HbA1c is not calculated from a single blood sugar reading but is measured directly in a
laboratory by determining the percentage of hemoglobin in red blood cells that has glucose
attached to it.

Because red blood cells live for about 120 days, the result reflects your average blood
sugar over the previous 2 to 3 months, with greater weight given to the most recent few
weeks.

Full story:

https://www.quora.com/How-is-HB1AC-calculated

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