How does a blood glucose monitor work?

Chemical reaction: There is a sharp lance that gets a drop of blood. This goes onto a strip that undergoes a chemical reaction. The resultant color reaction is read my the meter and translated into the number that appears of the face of the meter and tells you the blood sugar is at that time.

A common method is: rate limited glucose passage from a blood sample into filter paper containing (1) an enzyme which converts glucose into gluconic acid along with (2) a pH indicator which changes color as a function of pH. The color, at a specific time, corresponds to the amount of glucose which has managed to pass through the rate limiting membrane covering the filter paper. DEXCOM G6 CGM currently best monitor.