What Does an HbA1c Measure? An important component in choosing a medicine for diabetes treatment will actually be what the HbA1c level is. To review what HbA1c is, it's actually the 3 month average of blood glucose levels. Glucose will actually be carried by red blood cells around the blood. So when we're measuring the average blood glucose levels, we're actually measuring the amount of glucose on those red blood cells that have a life expectancy of 3 months. By measuring the amount of glucose on these cells we can work out what blood glucose levels have been for not only the previous 24 hours, but the previous 7 days of the week, and the previous 3 months.