The OMER-BE study was a naturalistic, longitudinal, multicenter cohort study, in which 189 individuals with SUDs were followed up over a 6-month period in various residential and outpatient treatment modalities in the Dutch- and French-speaking regions of Belgium. Data collection for the study started in July 2022. Participants were recruited from 8 residential services (4 psychiatric treatment centers [PCs] and 4 therapeutic communities [TCs]) and 9 outpatient treatment services.

Specialized wards of PCs offer long-term (3 to 6 months) residential care that provides intensive medical and psychological support, addressing SUDs and in some cases co-occurring mental health disorders. Treatment consists of group counseling, psychoeducation, individual psychotherapy, and occupational activities.

TCs for addictions have a long history and were set up for individuals with SUDs, complementing traditional mental health care services that were traditionally not open to persons with drug problems [31]. In a TC, individuals with SUDs live together in a structured environment, typically for a period of 6 to 12 months, aiming for positive changes that lead to a drug-free life in society. The TC approach centers around the concept of “community as a method,” thus highlighting the influential role of peers and the power of mutual support in fostering recovery [32,33].

Outpatient treatment services provide more autonomy to service users and offer various nonresidential care options consisting of drug-free counseling interventions and harm-reduction approaches like opioid agonist therapy (OAT) and needle exchange programs. OAT refers to the use of opioid replacement medication such as methadone or buprenorphine to help manage withdrawal symptoms and reduce craving, often combined with some form of counseling and social support. Drug-free counseling focuses on psychosocial interventions, such as motivational interviewing and cognitive behavioral therapy, aimed at helping individuals develop coping strategies and support systems to become or remain abstinent.

The OMER-BE study aims to follow up a naturalistic cohort of service users with SUDs as they start a new treatment episode in a selected number of SUD treatment services, focusing on individuals with a primary alcohol or primary (illicit) drug problem. Eligibility criteria were (1) having a documented SUD (eg, a Diagnostic and Statistical Manual of Mental Disorders diagnosis of a SUD or previous treatment for a SUD), (2) being at least 18 years old, (3) being able to communicate in Dutch or French, and (4) having started treatment no longer than 21 days ago. First-time users, as well as returning service users, were considered eligible for this cohort study, as long as they started a new treatment episode during the recruitment period [34].

Upon treatment entry, service users were informed about the aims and design of the OMER-BE study through posters, leaflets, and staff members of the selected treatment facilities. During the initial meeting with the researcher, participants were informed extensively about the (follow-up) study and the implications of study participation and were asked for written informed consent to participate [34].

This study was granted ethical approval from the Medical Ethics Committee of the University Hospital Brussels on May 11, 2022 (UZ Brussel; BUN: 1432022000071). The participants were informed about the confidentiality of the data they provide, and written informed consent was acquired from all participants before being included in the study. Participants received a €10 (US $10.46) voucher for each completed assessment. All data are treated confidentially and reported anonymously. This study was conducted in accordance with the Declaration of Helsinki.

This paper presents baseline characteristics of the study cohort and comparative analyses between participants from different treatment modalities, as well as between participants who completed the 45-day follow-up and those who did not participate in the 45-day assessment. For categorical demographic and patient characteristics, significant differences between treatment modalities were assessed using either the chi-square test or Fisher exact test when the data did not meet the assumptions required for the chi-square test. For continuous variables, we used ANOVA to assess group differences. Welch 2-tailed t test was used when the assumption of equal variances was not met. Post hoc comparisons between groups were made using the Tukey test when variances were equal or the Games-Howell test when the assumption of equal variances was violated. Initial power analyses indicated that a minimum of 159 participants were needed to detect moderate effects (with α=0.05 and 80% power). Statistical analyses were conducted using SPSS Statistics (version 29; IBM Corp).

