An A1C test measures how high your blood-glucose levels have been over a 2-3 month period by analyzing hemoglobin. Hemoglobin is a small protein that lives in the red blood cells of your body and carries the memory of any glucose it encounters in the blood stream for the three months of the blood cell’s life.

The A1C test is an excellent general indicator of how well you have controlled your blood sugar over an extended period of time. It can be used to help you and your health care team better manage your diabetes. The test is also known as HbA1c, glycohemoglobin, glycated hemoglobin, glycosylated hemoglobin, GHb and diabetic control index.

The A1C test works by measuring the amount of blood glucose in the hemoglobin. Hemoglobin is a protein in the red blood cells that carries oxygen from the lungs to all the cells in the body. If a person has diabetes, excess glucose enters the red blood cells and attaches itself to the hemoglobin; this is called glycosylation.

Imagine the glucose as a foreigner lost in a strange city (the red blood cells) who has to cling to a local (the hemoglobin) to find its way around. The more glucose there is in the blood, the more hemoglobin is glycated by glucose. The glucose stays with the hemoglobin until the red blood cell dies after 2-3 months. In the A1C blood test, hemoglobin is drawn out of the body in a blood sample and analyzed to see how much of it has been glycated: the results indicate how high your glucose levels have been for the past three months.

Another way to understand it is to think of your red blood cells as score keepers remembering the number of “glucose hits” to the hemoglobin. An A1C test is like a three month print-out of this score board. When you read the score board you can tell how high your blood-glucose levels are based on the amount of glucose-containing hemoglobin. If you have a high blood-glucose level, you will have a “high scoring” A1C.