What evidence syntheses reveal about PROSPERO, INPLASY, OSF, the Research Registry, and protocols.io: a meta-research study.

Background: This meta-research study examined how protocol registration information is reported in evidence syntheses published from 2020 to 2025 and registered in PROSPERO, INPLASY, OSF, the Research Registry, or protocols.io. Methods: We analyzed 4,750 studies covering various evidence synthesis types. Data were collected on registry use, reporting transparency, and accessibility features. Statistical comparisons included effect sizes with 95% confidence intervals. Results: PROSPERO remained the most widely used, with registrations from 70 countries. Among studies registered on non-PROSPERO platforms, more than 90% were found in INPLASY and OSF. Compared with PROSPERO, both registries demonstrated stronger reporting practices, with higher protocol status updates in INPLASY and more hyperlinks in OSF. However, a hyperlink did not always ensure public availability, as several OSF protocols required author authorization. Protocols in PROSPERO were associated with multiple publishers. In contrast, INPLASY protocols were more frequently linked to open-access journals, particularly those published by Frontiers and MDPI. Conclusion: Although PROSPERO remains the reference registry, INPLASY and OSF are playing an increasingly important role in promoting openness and accessibility. Researchers are encouraged to search multiple registries, especially PROSPERO, INPLASY, and OSF, before starting a new study to minimize the duplication of efforts. Systematic review registration: https://www.doi.org/10.37766/inplasy2025.6.0114, identifier INPLASY202560114.