Comparable reproductive outcomes in open versus closed oocyte vitrification systems: a prospective, paired study on the same genetic background and stimulation protocol

To compare in the same genetic background and stimulation regimen the laboratory and reproductive outcomes of the oocyte vitrification/warming open system Cryotop® (Kitazato) to the closed system Rapid-i® (Vitrolife). Prospective paired study carried out between July 2014 and November 2016. The study was approved by the local IRB and included a cohort of 83 oocyte donors providing a minimum of 12 mature oocytes (MII) at retrieval. In each case, 6 MII were vitrified using Cryotop® and assigned to one recipient and other 6 MII were vitrified using Rapid-i® and assigned to different recipient. ICSI was performed in all the cases, and embryos were transferred at D+2 or D+3. Oocyte survival rates, fertilization rate and embryo morphology, as well as reproductive outcomes (biochemical, clinical, ongoing pregnancy and live birth rates) between the two systems were analyzed by t-tests, Mann-Whitney test, and Chi2 tests as appropriate. Recipient characteristics were similar in both groups with regard to age (41.2±4.7) and BMI (23.8±4.0). Oocytes vitrified with Rapid-i had higher survival rates (93.7% vs 86.2 %, p<0.001). The fertilization rate was lower using Rapid-i (58.5% vs 69.2% p<0.001). The number of ICSI with total fertilization failure was 9 (7.2%) for Rapid-i and none for Cryotop (p=0.035). The percentage of cycles with frozen embryos was higher in Rapid-i (50.60% vs 45.26%, p<0.001). On average, the number of embryos transferred in the two groups was 1.8±0.4. Biochemical (50.7% vs 41.4%), clinical (45.1% vs 39.1%) and ongoing (45.7% vs 33.3%) pregnancy rates were not different (p>0.05) and were live birth rates (38.2% vs 32.3%, p>0.05). The participants of this prospective study are oocyte donors, young women in good reproductive health; care should be exerted in translating our results to other populations. Nevertheless, our results suggest that both closed and open oocyte vitrification systems offer comparable laboratory and reproductive outcomes up to live birth.