| Summary: | Abstract Background Registration of clinical trials is critical for promoting transparency and integrity in medical research; however, trials must be registered in a prospective fashion to deter unaccounted protocol modifications or selection of alternate outcomes that may enhance favorability of reported findings. We assessed adherence to the International Committee of Medical Journal Editors’ (ICMJE) prospective registration policy and identified the frequency of registrations occurring after potential observation of primary outcome data among trials published in the highest-impact journals associated with US professional medical societies. Additionally, we examined whether trials that are unregistered or registered after potential observation of primary outcome data were more likely to report favorable findings. Methods We conducted a retrospective, cross-sectional analysis of the 50 most recently published clinical trials that reported primary results in each of the ten highest-impact US medical specialty society journals between 1 January 2010 and 31 December 2015. We used descriptive statistics to characterize the proportions of trials that were: registered; registered retrospectively; registered retrospectively potentially after initial ascertainment of primary outcomes; and reporting favorable findings, overall and stratified by journal and trial characteristics. Chi-squared analyses were performed to assess differences in registration by journal and trial characteristics. Results We reviewed 6869 original research reports published between 1 January 2010 and 31 December 2015 to identify a total of 486 trials across 472 publications. Of these 486 trials, 47 (10%) were unregistered. Among 439 registered trials, 340 (77%) were registered prospectively and 99 (23%) retrospectively. Sixty-seven (68%) of these 99 retrospectively registered trials, or 15% of all 439 registered trials, were registered after potential observation of primary outcome data ascertained among participants enrolled at inception. Industry-funded trials, those with enrollment sites in the US, as well as those assessing FDA-regulated interventions each had lower rates of retrospective registration. Unregistered trials were more likely to report favorable findings than were registered trials (89% vs. 64%; relative risk (RR) = 1.38, 95% confidence interval (CI) = 1.20–1.58; p = 0.004), irrespective of registration timing. Conclusions Adherence to the ICMJE’s prospective registration policy remains sub-standard, even in the highest-impact journals associated with US professional medical societies. These journals frequently published unregistered trials and trials registered after potential observation of primary outcome data.
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