A technical bias: Differences in cooling rates prevent ampoules from being a reliable index of stem cell cryopreservation in large volumes

Abstract

Ampoule tests are commonly used as an index of the cryopreservation efficiency of marrow stem cells in bags. We have studied the recovery of hematopoietic progenitor cells (CFU-GM, BFUe) in 52 ampoules and compared it to the recovery in 83 standard bags. Our data showed significantly deficient CFU-GM and BFUe recoveries (respectively 47 ± 31% and 31 ± 30%) in ampoules when compared to bags (respectively 72 ± 22% and 64 ± 19%; P < 0.001). Moreover, a good progenitor cell recovery (⩾50%) was observed in only 46% of frozen ampoules versus 100% observed in frozen bags ( P < 0.05). We were able to relate this nonoptimal recovery to an excessively rapid freezing rate of −9 °C/ min following the release of fusion heat which occurred in ampoules, while the freezing rate was constantly maintained at −2 °C/min in the corresponding bags. We therefore conclude that the cooling conditions have to be carefully controlled to ensure that the bags and ampoules are both cooled under the same conditions. Otherwise, ampoules would not be a realiable index of the true progenitor cells' cryopreservation efficiency in bags.