Which laboratory test is primarily used to diagnose diabetes mellitus?

The fasting plasma glucose test is vital for diagnosing diabetes mellitus because it measures blood sugar levels after the patient has not eaten for at least eight hours. A reading of 126 mg/dL or higher indicates the presence of diabetes. This test is widely utilized due to its reliability, ease of administration, and the fact that it can identify individuals who may have prediabetes as well.

While other tests such as the random plasma glucose test and the oral glucose tolerance test also play important roles in diagnosing diabetes, they are used in specific situations. The random plasma glucose test does not require fasting and can be used in emergencies or to confirm diabetes but is less precise for an initial screening in an asymptomatic patient. The oral glucose tolerance test is often used for diagnosing gestational diabetes and may also identify type 2 diabetes, but it is more complex and less commonly used as a routine diagnostic tool.

The Hemoglobin A1c test assesses average blood glucose levels over the previous three months, serving as a useful tool for monitoring long-term glucose control rather than for initial diagnosis.

Thus, the fasting plasma glucose test is a primary and standardized approach for diagnosing diabetes mellitus, making it a crucial tool in the clinical assessment of a patient’s glucose metabolism.