Future analyses will center on longitudinal changes in PROMs and PREMs, using repeated measures and mixed model analyses. The role of mediating and moderating variables (eg, socioeconomic status, comorbidity, and recovery strength) will be assessed, along with differences between treatment modalities. All statistical analyses will be conducted using R studio (Posit PBC) and SPSS Statistics (IBM Corp).

The use of PROMs and PREMs in SUD treatment services is limited to date, and no systematic monitoring system of patient-reported outcomes in Belgium is currently available [34]. Therefore, the OMER-BE study was set up to assess PROMs and PREMs systematically and to improve the quality of SUD services through the routine measurement and monitoring of patient-reported outcomes and experiences at regular times during and after treatment.

Comparisons across the three treatment modalities at baseline revealed several noteworthy differences and similarities. While no significant differences were observed in age, sex, living situation, or country of birth, significant variations in education levels, history of SUD treatment, engagement in OAT, and primary substance of problematic use were observed. Regarding co-occurring mental health disorders, no significant differences were found in PTSD, depression, anxiety, and stress scores between the treatment modalities. However, significant differences were noted in ADHD scores, with the TC group scoring significantly higher than the outpatient group. These differences can be attributed to several factors. TCs are often more suitable for individuals with more severe ADHD symptoms, as these environments provide the structured support and comprehensive care needed to manage both SUDs and comorbid conditions. These settings allow for intensive monitoring and support, which may be crucial for individuals struggling with ADHD symptoms, such as impulsivity [55].

Analysis of PROM scores showed no significant differences between treatment modalities in general health and quality of life. However, significant differences were found in recovery strengths, with participants in the outpatient group scoring lower overall than those in the other groups. These differences, particularly noted in the subdomains of “Substance Use,” “Self-care,” and “Outlook on Life,” suggest that the structure of the treatment environment plays a role in shaping recovery trajectories. For example, we observed that participants in residential treatment score significantly higher on the “Substance Use” subscale compared to those in outpatient treatment, suggesting lower levels of substance use among those in residential settings. This difference can be attributed to the structured and controlled environment of residential treatment settings, where strict measures are often in place to encourage abstinence and limit access to substances [56]. The supervision and supportive community in residential facilities may also play a crucial role in reducing substance use, which is less enforceable in outpatient settings where individuals have more autonomy and access to substances. The significant difference in the “Self-care” subscale, with residential participants scoring higher, may reflect the comprehensive care and support provided in residential settings. These environments typically offer structured daily routines and more intensive psychological and physical care [56]. In contrast, outpatient treatment often places a greater emphasis on self-management [57], which can be challenging for individuals with limited resources or support networks. Participants in residential treatment also scored significantly higher on the “Outlook on Life” subscale, which may be attributed to the supportive environment of residential treatment settings, where participants are provided with continuous care and peer support, all of which contribute to a more positive and optimistic outlook on life. Notably, the group difference in the “Relationship” subscale was close to significance. As participants were assessed in the first weeks of treatment, it is possible that participants in residential facilities did not have enough time to build meaningful relationships with other service users and providers.

In addition to differences between treatment modalities, comparisons between participants who completed the 45-day follow-up and those who did not participate in this assessment revealed several significant differences. Participants who completed the 45-day follow-up were significantly older, had a higher education level, were more likely to live alone, and were more likely to have a mother born in Belgium. Additionally, they scored higher on the “Material Resources” domain of the SURE, which includes questions about stable housing, a steady income, and effective financial management. These findings are in line with studies that suggest that factors such as lower education level, younger age, unemployment, and financial instability are associated with higher attrition in follow-up assessments [58,59]. Furthermore, some studies have found that factors such as low socioeconomic status are associated with limitations in people’s ability to use electronic devices [21,60,61], which may contribute to reduced participation in follow-up assessments. The higher completion rates among individuals living alone could indicate that they experience fewer external obligations that interfere with follow-up participation. The differences observed in maternal country of birth suggest that cultural or socioeconomic factors may also play a role in study retention, such as language barriers or differing levels of trust in research participation. Furthermore, alcohol was more frequently reported as the primary substance of use among completers and individuals with alcohol as primary substance of use had a lower attrition rate in comparison to the total sample (28% vs 36.6%). The higher prevalence of alcohol as a primary substance among completers may reflect differences in treatment pathways and more stable living situations that can be linked to socioeconomic factors and higher education levels.

