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A core outcome set is a minimum set of outcomes that should be measured and reported in all clinical trials undertaken in a specific health condition
Implementation of core outcomes sets would improve the quality of evidence based knowledge worldwide, making it more useful to decision makers and reducing waste
Resources and initiatives are available to help researchers identify, find, and appraise core outcome sets for their research studies
Minimum standards for developing a core outcome set have been agreed, and a more comprehensive handbook on these issues is available
Jamie Kirkham and Paula Williamson outline the importance of core outcome sets in medical research, and make suggestions on how to identify, select, and develop them as well as highlight some of the barriers to their use
A core outcome set helps researchers to measure a consistent set of outcomes in studies of a health condition.1 Examples of core outcomes sets for acute respiratory failure survivors and eczema are provided in table 1. When a core outcome set has been agreed, it is hoped that researchers will use it in all studies for the particular condition for which it was developed, alongside any other outcomes if they wish.
The need for core outcome sets
When researching the same health condition, many studies can measure and report different outcomes or do not consider their relevance to a wider range of stakeholders such as patients and health service users. These problems (a form of avoidable research waste2) make evidence synthesis methods (eg, meta-analysis) very difficult to facilitate, such that conclusive recommendations about the effectiveness of interventions using patient centred outcomes cannot be made. Without core outcome sets, some important outcomes might be missing in some studies, and therefore evidence synthesis cannot immediately include results for those outcomes from all studies, potentially excluding thousands of patients who have provided their time and consent to contribute towards research as best as possible. Use of a core outcome set improves the consistency and quality of information about which interventions work or not, thus helping people make better choices about healthcare options.
How to find and assess existing core outcome sets
The COMET (Core Outcome Measures in Effectiveness Trials) Initiative provides a publicly available, free-to-use, and searchable repository of core outcomes sets that is kept updated through regular systematic review updates, citation alerts, and notifications from core outcome set developers (https://comet-initiative.org/). The database contains nearly 500 published studies on core outcome sets (and nearly as many ongoing studies) that can be searched by disease name as well as other key elements.
Assessing the quality of a core outcome sets can be tricky because methods for developing one often vary by health condition. Perhaps a good set is one that is routinely used, but this cannot be easily determined at the point at which it is first developed. To help users assess core outcome sets, a set of standards (COS-STAD; core outcome set standards for development) has been established to determine whether the set has been developed using reasonable methods.3 These recommendations focus on three key domains: the scope, the stakeholders, and the consensus process (table 2). Even if all standards are not met or the scope of the core outcome set is not an exact match to the study question, users should consider whether the recommended core outcomes (at least in part) are useful. Most core outcome sets so far have been developed for research, but researchers of a recent review observed a noticeable increase in the development of these sets that could be applied to research and routine care settings. In 2019, 12% of published core outcome sets were developed to apply to both, compared with 60% of ongoing core outcome sets registered in the COMET database.4 To help improve the uptake of core outcome sets further, researchers are encouraged to search for a relevant set for their study, and to specify why one was not used or partially used if one was available.
Planning a development study
As part of good research practice, developers of core outcome sets should register their study with the COMET Initiative, which will then be added to the database as an ongoing study.5 If no core outcome set is registered on the database, there is a good chance that it does not exist or is in the planning stages. Researchers might want to consider developing a core outcome set as part of feasibility work for a research study, and requires both planning and funding. A good starting point for developing a core outcome set is the previously mentioned COS-STAD recommendations (table 2).3 As well as for appraisal purposes, these recommendations also encourage developers to achieve at least the minimum set of standards for development. The COMET Handbook provides more detailed guidance.6 Specific initiatives exist for some healthcare conditions, such as OMERACT (outcome measures in rheumatology) for rheumatology7 and the CHORD COUSIN Collaboration (C3) for dermatology.8
Encouraging uptake of core outcome sets
The long term benefits of core outcome sets will only be achieved if they are used. The lack of awareness that such sets exist and the methodological rigour behind the development process are two main reasons why core outcome sets are not used.9 With informative articles such as this Methods Primer, as well as many other dissemination activities by key initiatives for core outcome sets more generally, we hope to raise the awareness of core outcome sets. However, a systems approach is needed, such that developers of core outcome sets can promote their own set through more active dissemination strategies and through post-development engagement with their relevant channels or networks and stakeholders to ensure that their set is used.4
A good example of successful uptake of a core outcome set is in rheumatoid arthritis, where over 80% of drug trials in this field are now using the full core outcome set.10 Potential methodological shortcomings in the development of core outcome sets can be overcome by adhering to relevant guidance such as COS-STAD. For example, stakeholder involvement (especially patient involvement) are frequently cited as the reason for not using a core outcome set. This factor is included as a minimum standard under the COS-STAD guidance, and in the latest review of core outcome set studies, 85% of studies now involve public representatives compared with 17% in 2014.11 Implementation of a core outcome set requires the measurement instruments to measure the core outcomes. Selecting measurement tools is beyond the scope of this article, but a practical guide on how to select such tools for core outcomes is available.12
This article sets out the principles of why core outcome sets are needed and provides some information on how medical researchers can select and appraise existing core outcomes. It also provides some simple steps to consider regarding development. The COMET Handbook6 brings together a comprehensive discussion on current thinking and methodological research regarding the development, implementation, evaluation, and updating of core outcome sets.
Contributors JJK had the idea for this article, discussed it with PW, and wrote the initial draft. PW reviewed the draft and made revisions and additions. JJK and PW both approved the final version.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial, or not-for-profit sectors.
Competing interests We have read and understood the BMJ policy on declaration of interests and declare the following interests: JJK is the research methods lead for the CHORD COUSIN Collaboration (C3), and PW is a member of the COMET management group.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review Commissioned; externally peer reviewed.