You’ve pulled out your logbook and are taking off your jacket to bare your upper arm for the blood pressure cuff, when the nurse walks in and asks you to hold out a finger. “Does it matter that I had breakfast this morning?” you ask, trying to remember if you were supposed to fast before coming in, as she pricks your finger and collects a blood sample. “No, it doesn’t,” she says. “There; all done. The doctor will be in shortly to discuss your result.” And, indeed, several minutes later, your doctor walks in and says with a smile, “Looks like things are coming together for you. You’re at 6.8%.”
For some people, the doctor’s words would be enough for them to realize that the fingerstick in the imaginary scenario above was for a glycosylated hemoglobin (HbA1c) test. The HbA1c test gives an indication of your blood glucose control over the previous 2–3 months and is an important part of your diabetes-care regimen. This article discusses what the test is, why it’s important, and how it’s used to help better blood glucose control.
Figuring out how the HbA1c test can help with your blood glucose control starts with understanding a bit about the test and what it measures.
Hemoglobin. Hemoglobin is a molecule found in great quantities in each of the body’s red blood cells. As red blood cells travel through the circulatory system, the hemoglobin molecules join with oxygen from the lungs for delivery to the peripheral tissues, where they exchange it for some of the carbon dioxide destined for release to the lungs. The hemoglobin molecule is made up of two pairs of protein chains (two alpha chains and two beta chains) and four heme groups (iron-containing structures that act as the site of oxygen attachment and give red blood cells their color). Adults usually have a variety of types of hemoglobin, each with slightly different properties. The type of particular interest to people with diabetes is called HbA1c.
Besides carrying oxygen, hemoglobin molecules were discovered to have a secondary property that could be used to monitor blood glucose levels, namely the ability to join with glucose. Unlike cells that have insulin-controlled gating mechanisms to regulate how much glucose enters cells (such as muscle and liver cells), red blood cells allow glucose from the blood to freely enter and leave. The concentration of glucose inside a red blood cell is therefore the same as its concentration in the blood. The level of glucose in the blood affects how much glucose is available to bind to hemoglobin.
Once bound to hemoglobin, the sugar molecules mostly remain attached for the life of the red blood cell, which averages about 120 days. Your blood cells don’t all die at the same time: New blood cells are constantly being created, and younger cells outnumber older cells. Because the red blood cells in a blood sample used for an HbA1c test are a mixture of cells of different ages, the test gives a “weighted” average of recent blood glucose levels. This average is heavily influenced by more recent blood glucose levels because of the greater number of younger red blood cells; blood glucose levels from the past three months determine most of an HbA1c test’s result. In fact, blood glucose levels in the 30 days before the test determine roughly half of the HbA1c test’s result. Therefore, the HbA1c test is often said to give an indication of blood glucose levels for the previous 2–3 months.
HbA1c test results are given as a percentage that indicates the percentage of your HbA1c molecules that are linked to glucose molecules. A chart like “Blood Glucose Correlations” can help you to figure out what your average blood glucose levels were that caused your HbA1c result.