P-138 Protein concentration of 5 mg/mL in recovery medium for thawed blastocysts is equivalent to 10 mg/mL in post-thaw survival and pregnancy rates
Authors: Patel J , VerMilyea L , Go K 
Published: June 2023

Abstract

Study question
Are comparable survival and pregnancy rates obtained when recovery media are supplemented with 5mg/mL protein versus customary 10mg/mL protein in Frozen Embryo Transfer cycles?

Summary answer
Recovery medium containing 5mg/mL protein compared to 10mg/mL yielded similar outcomes in blastocyst survival and pregnancy rates.

What is known already
Accepted practice has been to culture thawed blastocysts in medium with the same protein concentration as the vitrification media (20% protein) to minimize osmotic stress and optimize survival and implantation rates. The potential to acquire similarly positive outcomes with supplementation of only 5 mg protein/ml has been explored and demonstrated, challenging the requirement for high protein levels in recovery media.

Study design, size, duration
From July through August 2022, a prospective study was conducted in a university laboratory to compare the survival and implantation rates in Frozen Embryo Transfer cycles using vitrified blastocysts recovered in Continuous Single Culture–NX Complete (CSCM-NXC) (5mg/mL Human Serum Albumin (HSA)) or global HP[LW1] (8.8mg/mL HSA and 1.2mg/mL α-and ß-globulins). 50 blastocysts were warmed per condition. Comparative analyses included survival and implantation rates, beta HCG levels, and clinical pregnancy rates by fetal cardiac activity.

Participants/materials, setting, methods
To evaluate the effect of protein level in recovery medium for vitrified blastocysts, CSCM-NXC and global HP, with 5mg/mL and 10mg/mL total protein, respectively, were compared. Blastocysts were vitrified in Irvine Scientific Vit Kit – Freeze and were warmed in Irvine Scientific Vit Kit – Warm. Minimum recovery time was the same for all blastocysts (30 minutes), and is defined as the time from completion of the warming procedure to the time of embryo transfer.

Main results and the role of chance
In both study arms, one blastocyst was removed from the data analysis due to embryo transfer cancellation that was not related to embryo survival or suitability for transfer, resulting in N = 49 for each group. Chi-square analysis was performed to assess statistical significance of all compared parameters, and a p value &lt; 0.05 was considered significant. Average patient age (years) for CSCM-NXC was 36.1 and for global HP, 35.0. There was no difference between blastocysts warmed in CSCM-NXC and global HP in average recovery time, i.e., 3.8 and 3.9 hours, respectively, or survival rates, 98.4% and 98.1% respectively. Comparison of patients with blastocysts warmed in CSCM-NXC reflected a trend towards higher outcomes compared to those in global HP with initial positive beta HCG rates of 64.6% vs 56.3% and ongoing clinical pregnancy rates of 52.1% vs 45.8%, respectively. Implantation rates were 56.1% and 57.7% in CSCM-NXC and global HP, respectively. Incidence of monozygotic twinning of 0% in CSM-NXC and 3.3% in Global HP was observed.

Limitations, reasons for caution
As this is a challenge to accepted formulation of recovery media for vitrified blastocysts, specifically with respect to protein supplementation - a strategy for minimizing osmotic shock and potential damage to warming embryos - more expansive and robust studies must be undertaken to increase the statistical power of this study.

Wider implications of the findings
As frozen embryo transfers are utilized more routinely in assisted reproductive treatment, the clinical finding that a commercial recovery medium containing of 5mg/mL protein, the standard for routine embryo culture, can support the recovery of vitrified embryos and lead to successful pregnancy could increase the efficiency in the laboratory.

Trial registration number
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