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BackgroundPatients with comorbid depression and personality disorders suffer from a heavy disease burden while tailored treatment options are limited, accounting for a high psychological and economic burden. Little is known about the effect of treatment dosage and type of psychotherapy for this specific co-morbid patient population, in terms of treatment-effect and cost-effectiveness. This study aims to compare treatment outcome of 25 versus 50 individual therapy sessions in a year. We expect the 50-session condition to be more effective in treating depression and maintaining the effect. Secondary objectives will be addressed in order to find therapy-specific and non-specific mechanisms of change.MethodsIn a mono-center pragmatic randomized controlled trial with a 2 × 2 factorial design, 200 patients with a depressive disorder and personality disorder(s) will be included. Patients will be recruited from a Dutch mental health care institute for personality disorders. They will be randomized over therapy dosage (25 vs 50 sessions in a year) and type of therapy (schema therapy vs short-term psychodynamic supportive psychotherapy). The primary clinical outcome measure will be depression severity and remission. Changes in personality functioning and quality of life will be investigated as secondary outcomes. A priori postulated effect moderators and mediators will be collected as well. All patients are assessed at baseline and at 1, 2, 3, 6, 9–12 months (end of therapy) and at follow up (6 and 12 months after end of treatment). Alongside the trial, an economic evaluation will be conducted. Costs will be collected from a societal perspective.DiscussionThis trial will be the first to compare two psychotherapy dosages in patients with both depression and personality disorders. Insight in the effect of treatment dosage for this patient group will contribute to both higher treatment effectiveness and lower costs. In addition, this study will contribute to the limited evidence base on treating patients with both depression and personality disorders. Understanding the processes that account for the therapeutic changes could help to gain insight in what works for whom.Trial registrationThis trial has been registered on July 20th 2016, Netherlands Trial Register, part of the Dutch Cochrane Centre (NTR5941).

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