--is for glycated hemoglobin, a long term measure of glucose levels in the blood, considered to be a more accurate long term measure of those levels than just a random fasting blood glucose level.

The test measures the percentage of red blood cells that have hemoglobic glycation; since, once red blood cells have had their hemoglobin glycated, they stay that way until their demise, and since red blood cells have a life span of about two three months, this percentage is a reflection of average blood glucose levels in the body over the past few months. Generally, a level of 4 to 5.6 percent is considered normal/optimal; levels from 5.7 to 6.4 are considered to reflect impaired glucose metabolism, and levels of 6.5 percent and above frank diabetes.

The test results can vary and be somewhat less accurate if one has high turnover of red blood cells, which is hereditary, if one has experienced recent blood loss such as through surgery, if one has chronic liver or kidney disease, or if one takes high dose vitamin C.