Continuous glucose monitoring system principles

Current methods of continuous glucose monitoring are based on the measurement of the electrical current passing through the subcutaneously inserted sensor. The sensor is composed of three very thin electrodes. Two electrodes measure the glucose concentration in the interstitial fluid and the third electrode eliminates the “electrical noise” caused by the presence of other substances than glucose. The electrodes are made of two different metals or metal alloys that are usually well accepted by the human body.

The value of the electrical current is directly proportional to the glucose concentration in the interstitial fluid and can be converted into the glucose value using formula
       SG = ISIG x CAL        {1}
where SG is the interstitial (sensor measured) glucose value in mmol/l or mg/dl,
ISIG is the sensor current in nA,
CAL is a time variable constant called calibration factor in mmol/l/nA or mg/dl/nA.

Because there is no direct method to measure glucose concentration in the interstitial fluid, glucose value measured in the capillary blood is used to determine the calibration factor instead. The calibration factor is calculated as
       CAL = BG / ISIGref      {2}
where BG is the blood glucose value from a finger stick reading in mmol/l or mg/dl,
ISIGref is the referential sensor current in nA.

As the glucose profiles in interstitial fluid are slightly delayed after the glucose profiles in blood, ISIGref may not correspond to the sensor current at the time of calibration. E.g. ISIGref may relate to a value 10 minutes in the past.

To ensure the sensor stability and to adjust the slight variation of the calibration factor value, sensors usually require to be re-calibrated every 12 hours.