The impact of the cryopreservation period on the <scp>CD34</scp> + cell viability of cryopreserved cord blood units

BACKGROUND AND OBJECTIVES: Cord blood (CB) is an important source for haematopoietic stem cell transplantation because of its long-term cryopreservation. However, concerns remain regarding the potential decline in CB quality with prolonged cryopreservation. This study aimed to evaluate the post-thaw viability of CD45+ and CD34+ cells in CB units and to investigate how their viabilities change over prolonged cryopreservation. MATERIALS AND METHODS: We enrolled 726 CB units that underwent pre-transplantation examination from May 2006 through June 2023. Post-thaw viabilities of CD45+ and CD34+ cells were analysed in relation to cryopreservation period and across 4-year cryopreservation groups. RESULTS: The mean post-thaw viabilities were 60.5% for CD45+ cells and 91.2% for CD34+ cells. CD45+ cell viability showed a positive correlation with the cryopreservation period (r = 0.304, p < 0.001); however, no consistent trend was observed among the cryopreservation period groups. In contrast, CD34+ cell viability showed a significant negative correlation with the cryopreservation period (r = - 0.306, p <0.001) and decreased, with units cryopreserved for 1-4 years and 5-8 years having significantly higher viabilities than those cryopreserved for 9-12 years and 13-16 years. CONCLUSION: Our findings demonstrate that the post-thaw viabilities of CD45+ and CD34+ cells in cryopreserved CB units exceed internationally accepted standards, with CD34+ cell viability showing a tendency to decline with prolonged cryopreservation. These results underscore the importance of securing an adequate CD34+ cell dose at the time of cryopreservation to maintain the long-term quality and clinical utility of CB units.