Objective: Prospective registration of clinical research has been widely implemented and advocated for many reasons: to detect and mitigate publication bias, selective reporting, and undisclosed changes in the determination of primary and secondary outcomes. Prospective registration allows for public scrutiny of trials, facilitates the identification of gaps in research, and supports the coordination of efforts by preventing unnecessary duplication. Retrospective registration undermines many of these reasons but is commonly found. We provide a comprehensive analysis of retrospective registration and the reporting thereof in publications, as well as factors associated with these practices. Design: For this cross-sectional study, we used a validated dataset of trials registered on ClinicalTrials.gov or DRKS, with a German University Medical Center as the lead center, completed between 2009 and 2017, and with at least one peer-reviewed results publication. We extracted all registration statements from all results publications of retrospectively registered trials, including mentions and justifications of retrospective registration. We analyzed associations between key trial variables and different registration and reporting practices. Results: In our dataset of 1927 trials with a corresponding results publication, 956 (53.7%) were retrospectively registered. Of those, 2.2% (21) explicitly report the retrospective registration in the abstract and 3.5% (33) in the full text. In 2.1% (20) of publications, authors provide a justification/explanation for the retrospective registration in the full text. Registration numbers were significantly underreported in abstracts of retrospectively registered trials (p < 0.001). Publications in ICMJE member journals had higher rates of both prospective registration and disclosure of retrospective registration, although not statistically significant. Publications in journals claiming to follow ICMJE recommendations showed lower rates compared to non-ICMJE-following journals. Conclusions: In contrast to ICMJE guidance, retrospective registration is disclosed and explained only in a small number of retrospectively registered studies. Lack of disclosure might lead readers to wrongly interpret the registration as a quality criterion that, in the case of a retrospective registration, rather describes a concern. Disclosure of the retrospective nature of the registration would require 1-2 additional sentences in the manuscript and could be easily implemented by publishers.

The authors have declared no competing interest.

This work was partly funded under a grant from the Federal Ministry of Education and Research of Germany (Bundesministerium fuer Bildung und Forschung - BMBF) [01PW18012]. The funder was not involved in the study design, data collection, analysis, or interpretation, writing of the manuscript, or the decision to submit for publication.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.

Yes