Predictors of recruitment and retention in randomised controlled trials of behavioural smoking cessation interventions: a systematic review and meta-regression analysis.

AIM: To investigate predictors of participant eligibility, recruitment, and retention in behavioural randomised controlled trials (RCTs) for smoking cessation. METHOD: Systematic review and pre-specified meta-regression analysis of behavioural RCTs for smoking cessation including adult (≥18 years old) smokers. The pre-specified predictors were identified through a literature review and experts' consultation and included participant, trial and intervention characteristics and recruitment and retention strategies. Measurements included eligibility rates (proportion of people eligible for the trials), recruitment rates, retention rates, and differential retention rates. RESULTS: 172 RCTs with 89,639 participants. Eligibility [median 57.6%; IQR 34.7-83.7], recruitment [median 66.4%; IQR 42.7-85.2] and retention rates [median 80.5%; IQR 42.7-85.2] varied considerably across studies. For eligibility rates, the recruitment strategy appeared not to be associated with eligibility rates. For recruitment rates, use of indirect recruitment strategies (e.g., public announcements) [OR 0.30, 95% CI 0.11-0.82] and self-help interventions [OR 0.14, 95% CI 0.03-0.67] were associated with lower recruitment rates. For retention rates, higher retention was seen if the sample had ongoing physical health condition/s [OR 1.66, 95% CI 1.04-2.63] whereas lower retention was seen amongst primarily female samples [OR 0.83, 95% CI 0.71-0.98] and those motivated to quit smoking [OR 0.74, 95% CI 0.55-0.99], when indirect recruitment methods were used [OR 0.60, 95% CI 0.38-0.97], and at longer follow-up assessments [OR 0.83, 95% CI 0.79-0.87]. For differential retention, higher retention in the intervention group occurred when the intervention but not comparator group received financial incentives for smoking cessation [OR 1.35, 95% CI: 1.02-1.77]. CONCLUSIONS: In randomised controlled trials of behavioural smoking cessation interventions, recruitment and retention rates appear to be higher for smoking cessation interventions that include a person-to-person rather than at-a-distance contact; male participants, smokers with chronic conditions, smokers not initially motivated to quit and shorter follow-up assessments seems to be associated with improved retention; financial incentive interventions improve retention in groups receiving them relative to comparison groups.