Universal post-warming dilution of vitrified embryos: impact of different vitrification/warming kits, warming volume...

Is the universal warming protocol efficient to warm vitrified human blastocysts when combining different brands of vitrification and warming kits, when reducing warming droplet volume, and when using a single rehydration step (universal single-step warming)?

Two retrospective longitudinal cohort studies (CS1 and CS2) were undertaken, including a total of 3071 vitrified/warmed blastocysts. In CS1, embryos were vitrified/warmed with various combinations of different kits, and the universal warming protocol was performed with fixed volumes of warming solutions: 4 ml of 1M non-permeating solute (NPS) at 37°C; and 300 µl of 0.5M NPS and 300 µl of washing solution at room temperature. In CS2, embryos were vitrified/warmed with various combinations of different kits. Warming was performed using single-step elution in 4 ml of 1M NPS at 37°C, or a small droplet (500 μl) of 1M NPS solution at 37°C, and subsequent washings at 37°C. The primary outcome measure was cryosurvival rate, and the secondary outcome measures were clinical pregnancy rate and embryo implantation rate (also live birth rate and neonatal outcome for CS1 alone).

The cryosurvival rate varied between 99.5% and 100%, and was comparable between the study groups. Clinical pregnancy and embryo implantation rates were comparable between all groups in both studies. Live birth rate and neonatal outcome were comparable between the groups in CS1.

It is possible to combine various kits for vitrification/warming, and universal warming can be efficient when reducing the volume of the first warming solution. This study demonstrated, for the first time, that it is possible to perform universal single-step warming with any brand of warming kit.