Intended for healthcare professionals


Untapping the hidden value of clinical trial registries

BMJ 2022; 377 doi: (Published 26 April 2022) Cite this as: BMJ 2022;377:o1058

Linked Research Methods & Reporting

Searching clinical trials registers: guide for systematic reviewers

  1. Kylie Hunter, senior evidence analyst
  1. NextGen Systematic Reviews Team, NHMRC Clinical Trials Centre, University of Sydney
  1. Twitter @KylieEHunter

Systematic reviews provide a summary of all relevant evidence on a research topic. Since around 50% of biomedical evidence is never published,1 researchers conducting systematic reviews will find that only searching through bibliographic databases is insufficient for their purpose. Systematic reviewers must also search for unpublished evidence to ensure the validity and reliability of their review. Clinical trial registries are a key resource for this process.

Despite this, searching clinical trial registers is often an afterthought for systematic reviewers due to both a poor understanding of the benefits and because searching registers can be a challenging and complex task. To date, there has been little guidance on how to search registers effectively. Consequently, they are often used suboptimally, inconsistently, or, worse still, not at all.

In the absence of clear and standardised guidance, researchers are left with many unanswered questions and decisions to make when searching registers. For instance, is it sufficient to search the World Health Organization’s International Clinical Trials Registry Platform, or should other registers be used? Can a Medline search string be adapted for a register search? Will Boolean operators work (e.g. ‘AND’, ‘OR’, ‘NOT’)? Should filters be used? Can records be exported to Excel/Endnote/other software? The list goes on.

As a systematic reviewer, associate convenor of the Cochrane Prospective Meta-Analysis Methods Group, and a longstanding project officer with the Australian New Zealand Clinical Trials Registry (ANZCTR), I have a lot of experience searching registers and an in-depth appreciation of how valuable they are as a resource. Given my background, I was increasingly asked by other researchers for my help with their register searches. I realised how wide reaching the uncertainty was around this area of research practice, and that the methods, tips, and tricks I had developed over the years would be useful to share in a paper. I was also keen to see the benefits of these untapped resources reaped, particularly since I had invested so many years helping to develop one (the ANZCTR).

I began by assembling an expert team and developing methods to form the guidance paper. I consulted widely with a variety of key stakeholders so that the guidance would be broadly applicable and comprehensive, and searched the literature to bring together any existing guidance. Overwhelmingly, stakeholders told me how much this guidance was needed to fill a knowledge gap.

Since putting the guidance paper together, I have already referred back to it many times both to ensure my own searches are standardised and to offer advice to researchers that have approached me. I hope that others will find it similarly helpful and that they may be encouraged to use registers for a broader range of purposes.

Moving forward, I envisage registries will increasingly be used as a valuable source of unpublished data, leading to more valid systematic reviews that are not prone to publication bias. I hope that trial registries will also play a vital part in facilitating collaboration, by enabling trialists to identify similar studies with which they may coordinate their research efforts to avoid unnecessary duplication and research waste. By providing an overview of trial activity, registries may also guide the need to update existing reviews and inform research prioritisation to ensure best allocation of resources. A broadening awareness of these valuable uses of trial registries may make them a researcher’s first stop for accessing emerging evidence in the timeliest manner.


  • Competing interests: I have read and understood the BMJ Group policy on declaration of interests and declare the following interests: I receive research funding support via two scholarships administered by the University of Sydney (Postgraduate Research Supplementary Scholarship in Methods Development (SC3504), and Research Training Program Stipend (SC3227)). I worked with the Australian New Zealand Clinical Trials Registry (ANZCTR) from 2009 to 2020 in a project officer/research role; I remain peripherally involved in ANZCTR-related research. I am associate convenor of the Cochrane Prospective Meta-analysis Methods Group.

  • Provenance and peer review: Not commissioned; not peer reviewed.