How is HbA1c measured - from which blood component?

HbA1c (hemoglobin A1c or glycated hemoglobin) is formed when glucose in the bloodstream binds non-enzymatically to hemoglobin within red blood cells (RBCs). Because RBCs have a lifespan of approximately 120 days (3 months), HbA1c reflects the average blood glucose concentration over the preceding 2-3 months. The test measures the percentage of hemoglobin molecules that have glucose attached. It provides a superior long-term measure of glycemic control compared to a single fasting blood glucose test.