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Single-Step Blastocyst Warming Validated, Saves Time Without Compromising Outcomes

Key findings

  • In 2021, Manns and colleagues described an "ultra-fast" blastocyst warming protocol that requires only one step before transfer to a culture medium
  • This study, the first comprehensive external validation of the single-step protocol (SS), found that when 200 research blastocysts were divided among SS and standard warming (SW), survival and re-expansion rates were similar in the two groups
  • After the SS protocol was adopted into routine practice at the Massachusetts General Hospital Fertility Center, the team evaluated the protocol on transfer outcomes with 752 blastocysts evenly divided between SS and SW
  • The two groups were similar in rates of implantation, biochemical pregnancy, clinical pregnancy, spontaneous abortion, and live birth and in birth weight; the multiple gestation live birth rate was slightly higher with SS (OR, 1.05; p=0.027)
  • As IVF volumes increase nationwide, the greater efficiency of SS may allow labs to perform more frozen embryo transfer procedures per day without expanding staff and could result in shorter time to pregnancy, as there may be fewer delays in cycle starts

In 2021, an abstract in Fertility and Sterility introduced an "ultra-fast" blastocyst warming protocol that requires only one step before transfer to a culture medium. When the authors randomly assigned 71 blastocysts to the new technique or standard warming, there were no differences in initial survival or re-expansion of embryos after 24 hours.

Massachusetts General Hospital researchers performed the first comprehensive external validation study of the streamlined protocol. Victoria S. Jiang, MD, a clinical fellow in the Mass General Fertility Center, Charles L. Bormann, PhD, director of the Vincent Embryology Laboratory, and colleagues report in the Journal of Assisted Reproduction and Genetics that compared with standard warming, the single-step technique provided similar outcomes for nearly all parameters studied.

Methods

The study was conducted in two phases at the Mass General Fertility Center. Both phases compared the two techniques:

Standard warming protocol (SW)

  1. Exposure to thaw solution (1 M sucrose, 20% dextran serum supplement [DSS], and gentamicin in an M-199 HEPES-buffered medium) at 37°C for 1 minute
  2. Transfer to dilution solution (0.5 M sucrose, 20% DSS, and gentamicin) for a 4-minute rinse at room temperature
  3. Two 4-minute rinses in wash solution (20% DSS and gentamicin) at room temperature
  4. The sequential transfer of each blastocyst through five 25-uL drops of Continuous Single Culture-NX Complete culture media overlaid with Ovoil, then incubation under the same conditions for 1–4 hours prior to embryo transfer

Single-step warming protocol (SS)

  1. Exposure to thaw solution as in SW
  2. Vigorous rinse through five 25-uL drops of the same culture media as in SW, then incubation under the same conditions for 1–4 hours prior to embryo transfer

Study Phase 1

Between April 2021 and April 2022, the researchers thawed 200 blastocysts donated for research and divided them equally between SW and SS:

  • Survival rate after initial warming—100% for all blastocysts in both protocols
  • Re-expansion at 2 hours—84% of blastocysts with SW and 82% with SS (p>0.99)
  • Re-expansion at 4 hours—94% and 94%

Neither the blastocyst stage nor biopsy status influenced the likelihood of thaw survival.

Study Phase 2

After confirming the safety and efficacy of SS, the embryology lab made a uniform change to using it for frozen embryo transfer. From April 2022 to December 2022, 376 vitrified blastocysts were warmed with SW and 376 with SS.

In both groups, the blastocyst survival rate was 100%. In adjusted analyses, there were no significant differences between groups in rates of implantation, biochemical pregnancy, clinical pregnancy, spontaneous abortion, or live birth or in birth weight. The SS group did show slightly higher odds of multiple gestation live birth, which persisted after adjusting for number of embryos transferred (OR, 1.05; p=0.027).

Optimizing Efficiency

In Mass General's experience, SS saves at least 15 minutes per cryothaw procedure. The capacity to increase daily productivity without sacrificing safety might allow laboratories to perform more frozen embryo transfer procedures per day. Ultimately, this may result in shorter time to pregnancy and improved patient satisfaction, as there will be fewer delays in cycle starts.

Increased operational efficiency also frees the embryologist for alternative tasks, such as fresh in vitro fertilization cycles, a welcome development as the demand for assisted reproduction increases nationally.

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