right after accepting your calibration input?
I am not quite sure what it represents – but it is NOT the calibrated number. The calibrated number is what will come out on the next regular tick after you have calibrated, whenever that is (within 5 minutes, obviously). The Dexcom algorithm corrects the calibration number by the drift it measures between the time you calibrated and the time of the next tick.
Calibration confirmation requested
In some cases, apparently when your calibration is far from the Dexcom algorithm expectation, your receiver device may actually ask you for a second calibration to confirm the first. These cases are rare.
In brief
So, as a simplification:
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if your calibration is higher than the Dexcom measurement, the algorithm appears to average the two;
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if your calibration is lower than the Dexcom measurement, the algorithm appears to adopt the new calibration as the new value.
Consequence: Dexcom values are normally lower than BG
Since the algorithm skews towards lower values (by going up half way when low, but all the way when high), your Dexcom generally reads lower than the measurement of your BG meter. Over the life of a sensor, the calibration pattern often shows regular readjustments upwards for the Dexcom values.
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