Defining hemoglobin A1c. Hemoglobin itself is a protein that resides in red blood cells. Its function is to convey oxygen from the lungs to the constituent parts of the body and–conversely–to move carbon dioxide from the body’s organs to the lungs for expulsion. When there is a surplus of sugar, it will cloak the red blood cells for their living duration, i.e. approximately 120 days. This is why the A1c measurement is a better long-term evaluation of blood sugar levels. A1c refers to glycated hemoglobin.