Blood glucose levels are measured by a procedure based upon the enzyme glucose oxidase.
Since the enzyme is used, it is very specific for D- glucose, and will not be subject to interferences from other molecules in the blood.
Glucose can be oxidized since it is a reducing sugar. Glucose oxidase enzyme catalyzes the oxidation of beta-D-glucose to D-gluconic acid. The alpha-D-glucose is converted to the beta form so that all of the glucose is measured at one time.
Diatomic oxygen from the air is the oxidizing agent acting upon the glucose reducing agent. During the reaction process the ring opens and the aldehyde on carbon is converted to D-gluconic acid. At the same time the oxygen in the presence of water is converted to hydrogen peroxide. So far all of the chemicals are colorless, so you would not be able to see the reaction taking place. We will now detect its color.
In most cases, hydrogen peroxide reacts with a second color producing chemical. For instance 2-methylaniline reacts with the hydrogen peroxide using an enzyme called peroxidase to produce a color forming chemical.
Concentration of the glucose is related to the intensity of color produced because the more the intensity, the higher the concentration of glucose. Therefore a simple color chart can be used to "read" the concentration of the glucose.