In this study, the effectiveness of a treatment protocol for a personalized approach to behavioral problems, based on the core needs of patients based on their specific personality traits will be investigated. Patients will be recruited from clinical departments of (top) specialist mental health care institutions. Both adult (18-64 years old) and older adults (≥ 65 years) patients with SMI will be included in this study. Primary outcome measures are the Neuropsychiatric Inventory-Questionnaire (NPI-Q) and Brief Symptom Inventory (BSI). Secondary outcome measures are the burden of care for the team and the patient's quality of life, measured with the NPI-Q and Mental Health Quality of life (MHQoL-7D), respectively. Furthermore, predictive factors are mapped for the primary and secondary outcome measures. The gathered NPI-Q data will then be used for a validation study, where the construct validity, internal consistency and inter-rater reliability of the NPI-Q in an SMI population will be assessed.

Individual or group psychotherapy for SMI is not always possible because several patients are unwilling or unable to accept such treatment. Mediation therapy is an alternative, in which interventions are used via a team of professionals to bring about behavioral change in the patient. Unfortunately, there are only a few guidelines and protocols available in the field of mediation therapy. Therefore, the aim of this study will be the development of CoMBI-SMI for the SMI-population. CoMBI-SMI is a combination of cognitive behavioral therapy based upon Beck's cognitive model of personality disorders and nursing interventions from the Nursing Interventions Classification. Triggers from the environment may create a behavioral problem that is maintained by its consequences. The behavioral problems are first identified by the team. Based on this analysis, a core need is selected from the CoMBI-SMI protocol and the associated nursing interventions are selected. This is described in a CoMBI-plan and evaluated cyclically.

Healthcare providers of the departments will be trained in CoMBI-SMI. The training will consist of an online theoretical part about the model followed by a Meet-The-Expert meeting (MTE), where healthcare providers can ask questions about the theoretical part and where CoMBI-SMI will be practiced on the basis of case studies. Quality criteria are linked to the online training, wherein the theoretical part is tested by means of case studies.

According to a stepped wedge design cross-overtime from condition A to CoMBI-SMI varies across participating teams. Condition A is treatment as usual (TAU) with measurements each 4 weeks and the effect is measured by the NPI-Q, BSI and the MHQoL-7D. To properly identify the core needs for the CoMBI-SMI approach, the personality questionnaires Personality Inventory for Diagnostic and Statistical Manual of Mental Disorders edition 5 (PID-5-Brief form+Modified) and the Level of Personality Functioning-scale brief 2.0 (LPFS-Brief form 2.0) will be used. Then, the CoMBI training takes place and CoMBI-SMI is performed with also measurements each 4 weeks and the effect is measured with the same outcome measures.