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BMJ. 2018 Nov 28;363:k4738. doi: 10.1136/bmj.k4738.

Impact of patient and public involvement on enrolment and retention in clinical trials: systematic review and meta-analysis.

Author information

Health Experiences Research Group, Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK
National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC), John Radcliffe Hospital, Oxford, UK.
Balearic Islands Health Research Institute (IdISBa), Palma de Mallorca, Spain.
Primary Care Research Unit of Mallorca, Balearic Islands Health Service, Palma de Mallorca, Spain.
Ciber de Epidemiologia y Salud Publica (CIBERESP), Madrid, Spain.
York Trials Unit, Department of Health Sciences, University of York, York, UK.
Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford, UK.
University of the West of England, Bristol, UK.
Oxford Academic Health Science Network, Oxford, UK.



To investigate the impact of patient and public involvement (PPI) on rates of enrolment and retention in clinical trials and explore how this varies with the context and nature of PPI.


Systematic review and meta-analysis.


Ten electronic databases, including Medline, INVOLVE Evidence Library, and clinical trial registries.


Experimental and observational studies quantitatively evaluating the impact of a PPI intervention, compared with no intervention or non-PPI intervention(s), on participant enrolment and/or retention rates in a clinical trial or trials. PPI interventions could include additional non-PPI components inseparable from the PPI (for example, other stakeholder involvement).


Two independent reviewers extracted data on enrolment and retention rates, as well as on the context and characteristics of PPI intervention, and assessed risk of bias. Random effects meta-analyses were used to determine the average effect of PPI interventions on enrolment and retention in clinical trials: main analysis including randomised studies only, secondary analysis adding non-randomised studies, and several exploratory subgroup and sensitivity analyses.


26 studies were included in the review; 19 were eligible for enrolment meta-analysis and five for retention meta-analysis. Various PPI interventions were identified with different degrees of involvement, different numbers and types of people involved, and input at different stages of the trial process. On average, PPI interventions modestly but significantly increased the odds of participant enrolment in the main analysis (odds ratio 1.16, 95% confidence interval and prediction interval 1.01 to 1.34). Non-PPI components of interventions may have contributed to this effect. In exploratory subgroup analyses, the involvement of people with lived experience of the condition under study was significantly associated with improved enrolment (odds ratio 3.14 v 1.07; P=0.02). The findings for retention were inconclusive owing to the paucity of eligible studies (odds ratio 1.16, 95% confidence interval 0.33 to 4.14), for main analysis).


These findings add weight to the case for PPI in clinical trials by indicating that it is likely to improve enrolment of participants, especially if it includes people with lived experience of the health condition under study. Further research is needed to assess which types of PPI work best in particular contexts, the cost effectiveness of PPI, the impact of PPI at earlier stages of trial design, and the impact of PPI interventions specifically targeting retention.


PROSPERO CRD42016043808.


Conflict of interest statement

Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: no support from any organisation for the submitted work other than that described above; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

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