Embryology outcomes after oocyte vitrification with super-cooled slush nitrogen are similar to outcomes with conventional liquid nitrogen: a randomized controlled trial

Despite a more rapid rate of cooling, oocyte vitrification using a slush nitrogen protocol appears to result in similar embryology outcomes compared with oocyte vitrification using conventional liquid nitrogen.

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VOLUME 117, ISSUE 1, P106-114


Brent M. Hanson, M.D., Julia G. Kim, M.D., M.P.H., Sandra I. Suarez, B.A., Brian K. Ackerman, M.B.S., Cynthia E. Comito, M.T., Rosanna Pangasnan, B.A., Emre Seli, M.D., Kathleen H. Hong, M.D., Richard T. Scott Jr., M.D.



To determine whether the use of slush nitrogen (SN), a super-cooled form of nitrogen with a temperature from −207 to −210 °C, can improve oocyte survival after vitrification and warming compared with conventional liquid nitrogen (LN).


Randomized controlled trial.


Academic-affiliated private practice.


A total of 556 metaphase II oocytes from 32 oocyte donor cycles were included.


Donor oocytes were block randomized to undergo vitrification with either SN or LN. Vitrification was followed by warming, fertilization with donor sperm, embryo culture to the blastocyst stage, and preimplantation genetic testing for aneuploidy via trophectoderm biopsy with targeted next-generation sequencing.

Main Outcome Measure(s)

The primary outcome was oocyte survival after vitrification and warming. Secondary outcomes included rates of fertilization, usable blastocyst formation, and whole chromosome aneuploidy.


Half of the metaphase II oocytes (n = 278) were randomized to undergo vitrification with SN, whereas the other half (n = 278) were randomized to undergo vitrification with LN. There were no statistically significant differences noted in oocyte survival rate (85.3% vs. 86.3%), fertilization rate (84.0% vs. 80.0%), rate of usable blastocyst formation (54.3% vs. 55.7%), or rate of whole chromosome aneuploidy (9.4% vs. 11.7%) between the SN and LN arms, respectively.


The implementation of an SN oocyte vitrification protocol resulted in similar embryology outcomes compared with LN. The use of SN did not lead to any demonstrable improvement in oocyte survival after vitrification and warming.

Clinical Trial Registration Number


Fertility and Sterility

Editorial Office, American Society for Reproductive Medicine

Fertility and Sterility® is an international journal for obstetricians, gynecologists, reproductive endocrinologists, urologists, basic scientists and others who treat and investigate problems of infertility and human reproductive disorders.