I thought that A1C is a way to measure the average blood sugar a person has had over the past 3 months. But I just did an A1C test, and the form said that A1C measures an average over a 2 to 3 month time period. So my question is, what is the exact duration of time over which A1C measure an average?

I'd like as much numerical precision as possible.


This question cannot be answered. It is individual and based on, among many other things, red blood cell survival. To understand why this is so, you need to understand than A1C is more accurately called Hemoglobin A1c or glycated hemoglobin. It's a form of hemoglobin that is formed due to hemoglobin's exposure to plasma glucose.
It has been found that red blood cell (the cells that contain hemoglobin) survival has considerable variation, even in hematologically normal people.

Ultimately, however, it is more important to realize that HbA1c is a measure for glucose control over a few months, than to know how long exactly it "has been measuring for"

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