A young adult presents with polyuria, polydipsia, and weight loss. Which test would confirm the diagnosis of diabetes mellitus?

The diagnosis of diabetes mellitus can indeed be confirmed using multiple different tests, but the preferred method is often through the measurement of glycated hemoglobin (HbA1c). This test provides an average blood glucose level over the past two to three months, making it particularly useful for diagnosing diabetes as it reflects long-term glycemic control rather than a momentary value.

In the context of a presentation with polyuria, polydipsia, and weight loss, the implications of a high HbA1c level would indicate chronic hyperglycemia, thus supporting a diagnosis of diabetes mellitus. The HbA1c test is also advantageous because it does not require fasting and can provide more consistent results compared to single measurements of blood glucose levels.

Other tests like fasting blood glucose, oral glucose tolerance test, and random blood glucose can also diagnose diabetes, but more specific criteria and sometimes additional context (like symptoms or other glucose levels) are required. Thus, while these tests are clinically important, the HbA1c is often regarded as the most reliable indicator for confirming a diagnosis of diabetes mellitus in a clinical setting.