Clinical trial publications now linked for improved searchability


In a huge step forward for transparency and searchability, Linked Clinical Trials became a reality ( in the CrossMark system ( on 17 May 2016. CrossMark now collects the trial registry and trial registration number (TRN) of any clinical trial referenced in a paper. These metadata are used to link related publications: when viewing the CrossMark record for a paper on a clinical trial, you’ll see the records of every other paper linked to the same TRN. For the first time, Linked Clinical Trials allows the full history of a clinical trial, across journals and publishers, to be easily found and read.

Clinical trialists publish multiple pieces of work on the same trial, from protocols and statistical analysis plans to results papers, secondary analyses, and economic evaluations. As each piece is often published in a different journal and author lists change over the lifetime of the trial, finding a trial’s full history is challenging. TRNs offer a unique identifier for each trial. However, authors have been slow to follow the CONSORT requirement of listing TRNs in abstracts, making this an unreliable route for searching.

CrossMark’s Linked Clinical Trial system moves the responsibility of prominently displaying the TRN from the author to the publisher. Publishers already register each paper they publish with CrossRef ( when assigning a digital object identifier (DOI) ( Those also using the CrossMark system indicate whether any paper with a DOI is an update (correction, retraction, or withdrawal) of another. They can choose to upload additional optional metadata, like funding information. The CrossMark icon attached to these publications indicates whether the reader has accessed the most up-to-date version of the paper or whether an update exists. Now the CrossMark record will also include links to papers related to the same clinical trial.

For the Linked Clinical Trial system to truly make a difference in the way we view and search clinical trial information, every publisher needs to upload the new pieces of metadata for each of their records. At present, only BioMed Central, PLoS, and the NIHR Library have committed to the project, which is an optional part of the CrossMark metadata system. For now, you’ll only see papers published with these three publishers in the CrossMark Linked Clinical Trial record. Similarly, not all publishers use even the most basic version of CrossMark yet.

Clinical trialists now have a new factor to consider when choosing a venue for publication. Will your journal choice help or hinder the researcher or systematic reviewer following an evidence thread?

For those interested in the history of the Linked Clinical Trials project, read Iain Chalmers and Doug Altman’s 1999 paper on threaded publications ( and Daniel Shanahan (BioMed Central)’s posts on the CrossMark project ( and its launch.