Objective: Patients with both a substance use disorder and comorbid mental disorder, or dual diagnosis, form a heterogeneous group with complex treatment needs. Dual diagnosis patients have worse treatment outcomes compared to patients with substance use disorders alone. There is a lack of research focusing on predictors of treatment outcome in dual diagnosis patients. The aim of this study was to identify patient characteristics associated with abstinence in dual diagnosis patients. Methods: In a cohort design, we examined associations between baseline demographic and clinical patient characteristics and abstinence at 14-month follow-up in 102 dual diagnosis patients undergoing treatment in an addiction-psychiatry service in Amsterdam. Immigration, employment, housing, age, emotion regulation, psychiatric symptoms, and frequency and nature of substance abuse were examined as possible predictors of abstinence. Patients were considered abstinent if they had refrained from any substance use in the 30 days prior to the follow-up assessment, as measured with the Timeline Follow Back method. Results: In a multiple logistic regression analysis corrected for treatment utilization, being a first- or second-generation immigrant was associated with lower odds of abstinence (OR = 5.13; 95% CI [1.63, 18.18]), as well as higher levels of depressive and anxiety symptoms at baseline (OR = 0.90; 95% CI [0.81, 0.99]). Gender, age, housing, employment, emotion regulation, frequency and nature of substance abuse, and the psychiatric symptoms positive symptoms, negative symptoms, and manic excitement and disorganization did not show a significant relationship with abstinence. Conclusions: The results indicate that being a first- or second-generation immigrant and experiencing more symptoms of depression and anxiety are predictive of non-abstinence. Attention for linguistic and cultural congruence of dual diagnosis treatments may enhance abstinence in immigrants. Additionally, addressing symptoms of depression and anxiety in dual diagnosis treatment may be beneficial for the maintenance of abstinence. More research is needed on factors influencing treatment success in dual diagnosis patients.
Keywords: Dual diagnosis; abstinence; predictors; substance use disorder; treatment outcome.