Final answer:
The A1C test is used to measure average blood glycemic levels over several months, which is crucial for the diagnosis and management of diabetes.
Step-by-step explanation:
The test used to measure blood glycemic levels over several months is the A1C test. This test is an essential tool for diagnosing and managing diabetes mellitus as it provides an average of your blood glucose levels over the past two to three months. The A1C test gives a clear picture of how well blood glucose is being managed, which is crucial for patients with diabetes to prevent complications. In contrast, the glucose tolerance test (GTT) measures the body's response to sugar over a shorter period, and other tests like insulin and glucagon tolerance tests are also used to assess the functionality of the pancreas and the body's response to these hormones.