The A1c test (or hemoglobin A1c or glycohemoglobin test) is a blood test that shows the average level of blood sugar (glucose) over the past 2 to 3 months. Most doctors think that this is the best way to check how well a person is managing their diabetes. The test may also be used to diagnose diabetes or prediabetes. This test may not be appropriate for everyone because many things can affect the lifespan of red blood cells, such as the second or third trimester of pregnancy, a recent blood loss or a blood transfusion, sickle cell disease, hemodialysis, or erythropoietin (ESA) medicine.

The A1c test result also can be used to find the estimated average glucose, or eAG. A1c and eAG show the same thing in two different ways. A1c is shown as a percentage, while eAG uses the same units (mmol/L) as a glucose meter.