A systematic review of core outcome set development studies demonstrates difficulties in defining unique outcomes

Objectives: Core outcome set (COS) development often begins with a systematic review to identify outcomes. Reviews frequently show heterogeneity in numbers of outcomes reported across trials. Contributing to this is a lack of a uniform definition for an outcome. This study proposes a first working definition for a unique trial outcome to support reporting a quantitative assessment of outcome reporting heterogeneity (ORH). Study Design and Setting: Eligible COS literature (development papers, protocols, and reviews) were identified using the COMET database, Ovid MEDLINE, and PubMed. Outcome numbers, definitions, timing, and grouping methodology were examined. Results: One hundred and thirty two studies were included. 82 (88.1%) studies (excluding protocols) reported a total number of unique outcomes (median, 82; range, 12-5776; IQR, 261). Timing of assessment was reported in 32 (31.4%) studies. Methods to group similar outcomes were reported in 8 (7.8%) articles. No study defined how outcomes were agreed as different and how final numbers of unique outcomes were determined. It is proposed that a unique outcome requires original meaning and context. Thus ORH is suggested to be the reporting of multiple unique outcomes across trials related to one health care condition. Conclusion: This review identified inconsistencies in how authors define, extract, group, and count trial outcomes. Further work is needed to refine our proposed definitions to optimize COS development and allow a quantifiable measure of ORH. Crown Copyright (C) 2019 Published by Elsevier Inc.