Explanation

The criteria for eligible participants in a trial are important, partly because they should reflect the aim of the trial, and partly because they affect recruitment, attrition,(206-212) outcome event rates,(213, 214) and generalisability.(215)

Typical selection criteria relate to the nature and stage of the condition or disease being studied, to the exclusion of persons thought to be particularly vulnerable to harm from study participation, and to issues required to ensure that the study satisfies legal and ethical norms.

Reviews of two samples of 108 and 292 trial protocols from 2016 found that patient eligibility criteria were reported in 100% of protocols.(9, 10) The importance of transparent documentation is highlighted by evidence that the eligibility criteria listed in published trials often differ from those specified in the trial protocol.(216, 217) For example, a systematic review of articles comparing protocols and published reports (the vast majority being clinical trials or systematic reviews) found discrepancies regarding eligibility criteria in 25% to 57% of studies.(64)

Certain eligibility criteria may warrant explicit justification in the protocol, particularly when they limit the trial sample to a narrow subset of the population.(215, 218) The appropriateness of restrictive participant selection depends on the trial objectives.(219) When trial participants differ substantially from the relevant clinical population, the trial results may not easily be extrapolated to routine clinical settings.(216, 220)

Summary of key elements to address

● Specific inclusion and exclusion criteria defining the trial population to be randomised