Introduction
Mental disorders contribute to a significant global disease burden worldwide. Due to their high prevalence, chronic nature, and association with some physical disorders, mental disorders are associated with high disability and irreparable complications for the individual, family, and society [1]. However, it is estimated that only about 2% of people with mental disorders worldwide receive treatment [7]. Given the high burden of mental disorders and the need to increase access to evidence-based treatments [23], a program aimed at implementing an evidence-based intervention delivered by non-specialists is needed. Recent developments in psychotherapeutic interventions in the world have focused on transdiagnostic approaches [10]. Transdiagnostic approaches provide an integrated approach enabling service providers to address multiple problems within a single treatment program. It employs a unique technique and teaches clinicians how to make decisions about adapting therapy to the patient needs [9]. Transdiagnostic approaches provide a flexible and evidence-based treatment that is more efficient and cost-effective [24]. Studies have shown the effects of transdiagnostic interventions for different types of mental disorders [9, 25]. This study aimed to designed and evaluate a transdiagnostic intervention using the common elements treatment approach (CETA) for common mental disorders in Iranian adults.

Methods
This is a qualitative/quantitative study that was conducted in Semnan province, Iran, in four main stages. In the first stage, interviews were conducted using the free listing technique with 20 community members and 10 key informants to explore how local adults understand their own situation and describe their mental problems. The interviews continued until data saturation, and the content was analyzed in Dedoose software Version 6.1.18. In the second stage, the Yale-Brown obsessive-compulsive scale (Y-BOCS), the Beck Depression Inventory, the Beck Anxiety Inventory, and a performance evaluation checklist were used for designing a screening tool for mental disorders. In the third phase, the CETA-based transdiagnostic treatment program for mental disorders was prepared, and three training workshops were held for capacity building in healthcare providers. In the fourth stage, the effectiveness of the designed treatment program was evaluated by conducting a randomized controlled clinical trial (RCT) in 12 mental health centers in Semnan city on 459 subjects randomized to treatment (n=228) and control (n=231) groups using a balanced block randomization method (Figure 1). The initial assessment was conducted by trained healthcare providers using the above-mentioned tools to assess the eligibility of participants (n=520). The treatment group was also assessed after eight weeks of intervention, while the control group was assessed after 4 months.



Results
In the first stage, it was found that depression, anxiety, and obsessive-compulsive disorder (OCD) were the most common mental health problems. In the second stage, the psychometric properties of an initial screening tool with 101 items were assessed. After assessment, 25 items were removed, and the tool was finalized with 76 items. Cronbach’s alpha values for the subscales of depression, anxiety, and OCD were 0.92, 0.95, and 0.88, respectively. The cut-off points for these subscales were 14, 17, and 6, respectively. In the third stage, the program content was prepared, and three training workshops for 30 healthcare providers were held (two workshops on implementing the intervention program and one workshop on how to screen). In the fourth stage, the intervention was conducted for eight weeks by healthcare providers in the treatment group under the supervision of the research team. The control group received routine treatment for 4 months (once every two weeks). The results showed that the CETA-based transdiagnostic intervention was significantly more effective than the routine treatment in reducing the scores of depression (P<0.001), anxiety (P<0.001), and OCD (P<0.05) (Table 1).



Conclusion
The preliminary positive clinical outcomes suggest that the CETA may be a feasible, acceptable, and potentially effective alternative for addressing mental health problems, especially in some parts of Iran that do not have access to psychologists. This transdiagnostic approach allows non-specialized healthcare providers to select from a range of evidence-based therapy elements to address a variety of common and comorbid mental health problems in a primary healthcare center. Future research should include more RCTs using the CETA-based treatment program and with a longer follow-up period.

Ethical Considerations

Compliance with ethical guidelines
All ethical considerations were considered in this study. All participants declare their written or verbal informed consent. They were assured of the confidentiality of their information and were free to leave the study at any time. The study protocol was approved by the Research Ethics Committee of the National Institute for Medical Research Development (NIMAD) (code: IR.NIMAD.REC.1395.047) and the Research Committee of the University of Social Welfare and Rehabilitation Sciences (Code: IR.USWR.REC.1395.113). 

Funding
This study was funded by the National Institute for Medical Research Development (NIMAD) (Grant No.: 943114).

Authors contributions
Conceptualization, validation, data analysis, investigation: All authors; Resources: Katayoun Falahat, Zohreh Mahmoudi; Writing initial draft, editing & review: Monir Baradaran Eftekhari, Masoumeh Dejman, and Katayoun Falahat; supervision and project management: Monir Baradaran Eftekhari, Masoumeh Dejman, and Ameneh Setareh Forouzan. All authors read and approved the final draft.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The authors would like to thank all of the participants in this project for their cooperation.