@Limelight
Her is the answer.
The correct term for the general concept of "HbA1c" is " Glycated hemoglobin".
Hemoglobin in adults consists of Hemoglobin A (HbA), A2 and the traces of F ( fetal hemoglobin).
The HbA contains a small amounts of other hemoglobin's, so-called HbA1 or rapid hemoglobin's.
HbA1 consists of several fractions: HbA1a, HbA1b and HbA1c. HbA1c is the major fraction.
Glycated hemoglobin (HbA1c) is a connection between carbohydrates (glucose, sugar) and usually valine-amino acid in the hemoglobin molecule beta chain. There are two main steps in the chemical
reaction between the HbA and the carbohydrate's. In the first step is formed reversibly ( formed and transformed) a so-called aldimin-context ( "Schiff-base") , which is then slow (days) at a so-called "Amatory" rearrangement irreversible (no way back) is converted to a stable ketoamin. It is demonstrated that the conversion of 91% of the newly formed aldimin to stable ketoamin takes 22 days. The formation of Glycated hemoglobin is in progress whole lifetime off the erythrocyte's ,depending on the blood glucose level. High glucose level , the more Glycated hemoglobin will be formed. Glycated hemoglobin HbA1c, will be an expression of Glykemic control of glucose in the blood in the last 3 to 8 weeks lead sampling and are expressed usually as a fraction of total hemoglobin. The short-term increase and decrease in glucose levels will have little effect on the Glycated hemoglobin, as it will only affect the amount of aldimin that is removed before identification off the Glycated hemoglobin.
HBA1c is not an average estimate for BG it is an expression for the persons Glykemic control .
Look at this:
http://www.diabetes.co.uk/hba1c-to-blood-sugar-level-converter.html
https://en.wikipedia.org/wiki/Glycated_hemoglobin