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BACKGROUND: The role of GPs in recruiting or excluding participants critically underpins the feasibility, external validity and generalizability of primary care research. A better understanding of this role is needed. AIM: To investigate why GPs excluded potentially eligible participants from a large scale randomized controlled trial (RCT), to determine the proportion of patients excluded on account of trial eligibility compared with other reasons, and to explore the impact of such exclusions on the management and generalizability of RCTs. DESIGN AND SETTING: Secondary analysis of data from the CoBalT study, a multi-centre general-practice-based RCT investigating cognitive behavioural therapy as an adjunct to pharmacotherapy for treatment-resistant depression. METHOD: GPs were asked to screen patient lists generated from computerized record searches for trial eligibility and to provide narrative reasons for excluding patients. These reasons were coded independently by two researchers, with a third researcher resolving discrepancies. RESULTS: Thirty-one percent (4750/15,379) of patients were excluded at the GP screening stage, including 663 on patient lists that remained unscreened. Of the 4087 actively excluded patients, 67% were excluded on account of trial exclusion criteria, 20% for other criteria (half of which were comorbid conditions) and 13% without reason. CONCLUSION: Clear, comprehensive criteria, particularly with regards to comorbidities, are required for GPs to confidently screen patients for potential participation in research. Future studies should promote inclusivity and encourage GPs to adopt a liberal approach when screening patient lists. This would enhance the validity and generalizability of primary care research and encourage greater patient autonomy.

Original publication




Journal article


Fam Pract

Publication Date





364 - 370


Cognitive behavioural therapy, depression/mood disorder, medical comorbidity, mental health, practice-based research, primary care, randomized controlled trial, secondary data analysis., Adolescent, Adult, Aged, Cognitive Therapy, Comorbidity, Depressive Disorder, Female, General Practitioners, Guideline Adherence, Humans, Male, Middle Aged, Patient Selection, Practice Patterns, Physicians', Randomized Controlled Trials as Topic, Research Design, Young Adult