Vitrification Superior to Slow Freeze in Two Studies


WASHINGTON — Increased pregnancy rates and oocyte survival rates support the superiority of vitrification over the traditional slow-freeze cryopreservation technique, based on the results from several studies presented at the annual meeting of the American Society for Reproductive Medicine.

In vitrification, blastocysts briefly are immersed in a medium and then quickly frozen using liquid nitrogen. The debate over which method of cryopreservation yields the best clinical results continues, but emerging data from studies of vitrification appear to favor the fast-freezing method.

Early outcomes data from one study of 41 women showed that blastocyst vitrification yielded significantly more pregnancies than embryos that had been slow frozen.

In this study, Alicia L. Clifford and her colleagues at the Center for Reproductive Biology of Indiana in Indianapolis compared pregnancy rates from 30 patients who received embryos from slow-frozen eggs with 11 patients who received embryos from vitrified eggs.

Overall, 4 of 10 women in the vitrification group (40%) had positive fetal cardiac activity at 6 weeks, compared with 4 of 29 women (14%) in the slow-freeze group.

Fetal cardiac activity data were pending for one woman in each group at the time of the poster presentation.

The mean age was similar between the slow-freeze and vitrification groups (33 years vs. 31 years), and each group had an average of three embryos transferred.

In a second study of 84 oocytes from 42 women, oocyte survival rates were significantly higher when they were frozen using vitrification than slow frozen. Dr. Susan Sarajari and colleagues at the University of California, Los Angeles, and her colleagues thawed 44 oocytes that were slow frozen and 40 oocytes that were frozen using vitrification after 2 days in order to test their viability.

Overall, 17 of the 40 vitrified oocytes survived (42%), compared with 11 of the 44 slow-frozen oocytes (25%). The surviving oocytes from each group were selected for in vitro maturation (IVM), incubated, and then reassessed after 24 hours.

“The number and percent of oocytes that matured by IVM was also higher in the vitrification group,” the researchers noted, although the difference was not statistically significant (eight oocytes in the vitrification group vs. five in the slow-freeze group).

Cryopreservation of immature oocytes by any method has clinical application potential for women at risk of losing ovarian function for any reason and for women who simply wish to preserve their fertility, but larger outcome studies are needed to confirm the most effective techniques.

None of the authors of either study had any financial conflicts to disclose.

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