Continuous glucose monitoring system principles

Medtronic’s method of continuous glucose monitoring is based on measuring of electrical current of a subcutaneously inserted electrode called the sensor. The sensor current is measured every 10 seconds. The measured sensor current values are averaged once every five minutes and turned into the glucose value in interstitial fluid using formula
       SG = ISIGAVG x CAL      {Principles 1}
where SG is the interstitial (sensor measured) glucose value in mmol/l,
ISIGAVG is the averaged sensor current in nA,
and CAL is the calibration factor in mmol/l/nA.

As there is no direct method to measure interstitial fluid glucose value, glucose value measured in capillary blood is used to determine calibration factor instead. The calibration factor is required to be set at least every 12 hours and it is calculated as
       CAL = BG / ISIGAVG+10      {Principles 2}
where BG is the blood glucose value from a finger stick reading in mmol/l
and ISIGAVG+10 is the averaged sensor current in nA measured 10 minutes after the BG reading.

The method is based on following assumptions:

  • The blood glucose value is equal to the interstitial fluid glucose value with lag of 10 minutes. See section "Blood and interstitial fluid glucose profiles" for more details;
  • The electrical current of the sensor is linear function of the glucose value in interstitial fluid.