Which of the following is considered the best single criterion for diagnosing diabetes by ADA?

• A. Fasting plasma glucose
• B. HbA1c
• C. 2-hour OGTT
• D. Random plasma glucose

Answer: (A)

Diagnosing diabetes can be based on several different tests, but one criterion is favored in many clinical settings because it’s simple, widely available, and reliable. A fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or higher is considered the best single criterion because it requires minimal patient preparation beyond fasting, is inexpensive, easily standardized across laboratories, and has a clear cutoff that’s been well validated. If this level is reached on two separate occasions, it confirms diabetes.

HbA1c is also diagnostic (6.5% or higher) and has the advantage of not requiring fasting, but it can be affected by conditions that alter red blood cell turnover (like certain anemias or hemoglobinopathies), which can limit accuracy in some patients. The 2-hour OGTT after a 75 g glucose load is highly sensitive and can detect cases that fasting glucose misses, but it is more time-consuming and less convenient for routine use. A random plasma glucose is informative only when the patient has classic hyperglycemia symptoms; otherwise it’s less reliable for diagnosis.

So, while any one criterion can establish diabetes, fasting plasma glucose is typically viewed as the most practical single test for diagnosis in everyday practice.