Registered Trials, Protocols and Completed Studies: Why a Registration Is Not a Result

The short answer

A registration says that a study was planned or is under way. It does not say what the study found. A trial can be registered, recruiting, finished, stopped early or never reported, and the registration alone cannot tell you which outcome, if any, was good news. In the Research Library, a registered trial or systematic-review registration carries no result unless it is completed, and even then the registration record holds no result.

Protocol, registration and results are three different things

Every clinical trial has a protocol, or action plan, for conducting the trial. [3] For registration purposes, a clinical trial is any research study that prospectively assigns human participants or groups of humans to one or more health-related interventions to evaluate the effects on health outcomes. [2]

Why registries exist

Trial registers are an important source of information about completed, terminated and ongoing trials. [4] This matters because some completed or terminated studies are never published, and statistically significant results have been documented to be linked with publication. [4] Looking only at published studies can therefore leave out part of the picture.

PROSPERO is a register for systematic reviews. It was developed at the UK Centre for Reviews and Dissemination to reduce unplanned duplication of reviews and give transparency to the review process, with the aim of minimizing reporting bias. [5] A registered review is a plan for a review. It is not a finished review.

Reading trial status and phase

A status such as Completed, Recruiting or Terminated describes where a study is in its life, not what it found. Be careful even with that: two studies of trial register data found records that were frequently inaccurate or out of date, and trial completion status was commonly inaccurate. [4]

Phase gives a rough idea of the stage. Early studies look at safety and dosing, and in Phase 2 the studies are not large enough to show whether a drug will be beneficial. [6] In the library, a phase is recorded for only about two thirds of trials, so a missing phase is common.

One trial, many reports

A single study can produce several documents: a registry entry, a protocol paper, a results article, a follow-up report. Systematic reviewers treat the study, not the report, as the unit of interest, and multiple reports of the same study should be linked together. [7] It should never be assumed that there is only one report per study. [7] If you count records, count studies, not entries, or you can count one trial twice.

Common misunderstandings

• "A trial exists, so there is evidence." A registered trial is a description of planned or ongoing work.
• "Completed means it worked." Completed describes the status, not the finding.
• "Ongoing review means conclusions are settled." An ongoing systematic review has no conclusions yet.
• "Many registrations mean a well-researched topic." Many registrations may mean many plans.

What this means in the Research Library

The library holds 8,558 registered clinical trials. Of these, 6,881 have the status Completed, and the rest are recruiting, active, terminated, withdrawn, suspended, unknown or other. A phase is recorded for about two thirds of trials. It also holds 8,032 systematic-review registrations: 833 are recorded as completed and 6,877 as ongoing. Registered trials and registrations are descriptions of planned or ongoing work unless completed, and a registration carries no result in the library. Search can look up an NCT number or a CRD number directly.

Related reading: evidence types and study designs, how to read Research Library records, and how to use the Research Library.