Hemoglobin (Hb) is the compound in the red blood cells that transports oxygen. One of the types of hemoglobin (Hb) is called HbA; HbA1c is a specific subtype of HbA. Glucose binds slowly to Hb and produces glycosylated Hb. There are several types of glycosylated hemoglobin measures (including total glycosylated Hb and HbA1), but HbA1c, which is formed when HbA is glycosylated, is now considered the best and standard measure.
The higher the blood sugar, the faster HbA1c will be formed, resulting in higher HbA1c levels. Red blood cells circulate 60-120 days, and the HbA1c level is in part affected by blood sugar levels over a three-month period. However, it is heavily weighted to levels over the past 45-60 days.