What test is considered the most sensitive for diagnosing diabetes mellitus?

The oral glucose tolerance test (OGTT) is considered the most sensitive for diagnosing diabetes mellitus because it evaluates the body’s ability to metabolize glucose more comprehensively than other tests. During the OGTT, a patient’s blood glucose levels are tested before and after the ingestion of a glucose-rich beverage, typically containing 75 grams of glucose. This allows for the assessment of how well the body responds to glucose over a specific duration (usually two hours), capturing the body’s peak response and detecting any abnormalities in glucose metabolism.

In contrast, the fasting plasma glucose test measures blood glucose levels after an overnight fast, which may miss cases of impaired glucose tolerance that the OGTT could identify. The A1C test reflects average blood glucose levels over the previous two to three months, providing a broader picture but may not capture recent fluctuations well. The random plasma glucose test can indicate high blood sugar levels but is less structured and often less definitive without confirmation.

The unique capacity of the OGTT to highlight insulin resistance and the body's response to glucose enables it to detect prediabetes and diabetes early, making it the most sensitive option among the tests available for diagnosing diabetes mellitus.