Over the last four decades, 27 studies meeting our inclusion criteria could be identified that examined treatment satisfaction in DD clients. This review shows that most DD clients report being satisfied with their treatment experience, reflected by average ratings close to the "satisfied" end of the scales used. This applied regardless of the differences in study location (i.e. US, UK, Australia or Honduras), treatment settings and types of interventions delivered. When comparing satisfaction ratings of dual and single diagnosis clients treated in the same setting (i.e. either mental health or substance misuse treatment), a large and well-designed study found that DD clients were significantly less satisfied than single diagnosis clients [36
]. Two smaller studies, however, showed that clients with co-morbid problems had similarly high satisfaction ratings as those with a single diagnosis [33
]. This inconsistency may be linked to differences in satisfaction instruments used (i.e. standardised vs. non-standardised), client profiles (e.g. the larger study included men only) and the small sample sizes in the two studies that found no differences in satisfaction ratings (N < 50).
If replicated in future studies, a finding that DD clients are less satisfied with standard (i.e. either mental health or addiction-focused) treatment than single diagnosis clients would support the common understanding that disease-specific treatment is inadequate to address the complex needs of the DD population. An integrated treatment model is usually favoured in discussions about which approach is the most beneficial for co-morbid clients e.g. [57
]. The question as to whether or not these benefits are also reflected in greater satisfaction levels was specifically addressed by seven studies included in this review. Of these, five offered evidence that integrated care yields greater client satisfaction than standard treatment [30
]. The five studies were all conducted in the US whereas the two other studies that found no significant differences in satisfaction by treatment approach were carried out elsewhere (UK and Honduras ) [38
]. In this context, however, it is important to bear in mind that of the seven studies identified only three assessed treatment fidelity and thus monitored if the integrated treatment condition was implemented as intended. These three studies consistently demonstrated higher satisfaction levels in the integrated treatment group compared to ratings from clients in standard care [30
Nine studies investigated which factors - other than treatment type - are associated with satisfaction among DD clients. Studies that examined client pre-treatment factors (i.e. demographics, primary substance of misuse and type of psychopathology) found no association with satisfaction ratings. In contrast, a number of treatment process and service-related variables were identified that appeared linked to satisfaction (e.g. client and staff outcome ratings, frequency of contact with treatment service, family and transportation assistance). In some studies though, it remained unclear whether or not potential confounders were taken into account, which needs to be addressed in future studies. Moreover, it would be important to examine the effect of variables that have been found to be associated with treatment satisfaction among single diagnosis samples in the past (e.g. access routes, treatment motivation and engagement, care-plan procedures, staff and service characteristics) [11
In terms of rigor, the 27 studies were diverse, and some had important methodological shortcomings. Only 13 studies used standardised measures to assess treatment satisfaction, and while the selected instruments have shown acceptable psychometric properties when used with single diagnosis treatment populations e.g. [11
], the scales' reliability and validity in clients with co-morbidity was reported by only two studies [36
]. DD clients might have different treatment expectations due to more complex needs than those with a single diagnosis. Thus, response patterns to a given set of questions might vary between populations with and without DD, and psychometric testing would be important to ensure meaningful interpretation of data. Similarly, only three of the studies that used a self-developed satisfaction scale provided psychometric information sufficient to permit reasonable evaluation of the instruments [30
Secondly, studies were restricted in their examination of potential confounders of satisfaction ratings. Only five studies reported explicitly that they controlled for any links between client characteristics and satisfaction levels [28
]. The lack of client control variables and other potential confounders (e.g. treatment process variables, practitioner characteristics) is of particular concern in those studies that compared satisfaction levels by type of treatment model: uncontrolled factors may affect clients' satisfaction ratings, which in turn distorts interpretations concerning actual treatment effects.
A third methodological difficulty concerns possible time-in-treatment effects on satisfaction ratings. In most of the reviewed studies, clients were at different treatment stages when satisfaction was assessed, with only ten studies taking the length of treatment exposure into account. Two of these reported client satisfaction at different treatment stages, with one showing stable high ratings throughout [40
] and the other study indicating a negative linear trend in satisfaction levels during the treatment course [56
]. Based on the latter, it could be assumed that clients' most urgent needs are addressed in the early treatment phase thus producing particularly high satisfaction levels early on in the programme. In later treatment phases though, possibly more persistent problem areas are targeted for which behaviour change and improvement is more difficult to achieve. Subsequently, studies examining satisfaction early in treatment may find higher satisfaction ratings than studies with later assessment schedules. However, at the same time it is plausible that clients who have spent more time in treatment may have experienced greater benefits overall and possibly show higher satisfaction levels than clients who have spent less time in the programme [68
]. In either case, having more information about potential time-in-treatment effects across the existing studies would have been useful.
The current review has highlighted some important gaps in our knowledge of treatment satisfaction among DD clients such as the influence of practitioner characteristics and treatment process variables as well as the effect of client satisfaction on different treatment outcomes. Clients' subjective evaluations have been recognised in both mental health and addiction treatment populations as key indicators of treatment quality and effectiveness e.g. [19
], and so this remains an important area of research. The review contributes a methodological framework of four key aspects that future studies should consider to overcome the limitations, namely: 1) employment of well-validated and comparable satisfaction assessment techniques, 2) selection of multiple measures that incorporate several treatment- and client-related factors, 3) controlling for potential confounders of satisfaction, including pre- and in-treatment factors (e.g. treatment readiness, frequency of service contact, substitute prescribing) and practitioner characteristics (e.g. work experience), and 4) the nature and extent of treatment exposure (e.g. assertive vs. standard care, length of treatment stay). Here, special attention should be paid to the assessment of treatment fidelity. This is particularly important for studies aiming to replicate the finding that integrated treatment - if implemented appropriately - yields greater client satisfaction than other treatment models. Furthermore, it would be vital for future studies to investigate links between satisfaction and other treatment process and outcome variables to demonstrate more clearly whether greater satisfaction among DD clients translates into better engagement and retention, lower relapse rates and reduced symptom severity. Finally, a more general point requires consideration: a recent review has shown that satisfaction studies disproportionally found positive accounts from clients throughout treatment modalities and client populations [70
]. In order to avoid misinterpretation of client ratings due to social desirability or other potential bias, safeguards should be applied in future studies, such as keeping assessments anonymous and comparing satisfaction ratings of treatment completers and dropouts.
A limitation of the current review is that no meta-analysis could be carried out. A quantitative synthesis of data could have taken into account small sample sizes and moderate - if not significant - effects thus providing further insight into the current evidence base. Depending on the growth of studies in this field, future reviews should include such analyses where possible.