What is the hemoglobin A1c test looking for?

This post is for Kristen, who wanted to know what was being tested for when a hemoglobin A1c test is done. Here goes...

Hemoglobin is the molecule in red blood cells that carries oxygen from our lungs to cells in the rest of our bodies, like our muscles. When we are babies, we have a type of hemoglobin called fetal hemoglobin, or hemoglobin F. As we get older, our bodies make adult hemoglobin, or hemoglobin A. 90% of the hemoglobin in a normal adult is regular hemoglobin A. The rest are modified hemoglobins called hemoglobins A1a1, A1a2, A1b, and A1c.

Hemoglobin A1c is formed when sugar in the blood sticks to (reacts with) hemoglobin A. Sugar reacting with a protein is called glycation, and all people have some glycated hemoglobin. Once a molecule of hemoglobin A in a cell turns into hemoglobin A1c, it stays that way as long as the blood cell is alive. A red blood cell lives for about 4 months, so this gives doctors a way to see how much glucose sticks to hemoglobin over time; most people have around 7% hemoglobin A1c.

People with diabetes can have more sugar in their blood, especially if their diabetes is untreated or if they have difficulty maintaining a steady blood glucose level. Because there is more glucose in their blood, more of it sticks to their hemoglobin A, turning it into hemoglobin A1c. This means that people who have a hard time keeping their blood sugar level stable will have more hemoglobin A1c than non-diabetic people. Because red blood cells live for a few months, testing a person's hemoglobin A1c is a good test for how stable someone with diabetes’s blood sugar is over a long period of time. This is in contrast to the blood glucose tests diabetic people take every day- those take a snapshot of their blood sugar at a given moment. The hemoglobin A1c test is more like a movie, in that it shows what happened to a person's blood glucose level over a long period of time. This is why doctors do hemoglobin A1c tests- they can obtain a single number that tells them how well a patient has been maintaining their blood sugar.

A hemoglobin A1c test is a relatively simple blood test. Blood is drawn, and the hemoglobin is separated from the blood cells. The glycation of hemoglobin A1c changes both its size and its interactions with other molecules. Therefore, it can be easily separated from the other forms of hemoglobin in the lab, and the amount can be tested.