Where to prospectively register a systematic review

Prospective registration aims to reduce bias in the conduct and reporting of research and to increase transparency. In addition, prospective registration of systematic reviews is argued to help preventing unintended duplication, thereby reducing research waste. PROSPERO was launched in 2011 as the first prospective register for systematic reviews. While it has long been the only option to prospectively register systematic reviews, recently there have been new developments. Our aim was to identify and characterize current options to prospectively register a systematic review to assist review authors in choosing a suitable register.To identify systematic review registers, we independently performed internet searches in January 2021 using keywords related to systematic reviews and prospective registration. "Registration" was defined as the process of entering information about a planned systematic review into a database before starting the systematic review process. We collected data on the characteristics of the identified registries and contacted the responsible party of each register for verification of the data related to their registry.Overall, we identified five options to prospectively register a systematic review: PROSPERO, the Registry of Systematic Reviews/Meta-Analyses in Research Registry, and INPLASY, which are specific to systematic reviews, and the Open Science Framework Registries and protocols.io, which represent generic registers open to any study type. Detailed information on each register is presented in tables in the main text. Regarding the systematic-review-specific registries, authors have to trade-off between the costs of registration and the processing time of their registration record. All registers provide an option to search for systematic reviews already registered in the register. However, it is unclear how useful these search functions are.Authors can prospectively register their systematic review in five registries, which come with different characteristics and features. The research community should discuss fair and sustainable financing models for registers that are not operated by for-profit organizations.