I am a Medical Technologist, and I have always read that the lifespan of a red blood cell is a 120 days. I was surprised to see it listed as approximately 90 days. Can you explain the discrepancy?

You are correct. Red blood cells last 120 days. However, if you are talking about interpreting hemoglobin A1c, you must take into account that glucose attaches to amino acids differentially (more to the younger cells than the older ones), and so there is not an equal attachment to any isolated red blood cell on any given day. So, the time weighted average glucose is not equal to 120 days of the RBC life span, but more closely represents what the RBC sees in the previous 30-60 days. So, some folks think that getting A1c determinations more often than 120 days provides additional information. The DCCT obtained A1c values monthly.

SB