First, a lot of experiments have proven that glucose in subcutaneous tissue corresponds with a stabilized blood sugar level.
Second, the absorption and metabolism of glucose are located in the cell, while molecules move between the blood and the cells through capillary vessels and tissue fluid. Therefore, glucose in subcutaneous tissue can much more represent the organism physiological concentration level, so it is more representative in clinical practice.
Third, the blood sugar concentration will change with changes in the body. When the body loses blood, the detection is static which only represents glucose value at a certain moment. However, monitors glucose changes throughout the whole day. In this way, subcutaneous tissue offers a more holistic and accurate picture than blood sugar.