Sensor instrumental delay

Medtronic’s continuous glucose monitoring system uses data smoothening algorithm to suppress variation of ISIG values during calculation of interstitial fluid glucose values from sensor current according to formula {Principles 1}. This variation is caused by changes in interstitial fluid circulation at both sensor and cannula insertion sites. The algorithm takes into account several last ISIG values and its purpose is to smooth graphs shown on the pump display as shown on Fig. Delay 1.

Fig. Delay 1.    Sensor glucose values smoothening

The figure shows example of ISIG values and their conversion to interstitial glucose values. If there could be no data smoothening, SG values calculated from ISIG would be in range from 135 to 145 mg/dl between 16:25 and 17:25. The data smoothening algorithm keeps them nearly constant at value of 140 mg/dl with exception of one value at 16:55.

On the other hand the data smoothening algorithm:

The total shift between the glucose profile in capillary blood (measured by meter) and the interstitial fluid (measured by sensor and displayed by pump) is approximately 18 - 20 minutes in my case. The total shift is composed of: