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Clinical trials are a way of finding out which treatments work best for patients. In most trials one group of patients receives the new treatment and the other group does not.

For trials to work, enough people need to agree to take part in the trial (enrolment of participants) and then stay in the trial until it has finished (retention of participants). In reality, both are often big challenges for people who run trials.

Poor enrolment and retention of participants often increase the time and cost of trials, can render trial results less useful and sometimes cause trials to fail completely. Involving patients, carers and the public in designing and running trials may increase the chances of successful enrolment and retention of participants, but it is unclear how often this leads to such improvements, or by how much. It is also unclear how any benefits might be influenced by the type of patient and public involvement (PPI) and the type of trial.

To try and answer these questions, a research team led by Dr Joanna Crocker at the University of Oxford carried out a systematic review, which is a type of research that aims to bring together the results of all studies addressing a particular research question that have been carried out around the world. We searched for all published studies which measured the impact of some sort of PPI (which we call a ‘PPI intervention’) on the enrolment or retention of participants in trials. PPI interventions could include additional non-PPI activities or materials which we could not separate from the PPI activities (for example, the involvement of other stakeholders in an advisory group).

We found 26 studies, most of which took place in North America and the UK, and most of which looked at the impact of PPI interventions on enrolment rather than retention. Patients and members of the public were involved at various different stages of the trial process: designing recruitment and retention strategies, developing materials for patients (such as information leaflets about the trial) and/or direct recruitment or retention of participants. We did not find any studies which looked at PPI in choosing the research question or designing the trial itself.

When we brought together the results of these studies, we found that on average, PPI interventions in clinical trials improved enrolment, especially when the involved people had personal experience of the health condition being studied. However, PPI interventions did not always lead to improved enrolment, so we need to better understand when and how they work. We are uncertain about the effects of PPI interventions on retention because too few studies looked at this.

Further research would help to find out:

  1. which types of PPI work best in different situations;
  2. whether PPI reduces the cost of enrolling and retaining participants in trials;
  3. the effects of PPI in earlier stages of trial design;
  4. the effects of PPI interventions specifically aimed at improving retention.  

Our project has produced some documents that you may find interesting to read.

The Patient and public involvement Intervention to enhance Recruitment and Retention in Surgical Trials (PIRRIST) project recently launched its PPI guidance for Chief Investigators of surgical trials.

This guidance is based on the findings of the four-stage PIRRIST project, feedback from surgical trial staff and PPI contributors at dissemination events across the UK, and consultation and user testing with a wide range of potential users.

Although it was developed for UK-based surgical trials, it is applicable to any type of UK-based clinical trial.

The guidance is free and available to download from the project website:


Impact of patient and public involvement on enrolment and retention in clinical trials: systematic review and meta-analysis
Joanna C Crocker, Ignacio Ricci-Cabello, Adwoa Parker, Jennifer A Hirst, Alan Chant, Sophie Petit-Zeman, David Evans, Sian Rees.
BMJ 2018;363:k4738