Predicting Participation in Psychiatric Randomized Controlled Trials: Insights From the STEP-BD

Objective: Differences between patients who do and do not participate in randomized controlled trials (RCTs) could diminish the generalizability of results. This study examined whether RCT participants differ from non-RCT participants who are recruited from the same patient and provider population. Methods: The Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD) was an observational study in which participants also could enroll in an RCT during exacerbations of acute depression. The odds that a patient was enrolled in the STEP-BD acute depression RCTs (pharmacotherapy or psychotherapy) were estimated by fitting logistic regression models to STEP-BD participants with acute bipolar depression (total N=2,222; RCT, N=413; observational arm, N=1,809). Predictor variables included demographic characteristics, clinical information (including severity scales and comorbidities), and study site. The extent to which site determined RCT participation was estimated by using the area under the receiver operating characteristic curve (AUC). Results: RCT participation was associated with having no insurance (odds ratio [OR]=1.58, 95% confidence interval [CI]=1.16-2.15), a Clinical Global Impression score indicating greater severity (severe versus mild: OR=1.52, CI=1.08-2.15), and site (predicted probability range 8% 231%). Site was the most significant predictor of RCT enrollment (model excluding site, AUC=.61, CI=.58-.64; full model, AUC=.70, CI=.67-.73). Conclusions: STEP-BD RCT participants differed from those in the observational arm in few clinical or demographic characteristics. Site was the strongest predictor of RCT participation. Future study is needed to understand site characteristics associated with RCT participation and whether these characteristics are associated with patient outcomes and to test these findings in usual-care settings.