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A blood glucose test measures the amount of a sugar called glucose in a sample of your blood. Glucose is a major source of energy for most cells of the body, including those in the brain. The hormones insulin and glucagon help control blood glucose levels.

The A1c test evaluates the average amount of glucose in the blood over the last 2 to 3 months. It does this by measuring the concentration of glycated (also often called glycosylated) hemoglobin A1c. Hemoglobin is an oxygen-transporting protein found inside red blood cells (RBCs). There are several types of normal hemoglobin, but the predominant form – about 95-98% – is hemoglobin A. As glucose circulates in the blood, some of it spontaneously binds to hemoglobin A. The hemoglobin molecules with attached glucose are called glycated hemoglobin. The higher the concentration of glucose in the blood, the more glycated hemoglobin is formed. Once the glucose binds to the hemoglobin, it remains there for the life of the red blood cell – normally about 120 days. The predominant form of glycated hemoglobin is referred to as HbA1c or A1c. A1c is produced on a daily basis and slowly cleared from the blood as older RBCs die and younger RBCs (with non-glycated hemoglobin) take their place. This test is used to monitor treatment in someone who has been diagnosed with diabetes. It helps to evaluate how well their glucose levels have been controlled by treatment over time. This test may be used to screen for and diagnose diabetes or risk of developing diabetes. In 2010, clinical practice guidelines from the American Diabetes Association (ADA) stated that A1c may be added to fasting plasma glucose (FPG) and oral glucose tolerance test (OGTT) as an option for diabetes screening and diagnosis. For monitoring purposes, an A1c of less than 7% indicates good glucose control and a lower risk of diabetic complications for the majority of diabetics. However, in 2012, the ADA and the European Association for the Study of Diabetes (EASD) issued a position statement recommending that the management of glucose control in type 2 diabetes be more "patient-centered." Data from recent studies have shown that low blood sugar (hypoglycemia) can cause complications and that people with risk of severe hypoglycemia, underlying health conditions, complications, and a limited life expectancy do not necessarily benefit from having a stringent goal of less than 7% for their A1c. The statement recommends that people work closely with their doctor to select a goal that reflects each person's individual health status and that balances risks and benefits.