How long is the “memory” of the A1C test?

Your A1C reading is not a simple average of all the blood glucose level ups-and-downs over the past 2 to 3 months. It is a weighted average. You never have a complete turnover of red blood cells all at once. Some are dying as others are just coming on board. There are always more young cells than old cells. About half are newer cells, ones formed within the last month. So your glucose levels in the last month count for about half of your A1C measure, and cells from the previous 2 to 3 months make up the other half of the measure.