Final answer:
A patient should fast for 12-14 hours before a Glucose Tolerance Test (GTT) is performed.
Step-by-step explanation:
A Glucose Tolerance Test (GTT) is performed to assess an individual's ability to tolerate glucose and diagnose diabetes mellitus. In preparation for the test, the patient should follow a carbohydrate-rich diet for three days prior to the experiment. On the day of the test, the patient should fast for 12-14 hours before the blood glucose is measured. Then, they are given an oral dose of 50-60 grams of glucose in 300 ml of water, and their blood glucose levels are measured every half hour for several hours to assess their ability to tolerate glucose.