Despite being innovative and pioneering, the proposed study also has some limitations. First, recruiting participants in outpatient facilities proved to be extremely challenging, leading to a limited number of participants in these settings. One significant obstacle was the presence of long waiting lists, resulting in a limited number of new treatment episodes and lower turnover rates compared to the residential services. The frequency of contact between potential participants and service providers was limited in many outpatient settings, as many service users have only weekly or biweekly appointments. This poses significant scheduling constraints, especially since participants were expected to complete the baseline questionnaires within three weeks of treatment initiation. Additionally, there was a high occurrence of no-shows for scheduled appointments with the researchers, which resulted in missed opportunities to engage potential participants. To address these challenges, several strategies were implemented. Flyers and posters were provided in waiting areas to inform potential participants about the study and proactive follow-up calls were made to confirm appointments. Moreover, regular contact was maintained with service providers (both through email and phone) to encourage participation. Despite these efforts, maintaining consistent engagement in outpatient facilities has been proven particularly challenging, resulting in some outpatient centers withdrawing from the study. Consequently, future comparative analyses will primarily focus on residential treatment services due to the limited number of outpatient participants. These challenges raise important considerations for the broader implementation of PROMs and PREMs in outpatient addiction services. While comprehensive assessments seem feasible in residential settings, these findings underscore the need for effective strategies for the structural implementation of PROMs and PREMs in outpatient services. Potential strategies to increase participation and retention may include broadening recruitment periods, simplifying assessment batteries, and using short-form PROMs and PREMs that could be completed during brief waiting periods before appointments. Additionally, codeveloping implementation strategies with service users and providers could further enhance feasibility and engagement in outpatient settings.

Second, recruiting facilities and participants in the French-speaking regions of Belgium proved to be more challenging than in the Dutch-speaking parts. This might be caused by differing attitudes toward the importance of outcome evaluation and measurement. Hence, beyond providing feasible measurement instruments and implementation strategies, there is a need to build service cultures that value the implementation of measurements to improve the quality of support. Organizing policies and funding that support the routine use of PROMs and PREMs might be an important step toward realizing this. Yet, this also includes overcoming practical challenges such as staffing shortages that hinder broader participation.

Finally, the study is limited by the lack of standardized diagnostic assessment of participants entering the study. Inclusion was based on proxy indicators (previous treatment for SUD, clinical assessment, or history of substance misuse), rather than on clinical diagnoses. This lack of structural diagnosis may have introduced variability in our findings and may limit generalization to other populations.

Despite these challenges, this study is one of the first studies to explore the assessment of PROMs and PREMs in SUD treatment services using the ICHOM SSA set of instruments. By systematically and routinely monitoring PROMs and PREMs, the project aims to empower service providers and give them tools to evaluate subjective treatment outcomes and experiences. This approach can provide service providers and policy makers with benchmarks for assessing outcomes and experiences during and after treatment across various treatment modalities. By applying an internationally validated tool, the study allows international comparison with similar interventions and treatment modalities globally. Future analyses will explore longitudinal changes in PROMs and PREMs, with a particular focus on the influence of baseline mediating and moderating variables (eg, socioeconomic status, comorbidity, and recovery strength) and differences between treatment modalities. The findings from this study will offer new perspectives and insights into the effectiveness of different SUD treatment modalities and their impact on diverse service user populations. This shift toward person-centered care not only supports better recovery outcomes but also fosters continuous improvement and innovation in SUD treatment services. Ultimately, systematically monitoring PROMs and PREMs has the potential to enhance the quality of care by offering a comprehensive understanding of service users’ needs and treatment effectiveness, extending beyond abstinence.