ObjectiveProspective registration has been widely implemented and accepted as a best practice in clinical research, but retrospective registration is still commonly found. We assessed to what extent retrospective registration is reported transparently in journal publications and investigated factors associated with transparent reporting.DesignWe used a dataset of trials registered in ClinicalTrials.gov or Deutsches Register Klinischer Studien, with a German University Medical Center as the lead centre, completed in 2009–2017, and with a corresponding peer-reviewed results publication. We extracted all registration statements from results publications of retrospectively registered trials and assessed whether they mention or justify the retrospective registration. We analysed associations of retrospective registration and reporting thereof with registration number reporting, International Committee of Medical Journal Editors (ICMJE) membership/-following and industry sponsorship using χ2or Fisher exact test.ResultsIn the 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 an explanation for the retrospective registration in the full text. Registration numbers were significantly underreported in abstracts of retrospectively registered trials compared with prospectively registered trials. Publications in ICMJE member journals did not have statistically significantly higher rates of both prospective registration and disclosure of retrospective registration, and publications in journals claiming to follow ICMJE recommendations showed statistically significantly lower rates compared with non-ICMJE-following journals. Industry sponsorship of trials was significantly associated with higher rates of prospective registration, but not with transparent registration reporting.ConclusionsContrary to ICMJE guidance, retrospective registration is disclosed and explained only in a small number of retrospectively registered studies. Disclosure of the retrospective nature of the registration would require a brief statement in the manuscript and could be easily implemented by journals.

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.

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