Hemoglobin is the oxygen-carrying part of the red blood cells. This blood test measures a special kind of hemoglobin that has been coated with glucose, the main sugar in the body. It is called glycosylated hemoglobin or HbA1c.

Red blood cells floating around the body occasionally bump into glucose, which sticks to hemoglobin in these cells. Normally, only a small amount of hemoglobin gets coated with glucose. A person with diabetes has a lot of glucose in their bloodstream, so his or her HbA1c level will be high. A person who does not have diabetes will have a low, or normal, level of HbA1c.

Red blood cells live about 100 days before getting recycled. Therefore, the test for glycosolated hemoglobin shows the average level of blood sugar for the past 3 months. High levels of HbA1c mean the blood sugar levels have been too high over the previous 3 months and need to be better controlled. If the test is in the goal range, the person with diabetes has achieved good glucose control.

Who is a candidate for the test?

Normally, this test is done only on people with diabetes. It is often done at least once or twice a year to see if the diabetes is being controlled.

How is the test performed?

A blood sample is taken from a vein in the forearm or the hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through the veins.

A very thin needle is gently inserted into a vein and the tourniquet is removed. Blood flows from the vein through the needle and is collected into a syringe or vial. The sample is sent to a lab and analyzed.

After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

Home test kits that can be done with a fingerstick and test strips that are sent to a lab will be available soon. There are also home monitors that measure fructosamine, which is the glycosylation of different blood proteins. Sometimes called the glycoprotein test, this measures control of diabetes over about a 2-week period of time.