Final answer:
Patients should fast for at least 12 hours before a glucose tolerance test, which is essential for diagnosing diabetes mellitus by assessing how well the body metabolizes glucose.
Step-by-step explanation:
A patient should be fasting for at least 12 hours, with water allowed, before undergoing a glucose tolerance test (GTT). In a GTT, initially, the patient's fasting blood glucose is measured to assess baseline levels. Subsequently, the patient is given an oral dose of glucose and blood glucose levels are estimated at regular intervals to evaluate the body's ability to metabolize glucose, which is crucial in diagnosing diabetes mellitus.
The test requires the patient to have a carbohydrate-rich diet for several days prior to the test and then to fast to ensure that the results are not influenced by recent food intake. Once fasted, the oral dose of glucose is administered, and the blood sugar response is monitored to determine if it rises and falls within a normal range. This helps in diagnosing if a patient is diabetic, pre-diabetic or has a normal tolerance to glucose.