Which laboratory test is crucial for the diagnosis of diabetes mellitus?

The Hemoglobin A1C test is crucial for the diagnosis of diabetes mellitus as it provides an overview of the average blood glucose levels over the past two to three months. This test measures the percentage of glucose that is attached to hemoglobin, which is a protein in red blood cells. A higher percentage indicates poor blood sugar control and is indicative of diabetes.

While other tests like the Fasting Blood Glucose and Oral Glucose Tolerance Test are also used for diagnosis, the A1C test has the advantage of not requiring fasting and can reflect long-term glucose management, making it a vital tool for both diagnosis and ongoing monitoring of diabetes. Its results help healthcare providers determine the effectiveness of a patient's treatment plan and can guide adjustments to medication or lifestyle changes.