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BACKGROUND: Our approach to advancing the treatment of psychosis is to focus on key single symptoms and develop interventions that target the mechanisms that maintain them. In our theoretical research we have found worry to be an important factor in the development and maintenance of persecutory delusions. Worry brings implausible ideas to mind, keeps them there, and makes the experience distressing. Therefore the aim of the trial is to test the clinical efficacy of a cognitive-behavioral intervention for worry for patients with persecutory delusions and determine how the worry treatment might reduce delusions. METHODS/DESIGN: An explanatory randomized controlled trial--called the Worry Intervention Trial (WIT)--with 150 patients with persecutory delusions will be carried out. Patients will be randomized to the worry intervention in addition to standard care or to standard care. Randomization will be carried out independently, assessments carried out single-blind, and therapy competence and adherence monitored. The study population will be individuals with persecutory delusions and worry in the context of a schizophrenia spectrum diagnosis. They will not have responded adequately to previous treatment. The intervention is a six-session cognitive-behavioral treatment provided over eight weeks. The control condition will be treatment as usual, which is typically antipsychotic medication and regular appointments. The principal hypotheses are that a worry intervention will reduce levels of worry and that it will also reduce the persecutory delusions. Assessments will be carried out at 0 weeks (baseline), 8 weeks (post treatment) and 24 weeks (follow-up). The statistical analysis strategy will follow the intention-to-treat principle and involve the use of linear mixed models to evaluate and estimate the relevant between- and within-subjects effects (allowing for the possibility of missing data). Both traditional regression and newer instrumental variables analyses will examine mediation. The trial is funded by the UK Medical Research Council (MRC)/NHS National Institute of Health Research (NIHR) Efficacy and Mechanism Evaluation (EME) Programme. DISCUSSION: This will be the first large randomized controlled trial specifically focused upon persecutory delusions. The project will produce a brief, easily administered intervention that can be readily used in mental health services. TRIAL REGISTRATION: Current Controlled Trials ISRCTN23197625.

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Antipsychotic Agents, Anxiety, Clinical Protocols, Cognitive Therapy, Delusions, England, Humans, Psychiatric Status Rating Scales, Research Design, Schizophrenia, Paranoid, Time Factors, Treatment Outcome