Trial registration and selective outcome reporting in 585 clinical trials investigating drugs for prevention of postoperative nausea and vomiting

Background: Selective outcome reporting in clinical trials introduces bias in the body of evidence distorting clinical decision making. Trial registration aims to prevent this bias and is suggested by the International Committee of Medical Journal Editors (ICMJE) since 2004. Methods: The 585 randomized controlled trials (RCTs) published between 1965 and 2017 that were included in a recently published Cochrane review on antiemetic drugs for prevention of postoperative nausea and vomiting were selected. In a retrospective study, we assessed trial registration and selective outcome reporting by comparing study publications with their registered protocols according to the ‘Cochrane Risk of bias’ assessment tool 1.0. Results: In the Cochrane review, the first study which referred to a registered trial protocol was published in 2004. Of all 585 trials included in the Cochrane review, 334 RCTs were published in 2004 or later, of which only 22% (75/334) were registered. Among the registered trials, 36% (27/75) were pro- and 64% (48/75) were retrospectively registered. 41% (11/27) of the prospectively registered trials were free of selective outcome reporting bias, 22% (6/27) were incompletely registered and assessed as unclear risk, and 37% (10/27) were assessed as high risk. Major outcome discrepancies between registered and published high risk trials were a change from the registered primary to a published secondary outcome (32%), a new primary outcome (26%), and different outcome assessment times (26%). Among trials with high risk of selective outcome reporting 80% favoured at least one statistically significant result. Registered trials were assessed more often as ‘overall low risk of bias’ compared to non-registered trials (64% vs 28%). Conclusions: In 2017, 13 years after the ICMJE declared prospective protocol registration a necessity for reliable clinical studies, the frequency and quality of trial registration in the field of PONV is very poor. Selective outcome reporting reduces trustworthiness in findings of clinical trials. Investigators and clinicians should be aware that only following a properly registered protocol and transparently reporting of predefined outcomes, regardless of the direction and significance of the result, will ultimately strengthen the body of evidence in the field of PONV research in the future