Hemoglobin A1c: A minor component of hemoglobin to which glucose is bound. Abbreviated HbA1c. HbA1c levels depend on the blood glucose concentration: The higher the glucose concentration in blood, the higher the level of HbA1c. Levels of HbA1c are not influenced by daily fluctuations in the blood glucose concentration but reflect the average glucose levels over the prior 6 to 8 weeks. Measurement of HbA1c is a useful indicator of how well the blood glucose level has been controlled in the recent past and may be used to monitor the effects of diet, exercise, and drug therapy on blood glucose in patients with diabetes. In healthy people without diabetes, the HbA1c level is less than 7 percent of total hemoglobin. Also known as glycosylated or glucosylated hemoglobin.