Blood glucose does not always correspond to glucose measured by the sensor.

Finger prick blood glucose readings and sensor-measured glucose will not always be the same, and are likely to differ. This is because the sensor measures glucose in the interstitial fluid (ISV), a thin layer of fluid around the cells of the tissues under the skin, not in the blood. The glucose levels in the ISV are 5 to 10 minutes behind changes in blood glucose. Glucose measurements based on ISV have been shown to be a reliable indicator of glucose levels.

Traditional blood glucose monitoring provides the user with glucose readings at specific times. Users don't get any information about how their glucose levels have changed or the direction their glucose levels are moving. Without such information, significant glucose fluctuations – the peaks and troughs – can easily be missed. A continuous or flash system (CGM or FGM) is a new way to take glucose readings. This allows the user to obtain a current glucose reading by just scanning with the reader above the sensor. In addition, it provides a complete picture of glucose concentration variations.