Methods and Findings: Outcomes identified from systematic reviews and patient interviews informed a questionnaire survey. Patients and health professionals were surveyed three times and asked to rate the importance of each item, on a 1-9 scale. Delphi methods provided anonymised feedback to participants. Items not meeting predefined criteria were discarded between rounds. Remaining items were discussed at consensus meetings, held separately with patients and professionals, where the COS was agreed. Data sources identified 2990 outcomes which were used to develop a 130-item questionnaire. Round 1 response rates were moderate, but subsequently improved to above 75% for other rounds. After rounds 2 and 3, 81 and 14 items were discarded, respectively, leaving 35 items for discussion at consensus meetings. The final COS included nine items: ‘weight’, ‘diabetes status’, ‘cardiovascular risk’, ‘overall quality of life’, ‘mortality’, ‘technical complications of the specific operation’, ‘any re-operation/re-intervention’, ‘dysphagia/regurgitation’, and ‘micronutrient status’. The main limitation of this study was that it was based in the UK only.
Conclusion: The COS is recommended to be used as a minimum in all trials of bariatric and metabolic surgery. Adoption of the COS will improve data synthesis and the value of research data. Future work will establish methods for the measurement of the outcomes in the COS.