A glucometer is a portable medical device used to determine blood glucose levels. It works by applying 0.3-1 microliter of blood to a test strip, which contains chemicals that react with glucose in the blood. The result is displayed in minutes in units of mg/dL or mmol/L. Glucometers are important for patients with diabetes who need to monitor their glucose levels several times a day to maintain healthy levels and treat high or low blood sugar promptly.