BACKGROUND: Trigger tools are widely used for detecting adverse events. Within the multicentre study Patient Safety in Paediatric Oncology, a trigger tool was created to address the unique needs of paediatric oncology. Although trigger tools are highly valued for detecting adverse events, concerns about their usability and reliability persist. Understanding the perspectives of medical record reviewers using these tools may provide valuable insights for improving their usability and reliability. This study aimed to explore the experiences of medical record reviewers involved in the development and use of a Paediatric Oncology Trigger Tool. METHODS: A descriptive qualitative case study was conducted to investigate the experiences of medical record reviewers participating in the development and use of the Paediatric Oncology Trigger Tool. Data were collected through a semi-structured focus group interview conducted via Zoom, involving six reviewers with varying levels of experience in paediatric oncology and trigger tool methodology. The interview was audio-recorded and transcribed verbatim. The written text was analysed in its entirety using reflexive thematic analysis. RESULTS: The analysis revealed an overarching theme of CONCLUSIONS: This study offers a comprehensive examination and clarity regarding the development and use of a patient safety instrument, a process marked by both challenges and facilitators from the perspective of medical record reviewers. The study underscores the need for resources, training and support during the review process to ensure the reliability and usefulness of the trigger tool.