Workshop Report: Scoping for the Development of a Proposal for an OECD Guidance Document on Fish Vitellogenin Assessment.

This report summarises discussions of an international workshop in May 2024, aiming to scope the development needs of a proposal for an Organisation for Economic Cooperation and Development (OECD) Guidance Document on vitellogenin (VTG) analysis and assessment. VTG, an egg-yolk precursor protein in oviparous species, is an informative marker of possible chemical-induced endocrine activity (particularly estrogenicity). Vitellogenin measurement is included in several standardised Test Guidelines published by the OECD and United States Environmental Protection Agency for the assessment of endocrine activity of chemicals in fish (and one in amphibians). However, in vivo VTG data across and within fish species can be highly variable and influenced by both technical and biological factors, affecting the reliability and interpretation of results. This poses a challenge for regulatory decision making and can trigger potentially unnecessary vertebrate studies or prevent necessary further investigations. The workshop brought together 42 experts from different sectors and geographies, to discuss three key aspects regarding VTG assessment: 1) technical/laboratory factors and VTG quantification; 2) data handling and statistics, and 3) interpreting the biological relevance of VTG responses. The workshop participants discussed the development and needs of a Guidance Document to provide harmonised recommendations, best practices, quality assurance criteria, and minimum reporting standards for VTG assessment in regulatory studies across the different test guidelines. Several areas were identified where further work and discussion are needed to inform on the development of the Guidance Document, such as the use of historical control data, the identification of potential outliers, the presentation of data, and analysis of case study examples. Provision of such a Guidance Document will further support the use of VTG as a relevant marker for a key aspect of endocrine activity assessment for regulatory decision making. A formal project was subsequently accepted onto the OECD Test Guideline workplan in April 2025.