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.
HbA1c can be used to measure disease state. The “Hb” stands for hemoglobin, and the “A1c” is simply the type of hemoglobin that glucose attaches to. Ninety percent of hemoglobin is the “A” or “Adult” type, and “A1c” is one of the minor components of hemoglobin, which also includes A1b, A1a1, and A1a2. The key reason why HbA1c is a better marker than glucose for chronic exposure is that blood glucose is very transient, whereas the half life of HbA1c is about 3 months. Although HbA1c is indicative of longer-term blood sugar levels up to 3 or more months, it is likely somewhat more weighted toward blood sugar levels during the previous 2 to 4 weeks.