trial registration

A trial's plan, including who will be enrolled and what will be measured, filed in a public register before the first participant joins, under a number that identifies it ever after.

The International Committee of Medical Journal Editors requires that a trial be entered in a public register at or before the time of first patient consent for enrollment, as a condition of being considered for publication. The register must be one of those in the World Health Organization's International Clinical Trials Registry Platform, or ClinicalTrials.gov, and in the ICMJE's words it must be accessible to the public at no charge, open to all prospective registrants, and managed by a not-for-profit organization. The trials this covers are defined broadly: any research project that prospectively assigns people to an intervention, with or without a comparison group, to study the relationship between a health-related intervention and a health outcome.

What gets filed is fixed rather than left to the sponsor. The WHO Trial Registration Data Set, version 1.3.1, sets 24 items that a full registration must carry, and the first of them is the name of the primary registry and the unique identifying number that registry assigns to the trial. That number is what a paper is quoting when it says a study is registered, an NCT number in the case of ClinicalTrials.gov. The same data set requires the primary outcome, which it says should be the outcome used in sample size calculations or the main outcome used to determine the effects, along with the secondary outcomes and the number of participants the trial plans to enrol.

The ICMJE states the purpose plainly: to prevent selective publication and selective reporting of research outcomes, to prevent unnecessary duplication of research effort, to help patients and the public know what trials are planned or ongoing, and to give ethics review boards considering new studies a view of similar work. What a registration number establishes is narrower than it can look. It shows that a record was filed before the trial began and can still be read, not that the trial ran as the record describes. The comparison that carries the weight is between the outcomes registered in advance and the outcomes the published paper reports.

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