Components
The two essential parts of a glucose meter are the testing strip (which is single-use) and the monitor itself. The testing strip is a small piece of paper that has certain chemicals on it, which allow it to react with sugar in the blood. The testing strip has a hard plastic base to give the strip stability and several chemical layers that are separated by spacers. One layer includes the enzyme glucose oxidase, another includes potassium ferricyanide, and yet another layer that has two electrodes to measure current flow. Between these layers are other layers that protect the chemical layers and allow the chemical layers to react with the blood. The glucose monitor measures electrical currents running through the test strip and translates that current into the amount of glucose in the blood.
Glucose Oxidase
When the end of the test strip is pressed to a drop of blood, the blood is pulled up the sides of the test strip via capillary action. As the blood flows into the test strip it comes into contact with the glucose oxidase layer. Glucose oxidase reacts with the glucose in the blood to make gluconic acid. Although this enzyme can react with other sugars, it does so much more slowly.
Ferricyanide and Electricity
Once gluconic acid has been produced by the glucose oxidase layer, the gluconic acid reacts with the potassium ferricyanide laye to form potassium ferrocyanide. Potassium ferrocyanide reacts with the metals on the electrode layer, which causes an electrical current to flow through the electrodes. The more glucose in the blood, the more gluconic acid is produced, which causes more ferrocyanide to be formed, which increases the electrical current. Thus, the glucose meter is able to interpret the electrical current and use that information to determine how much glucose was in the blood sample. This information is displayed on the readout. Although less common, some test strips don't generate an electrical current but instead have a layer that changes color in response to glucose. These strips use meters that measure the color change.
