Hemoglobin A1c is an unreliable measure of average glucose due to significant influence from red blood cell lifespan.

“I've largely discounted hemoglobin a1c in an absolute sense as a meaningful number I think it's you know it's directionally tolerable but mostly and I know that because now I've used CGM in so many patients with calibration and compared it to a1c and you realize that the a1c is really at the mercy of its most important assumption which is a red blood cell lives for 90 to 120 days anything that takes it outside of that range leads to an over or under estimation of the a1c and therefore an over or under estimation of the average glucose”