Registered trials address questions already answered with high-certainty evidence: a sample of current
Redundant trials exist despite previous findings of the same clinical question.
Existing high certainty evidence may not be considered when planning new trials.
Redundant trials should be avoided because of their ethical implications.
Objective: To identify clinical trials registered later than 2015, that study the effect of an intervention on a primary outcome whose ‘Certainty of Evidence’ (CoE) has already been rated ‘high’ in a Cochrane SR. Study Design and Setting: We searched the Cochrane Library for all SRs from 2015. We analyzed SRs of interventions and excluded withdrawn reviews or those with no Summary of Findings (SoF) table. We retrieved the GRADE CoE ratings of each SR’s primary outcomes in the SoF tables and identified those rated ‘high’. We searched the World Health Organization International Clinical Trials Registry Platform and ClinicalTrials to identify records of clinical studies that tackled those outcomes and were registered after the date of publication of the respective 2015 SR. Results: We selected 602 SRs. 81 contained a ‘high’ CoE rating on at least one primary outcome, totaling 152 primary outcomes rated ‘high’. We found 39 clinical trials registered for primary outcomes with evidence already rated as ‘high’ in a 2015 Cochrane SR. Conclusion: This study shows the existence of clinical trials registered to study primary outcomes whose CoE has already been rated ‘high’ in a Cochrane SR.