P-273 Heavy oil and conventional mineral oil produce similar Day 5 embryological outcomes. A prospective, single centre case study on 546 sibling 2PN zygotes
Authors: Meiring E R , Avo Santos M , Poli M 
Published: June 2023

Abstract

Study question
Does heavy oil improve embryological outcomes compared to conventional mineral oil?

Summary answer
Heavy oil produces no improvement in embryological outcomes.

What is known already
Oil overlays are integral part of the culture system as they buffer gas and temperature exchanges in the culture media and reduce the risk of contamination and evaporation. However, slight variations in their composition, oxidative status, storage and utilisation can have a direct effect on culture system performance, eventually resulting in reduced embryo development and subsequent decrease in clinical outcomes. In an attempt to improve the performance of oil overlays, new formulations have recently reached the market. Nonetheless, their positive impact on embryological outcomes, as well as best practice in handling them, are yet to be broadly recognised.

Study design, size, duration
This is a prospective case-study conducted as part of routine consumable validation procedures undertaken when evaluating the use of new products in the IVF laboratory of a public hospital. A total of 546 sibling 2PN zygotes derived from 52 treatments were cultured to Day5 of development. All cases attending the clinic between March 2022 and January 2023 and leading to the collection of more than 10 oocytes were enrolled in this validation exercise.

Participants/materials, setting, methods
Sibling oocytes from IVF and ICSI treatments were cultured under the same conditions until fertilisation check (IVF) or after ICSI. Oocytes from each treatment were then equally split between conventional mineral oil (CONTROLS) and heavy oil (CASES) study groups. All embryos were individually cultured in 25µL drops of single step media at 37 °C, 5% CO2 and 5% O2. Embryological outcomes on Day3 and Day5 were compared across the study groups using Chi-square test.

Main results and the role of chance
CONTROLS and CASES showed similar embryological outcomes, including fertilisation (for ICSI only; 77.2% vs. 79.77%, respectively p = NS), good quality Day3 (69.5% vs. 75.6%, p = NS), good quality Day5 (30.2% vs. 24.0%, p = NS), blastulation (58.9% vs. 53.9%, p = NS) and utilisation rates (34.9% vs. 29.5%, p = NS). However, a sub-analysis was performed on the basis of oil handling/storage method. When heavy oil was pre-aliquoted and stored in 15mL tubes prior to usage, negative effects were measured on embryological parameters. In this subset of cases (n = 28 treatments; n = 304 2PN zygotes), although fertilisation and good quality Day3 rates were comparable (80.0% vs 81.6% and 69.1% vs. 73.0% in CONTROLS and CASES, respectively), CASES revealed a significantly lower rate of good quality Day5 blastocysts (Gardner grade ≥B for ICM and TE and expansion ≥grade3) compared to CONTROLS (32.2% vs. 21.7%, p = 0.04). Moreover, CASES showed significant lower embryo utilisation (36.8% vs. 25.7%, CONTROLS and CASES, respectively, p = 0.03) and a trend towards lower blastulation rate (61.2% vs. 50.7%, respectively, p = 0.06). Although the equal split of sibling oocytes between the two groups has minimised population bias, multiple variables may have influenced the performance of the tested product. We note that adequate storage/handling of products is required to avoid suboptimal results.

Limitations, reasons for caution
Although designed with sibling oocytes/embryos, this study is limited in population size. Moreover, only one product brand has been tested, limiting the universality of the conclusions about heavy oil performance in IVF culture systems.

Wider implications of the findings
Introducing a new product in the IVF laboratory should undergo a reliable validation exercise. Despite the commercial claims, heavy oil failed to improve our results. Untested variations may have detrimental effects on clinical performance and, if implemented without control, their effects may become impossible to identify at later stages.

Trial registration number
not applicable