HbA1c (glycated hemoglobin) reflects average blood glucose over approximately 2–3 months. What is the basis for this time frame?

HbA1c is formed when glucose non-enzymatically binds to the hemoglobin A in red blood cells (glycation). Because red blood cells have a lifespan of approximately 90–120 days (2–3 months), HbA1c reflects the average blood glucose concentration over the preceding 2–3 months. Conditions that shorten red blood cell survival (hemolytic anemia, sickle cell disease) can falsely lower HbA1c, while conditions that prolong red blood cell survival (iron deficiency anemia) can falsely elevate it. The HbA1c test is used both to monitor glycemic control in known diabetics and to diagnose diabetes (HbA1c ≥ 6.5%).