BackgroundClinicians, patients, and policy-makers rely on published evidence from clinical trials to help inform decision-making. A lack of complete and transparent reporting of the investigated trial outcomes limits reproducibility of results and knowledge synthesis efforts, and contributes to outcome switching and other reporting biases. Outcome-specific extensions for the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT-Outcomes) and Consolidated Standards of Reporting Trials (CONSORT-Outcomes) reporting guidelines are under development to facilitate harmonized reporting of outcomes in trial protocols and reports. The aim of this review was to identify and synthesize existing guidance for trial outcome reporting to inform extension development.MethodsWe searched for documents published in the last 10 years that provided guidance on trial outcome reporting using: an electronic bibliographic database search (MEDLINE and the Cochrane Methodology Register); a grey literature search; and solicitation of colleagues using a snowballing approach. Two reviewers completed title and abstract screening, full-text screening, and data charting after training. Extracted trial outcome reporting guidance was compared with candidate reporting items to support, refute, or refine the items and to assess the need for the development of additional items.ResultsIn total, 1758 trial outcome reporting recommendations were identified within 244 eligible documents. The majority of documents were published by academic journals (72%). Comparison of each recommendation with the initial list of 70 candidate items led to the development of an additional 62 items, producing 132 candidate items. The items encompassed outcome selection, definition, measurement, analysis, interpretation, and reporting of modifications between trial documents. The total number of documents supporting each candidate item ranged widely (median 5, range 0–84 documents per item), illustrating heterogeneity in the recommendations currently available for outcome reporting across a large and diverse sample of sources.ConclusionsOutcome reporting guidance for clinical trial protocols and reports lacks consistency and is spread across a large number of sources that may be challenging to access and implement in practice. Evidence and consensus-based guidance, currently in development (SPIRIT-Outcomes and CONSORT-Outcomes), may help authors adequately describe trial outcomes in protocols and reports transparently and completely to help reduce avoidable research waste.