Why is HbA1c a marker for uncontrolled diabetes?

Hemoglobin A1c (HbA1c) represents the percentage of red blood cells that are glycated (i.e., glucose-bound). HbA1c can give an idea of a person’s average sugar levels over the past 3 months (the approximate lifespan of a red blood cell). When blood sugar levels are chronically elevated due to uncontrolled diabetes, red blood cells are exposed to high levels of glucose molecules. Glucose in the bloodstream will irreversibly attach itself to hemoglobin molecules at levels substantially higher than in blood without elevated blood sugar. The percentage of these glycated hemoglobin molecules can be measured, and the levels are used to assess how well an individual is managing their disease and whether they are at an increased risk for cardiovascular complications. The key reason why HbA1c is a better marker than blood glucose for how well diabetes is controlled is that blood glucose is changing all the time, whereas HbA1c represents average glucose control over time. Although HbA1c is indicative of average blood sugar levels around 3 or more months, it is likely somewhat more weighted toward blood sugar levels during the previous 2 to 4 weeks.