Preregistration Works: Increased Reporting Quality, Internal Validity, and Protocol Adherence in Animal Studies

Background: Transparency is essential for reliable research but is often compromised, contributing to irreproducibility. Preregistration can improve transparency, limit selective reporting, and prevent post hoc hypothesis changes. Despite the clear benefits of preregistration in multiple fields, its effect on reporting and quality in experimental animal studies has not been studied.Methods: We collected preregistered protocols from Preclinicaltrials.eu and Animalstudyregistry.org, along with their corresponding manuscripts and matched it with control papers to evaluate reporting quality (ARRIVE essential 10), internal validity (SYRCLE’s risk of bias tool) and deviations between protocols and manuscripts. Results: Preregistered papers (n=22) scored higher on reporting quality than controls (mean=0.794 vs 0.637 author-matched (n=13) and 0.593 journal-matched (n=20); p<0.001). They also displayed lower risk of bias, particularly for selection, performance, and detection biases. Consistency between preregistered protocols and manuscripts represented 47.6% of the answers, with highest consistency for primary outcomes, study type, and hypotheses. Frequent inconsistencies, 28.2% of the answers, also occurred, especially regarding secondary outcome reporting, bias-reduction measures, and animal numbers. Conclusions: Preregistration stimulates high quality reporting, internal validity and consistent reporting in animal research. However, reporting across the board could be improved and undisclosed deviations to protocols remain common. Enhancing protocol templates with more detailed structure may improve reporting practices. Next steps would be to validate these findings, optimise preregistration formats, and guide journal policies supporting transparent reporting.