Animation showing a molecule of glycated haemoglobin, found within a red blood cell erythrocyte. Glycated haemoglobin is a form of the oxygen_carrying complex haemoglobin red molecule, which has reacted with a sugar molecule, usually glucose glowing yellow, in the blood. The reaction is irreversible, so glycated haemoglobin levels rise during the lifetime of a red blood cell. There is a direct correlation between blood sugar levels and the levels of glycated haemoglobin, so measuring the levels of glycated haemoglobin in the blood can reveal the blood sugar levels in the preceding weeks. As such, it is a useful diagnostic test for diabetics, to check how blood sugar management has been progressing.