Red blood cells transport oxygen from the lungs to other parts of the body with the help of a protein called hemoglobin that is present in them.
Glucose in the blood links with (or glycates) hemoglobin found in the red blood cells. This forms the basis of the HbA1C or glycated hemoglobin blood test to determine the average blood sugar level over the prior 3 - 4 month period. An above normal level of glycated hemoglobin indicates that there was too much glucose in the blood. This blood test provides an estimate of the overall blood glucose level over a 120 day period as that is the life span of red blood cells.
Approximately 5% of hemoglobin is glycated in a person without diabetes while for a person who does not control blood sugar levels in a proper manner the level could rise to as high as 25%. The glycated hemoglobin test provides a good estimate to how well blood glucose levels are being managed though it is not possible to use it for determining large spikes in blood glucose levels that could be the result of a person having hyperglycemia and hypoglycemia as a result of the food consumed and the amount of medication taken.