The diagnosis of diabetes is to a large extent based on detecting elevated levels of sugar (glucose) in the blood. Hemoglobin A1c (HbA1c) is a laboratory measure frequently used for this purpose. The test is also useful to monitor treatment in patients with established diabetes.Approximately 8 percent of the US populations suffer from type 2 diabetes, with as many as 40 percent of those undiagnosed (1).s HbA1c reflects long-term control of blood glucose, it is often used to monitor the effects of diet, exercise, and drug therapy in people with diabetes. If blood sugar levels have been high in recent weeks, HbA1c will also be higher.