Hi keano
Heamoglobin has the ability to "attach" oxygen. that's why it's the "respiratory molecule"
At the site where oxygen should go, it can attach some other things, among them carbon monoxide. That's the poison mechanism. Oxygen can detach, when it's delivered. Carbon monoxide does not.
Glucose can also attach irreversibly. The more glucosse is present in the blood, the more glycylates ( attaaches to) the Hb. Red blood cells have a lifespan of about 120 days, so The HbA1c is the percentage of haemoglobin which is glycylated(SP?).modified by how many cells have been renewed in the period since the last test. The more recent cells also bias the reading towards the most recent month or so.
There are tables for converting BG averages to probable HbA1cs
I tend to have a higher Hb A1c than expected.