Sample preparation now (2025) and then: Lessons from 13 years of nanomaterial regulatory safety testing

The Organisation for Economic Co-operation and Development (OECD) has adapted or newly developed selected OECD test guidelines (TGs) and guidance documents (GDs) to address regulatory testing of nanomaterials. The "Guidance on Sample Preparation and Dosimetry for the Safety Testing of Manufactured Nanomaterials" (GSPD) is a cornerstone document in this context. It presents the general approach to sample preparation, whereas TGs and GDs often include information for the specific test. The GSPD addresses sample preparation of nanomaterials for testing physical and chemical properties, mammalian and environmental toxicity, and environmental fate. The third edition of the GSPD, published in 2025, integrates the substantial progress over the last decade in addressing issues related to sample preparation and dosimetry, i.e. the lessons learnt. The GSPD captures the progress in sample preparation protocols, noting that an overarching protocol is not available, and the deeper understanding of dosimetry and effective dose. Dose estimation is now underpinned by in silico tools. Across testing, medium composition is a key determinant of nanomaterial behaviour and should be systematically addressed. The sample preparation for physical and chemical properties includes one new endpoint, hydrophobicity. The GSPD reflects the growing consensus on the key practical issues for preparing nanomaterial samples in media for environmental studies. The information on sample preparation for in vitro testing is completely new, and the information on mammalian toxicity sample preparation reflects the leap in knowledge. The glossary has been significantly expanded. This paper provides an overview of the new elements in the updated GSPD.