P-449 Storage of vitrified oocytes up to 8 years does not affect pregnancy and live birth rates: analysis of 5,362 cycles
Authors: Torra M , Sorribes J , Rodríguez A , Vassena R 
Published: June 2022

Abstract

Study question
Does long-term storage of vitrified oocytes affect laboratory and reproductive outcomes after ICSI?

Summary answer
The time spent by vitrified oocytes in liquid nitrogen banks does not affect oocyte survival, fertilization rate nor reproductive outcomes up to live birth.

What is known already
Vitrification is the most efficient approach to oocyte cryopreservation and is commonly applied in oocyte donation programs. The survival rate can be &gt;90% and vitrified-warmed oocytes provide comparable reproductive results to fresh oocytes. However, even with modern cryobanking technology, there are potential temperature fluctuations caused by access to cryopreserved material, transportation, stocktaking events, and regular maintenance. While some studies indicate that a multiyear storage of vitrified oocytes does not affect pregnancy rates, others found decreased reproductive outcomes after as little as 6 months, and further investigations in large cohorts are needed to confirm the safety of long-term oocyte storage.

Study design, size, duration
Retrospective cohort study of 5,362 cycles with vitrified oocytes from oocyte donors carried out between 2013 and 2021. All cycles included fresh embryo transfer (ET). Oocyte vitrification and warming were carried using the Cryotop® protocol (Kitazato). We analyzed the effect of storage time of vitrified oocytes in liquid nitrogen vapors on laboratory outcomes (oocyte survival, fertilization rate (FR), percentage of viable embryos) and reproductive outcomes (pregnancy -biochemical and clinical- and live birth (LB) rates).

Participants/materials, setting, methods
Eight categories of storage time were established: 0-0.25 years (reference group, ≤3 months), 0.25-0.5, 0.5-1, 1-1.5, 1.5-2, 2-3, 3-4, and &gt;4 years. The effect of oocyte storage time on laboratory and reproductive outcomes was analyzed by linear and logistic regression, respectively. These multivariate analyses were adjusted by multiple factors: sperm origin (partner vs donor), sperm parameters, number of oocytes inseminated, number of embryos transferred and day of ET (2-3 vs 5), among others.

Main results and the role of chance
Mean recipient age was 42.0±4.6, while oocyte donors age was 26.1±4.6. The mean number of thawed oocytes was 8.0±2.5, all surviving oocytes were inseminated by ICSI with partner (84.9%) or donor sperm (15.1%). The oocyte storage time ranged from 3 days to 8.2 years (mean: 0.7 ± 0.9 years). We did not find significant differences in oocyte storage time between positive and negative pregnancy and LB, nor between different survival rates (&lt;100 vs = 100%), FR (&lt;70 vs ≥ 70%) and percentages of viable embryos (&lt;50 vs ≥ 50%) at univariate analysis (p &gt; 0.05 in all cases). The mean oocyte survival rate was 90.2%±14.7% and, after adjusting for confounders, did not decrease with longer storage time (i.e. 88.9% for time &gt;4 years, p = 0.963). Similarly, the linear regression model did not show a significant effect of storage time on FR, which was close to 70% in all categories (range: 67.6-70.6, p &gt; 0.05), nor a decrease in the percentage of viable embryos. Finally, reproductive outcomes were similar across storage times (p &gt; 0.05 for all categories when compared to the reference 0-0.25); specifically, long-term oocyte storage (&gt;4 years) did not affect the chances of clinical pregnancy (OR: 0.657 [0.395-1.092], p = 0.194) and LB (OR: 0.666 [0.393-1.128], p = 0.231).

Limitations, reasons for caution
These results cannot be extended to cycles using the patient’s own oocytes or involving a severe male factor (testicular spermatozoa or criptozoospermia), which were excluded. Due to the retrospective nature of the study, some uncontrolled variables could affect the results.

Wider implications of the findings
This is the first study evaluating the effect of long-term oocyte storage on a large cohort of patients undergoing oocyte donation, thus partially discarding the effect of the female factor. Reassuringly, these results indicate that long-term storage of oocytes is a safe option for young patients and oocyte banks.

Trial registration number
Not applicable