Storage of human oocytes in the vapor phase of nitrogen

Objective To evaluate the effectiveness of long-term vapor-phase nitrogen storage of vitrified human oocytes as a strategy for preventing the risk of cross-contamination due to direct contact with the liquid nitrogen (LN). Design Prospective randomized study. Setting Private infertility center, IVI, Valencia. Patient(s) Oocyte donors (n = 44) and recipients (n = 46). Intervention(s) Vitrification by the Cryotop method. Storage of vitrified oocytes in a vapor-phase nitrogen storage freezer and a traditional LN storage tank. Donation of the surviving oocytes and evaluation of fertilization, embryo development, and clinical results. Main Outcome Measure(s) Survival, fertilization, and cleavage rates. Embryo quality and clinical outcome. Result(s) Survival was 95.3% (vapor-phase nitrogen) and 94.5% (LN). Fertilization rates (73.1% and 71.7%) or cleavage on day 2 (95.6% and 94.7%), day 3 (84.5% and 79.9%), and blastocyst formation (54.7% and 53.9%) were similar between vapor-phase nitrogen and LN. Implantation, clinical, and ongoing pregnancy rates were similar for vapor-phase nitrogen (40.5%, 58.1%, and 48.8%, respectively) and LN groups (33.7%, 53.3%, and 46.6%, respectively). Conclusion(s) The vapor-phase nitrogen system permits the storage of oocytes vitrified, maintaining their potential to develop into competent embryos in a similar manner as those stored in a traditional LN freezer. This approach represents a practical alternative that prevents cross-contamination during the storage of vitrified samples.