Final answer:
To diagnose diabetes mellitus, a fasting blood glucose level should be 125 mg/dL or higher, or a random or OGTT should read 200 mg/dL or higher. The glucose tolerance test is especially useful for diagnosing those with normal to borderline fasting glucose levels.
Step-by-step explanation:
Diagnosing Diabetes Mellitus (DM)
To diagnose diabetes mellitus type I or II, a fasting glucose should be ≥125 mg/dL or a random or oral glucose tolerance test (OGTT) reading should be ≥200 mg/dL.
The fasting blood glucose level is considered normal below 100 mg/dL. A level between 100 and 125 mg/dL indicates pre-diabetes, and exceeding 125 mg/dL after fasting is indicative of diabetes. The glucose tolerance test is a valuable tool in confirming diabetes in those with fasting levels that are normal or close to normal, as a significant rise in blood glucose following an oral dose indicates intolerance to glucose, a hallmark of diabetes mellitus.
During an OGTT, a person is given a specific dose of glucose after fasting, and blood glucose is measured at intervals to assess how well the body can manage the glucose. If the body cannot return the blood glucose levels to normal within the expected timeframe, it suggests an impaired ability to regulate blood sugar, consistent with a diagnosis of diabetes mellitus.