Final answer:
Patients must fast before a Glucose Tolerance Test (GTT) and sometimes before an A1C test, both of which are used to diagnose and manage diabetes mellitus by assessing blood glucose regulation and control.
Step-by-step explanation:
Two tests that patients are commonly required to fast for before having their blood drawn are the Glucose Tolerance Test (GTT) and the A1C test. The GTT helps diagnose conditions like diabetes mellitus by measuring the patient's ability to regulate blood glucose levels after consuming a sugary drink post-fasting. The A1C test, on the other hand, reflects the patient's average blood sugar control over the past two to three months and can also be a part of the diagnostic criteria for diabetes mellitus.
For the GTT, fasting is typically required for 12 to 14 hours before the test. Following the fast, a specific amount of glucose is ingested and blood glucose levels are measured at intervals to assess glucose tolerance and determine if the patient has diabetes mellitus. It should be noted that a carbohydrate-rich diet is advised for three days before undergoing the GTT.
In contrast, the A1C test does not always require fasting, but it can be included as part of a fasting blood panel in some cases to provide a comprehensive picture of the patient's blood glucose management.