Which laboratory measure best reflects long-term average blood glucose control in diabetes?

• A. Hemoglobin A1c (HbA1c).
• B. Fasting plasma glucose.
• C. Oral glucose tolerance test.
• D. Random plasma glucose.

Answer: (A)

Long-term average blood glucose control is best reflected by the HbA1c test because it measures the amount of glucose that has permanently attached to hemoglobin over the life span of red blood cells. Since red blood cells circulate for about 120 days, HbA1c provides an integrated view of average glucose exposure over roughly two to three months. Higher average glucose leads to more glycation and a higher HbA1c percentage, which is why this test tracks chronic control rather than a single moment in time.

The other measures capture more immediate states: fasting plasma glucose is a snapshot after an overnight fast, the oral glucose tolerance test assesses how the body handles a glucose load at one point in time, and random plasma glucose reflects glucose levels influenced by recent meals. HbA1c remains the best indicator of long-term control, though it can be affected by conditions that alter red blood cell turnover.