The HbA1c test does not measure the amount of glucose in the blood, it compares the number of glycated hemoglobin molecules (ie. with glucose attached) to the total hemoglobin. The old DCCT standard expressed this as a percentage, the newer IFFC standard uses mmol(HbA1c)/mol(Hb). From the HbA1c value the average glucose concentration in the blood over the last approx 120 days can be estimated since the higher the glucose concentration the more hemoglobin molecules will have glucose attached.