Elongation index and retention rate of “fresh” versus “old” RBCs. Mean (SD) retention rates (black bars) in microbeads of an RBC population measured weekly during a 6-week storage period in blood bank storage conditions. Data from the 3 first (< 21) and 3 last (> 21) weeks of storage period were pooled. Mean (SD) elongation index of a RBC population upstream (●) and downstream (▲) from the microbead layer. Data from the 3 first (< 21) and 3 last (> 21) weeks of the 6-week storage period were pooled. In our experimental conditions, the fresh RBC population elongation index values range from 0.610 to 0.640 (horizontal dotted lines).