The A1C test is a blood test used to measure the average level of glucose in the blood over the last two to three months, and is used as a means of checking how well blood sugar levels are being controlled for a person with diabetes.t can also be used to make an initial diagnosis of diabetes.1. The reason why A1C, glycated hemoglobin, is an indicator of blood sugar levels is that some of the glucose in the blood binds to hemoglobin, the protein found in red blood cells and responsible for transporting oxygen around the circulation.