Outcome definitions in clinical trial registrations: a cross-sectional study

Importance: Defining outcomes completely in trial registrations limits opportunities for reporting bias. That is, an incomplete outcome definition might be compatible with multiple results, so investigators could conduct many analyses and calculate different effects that would all appear to be consistent with the planned outcome. Objective: To evaluate outcome definitions for registered randomized controlled trials (RCTs) to determine whether they were complete or incomplete. Design: A cross-sectional study of RCTs registered on ClinicalTrials.gov and published in International Committee of Medical Journal Editors (ICMJE) member journals from 1 January to 30 June, 2025. Setting: ClinicalTrials.gov and RCTs published in ICMJE member journals. Participants: Outcomes registered before or soon after participant enrollment began for eligible RCTs. Exposure: This study characterized outcome registration completeness without a formal exposure or intervention. Main Outcomes and Measures: The proportion of outcomes that were defined completely (i.e., included a specific measurement, specific metric, variable type, time point, and cutoff, if applicable). Results were explored by outcome priority (i.e., Primary, Secondary, Other outcomes), trial regulatory status for RCTs governed by the US Food and Drug Administration Amendments Act (i.e., “Applicable Clinical Trials” versus other RCTs), by funder (i.e., Industry, National Institutes of Health, Other governmental organizations, Other), and journal. Results: Among 210 RCTs including 2543 registered outcomes, only 6 RCTs (2.9%) defined all outcomes completely and 326 outcomes (12.8%) were defined completely. Most outcomes defined the specific measurement (72.7%) and metric (85.7%), and the cutoff where applicable (65.0%) but not variable type (22.4%). In exploratory analyses, outcome definitions were more complete for Primary outcomes compared with Secondary and Other outcomes (20.5% versus 12.2% and 9.2%, respectively). Outcomes in Applicable Clinical Trials were better defined than outcomes in other RCTs (15.5% versus 11.0%), and outcomes in RCTs sponsored by Industry or funded by the National Institutes of Health were better defined than outcomes in Other RCTs (17.0% and 19.6% versus 7.4%, respectively). There was no evidence of differences across journals. Conclusions and Relevance: Most RCTs published in ICMJE member journals did not define their outcomes completely in registrations. Complete outcome definitions in trial registrations could prevent reporting biases.