A1c is frequently used to help newly diagnosed diabetics determine how elevated their uncontrolled blood glucose levels have been. It may be ordered several times while control is being achieved, and then several times a year to verify that good control is being maintained.

The A1c test may be used to screen for and diagnose diabetes. However, A1c should not be used for diagnosis in pregnant women, people who have had recent severe bleeding or blood transfusions, those with chronic kidney or liver disease, and people with blood disorders such as iron-deficiency anemia, vitamin B12 anemia, and hemoglobin variants.

Hemoglobin A_1c measurements are of less value in stable diabetics, because urine and blood glucose determinations provide simple, economic, and reliable means for assessing glycemic control. In these patients, fasting glucose concentrations are fairly consistent from day to day and there is a significant correlation between hemoglobin A_1c and single fasting glucose levels. Although measurement of hemoglobin A_1c may provide a unique means of evaluating control in diabetics who have wide fluctuations in blood glucose levels, it is not a substitute for urine and blood glucose levels that are used to regulate insulin dose.

Depending on the type of diabetes that you have, how well your diabetes is controlled, and your doctor, your A1c may be measured 2 to 4 times each year. The American Diabetes Association recommends testing your A1c at least twice a year. When someone is first diagnosed with diabetes or if control is not good, A1c may be ordered more frequently.