A glucose test is a type of blood test used to determine the amount of glucose in the blood. It is mainly used in screening for prediabetes or diabetes. Patients are instructed not to consume anything but water during the fasting period. Caffeine will also distort the results. If the person eats during the period in which he or she is supposed to have been fasting then they may show blood sugar levels that may cause his or her doctor to think the person has or is at increased risk of having diabetes. In people already having diabetes, blood glucose monitoring is used with frequent intervals in the management of the condition.