Summary of main findings
This review documents that motivational interviewing in a scientific setting effectively helps clients change their behaviour and that it outperforms traditional advice giving in approximately 80% of the studies. No studies have reported motivational interviewing to be harmful or to have any kind of adverse effect; however, no study did explicitly aim to report this.
The meta-analysis shows significant effects of motivational interviewing for combined effect estimates of body mass index, total blood cholesterol, systolic blood pressure, blood alcohol concentration and standard ethanol content (Table 2). In particular, the magnitude of the decrease of body mass index, systolic blood pressure, blood alcohol concentration and standard ethanol content is of clinical relevance and implies that motivational interviewing can and should be used. The significant decrease for the combined effect estimate of total blood cholesterol is of less clinical importance. However, viewing an effect as clinically important or not, it is imperative to keep in mind that motivational interviewing is based on making the patients themselves aware of the potential for change in behaviour resulting in improved health parameters, which means that small changes may also be of interest if they mark the beginning of a changing process for the patient. The nature of changes in the patient is almost always related to both adherence to prescribed medication and to the type of changes the patient makes in their lifestyle. However, the effect of motivational interviewing in some problem areas; for example, weight loss and smoking cessation, is primarily brought about through a change in personal habits, whereas the effect on asthma primarily comes from adherence to prescribed medication. This meta-analysis provides evidence of the significant effect of motivational interviewing on many different areas of intervention.
The review has shown that motivational interviewing can be effective even in brief encounters of only 15 minutes and that more than one encounter with a patient increases the likelihood of effect.5,6,14,16,36,37,41,42,45
This review sheds new light on the assumption that the effectiveness of motivational interviewing depends upon the counsellor's profession. The effect was not related to the counsellor's educational background as medical doctor or psychologist. Therefore, there was no statistically significant difference in the percentage of studies obtaining an effect from motivational interviewing, whether it was performed by psychologists, psychiatrists, physicians or GPs. Use of motivational interviewing probably depends on other aspects such as duration and number of client–counsellor encounters. However, it would be reasonable to speculate that aspects such as training and experience of motivational interviewing methods and client–counsellor relationships also influence the effectiveness, even if this cannot be shown in this review. Five studies involved other health workers as counsellors; for example, nurses, midwives and dieticians, and another six studies partly involved other health workers in the counselling encounter. Only five of these 11 studies found motivational interviewing to be effective.8,9,13,14,19,22,24,30,57,65,67 This may to some extent be explained by the design of these studies, as most of them reported on the effect of only one encounter, they had follow-up periods shorter than 3 months, and they began with difficult subjects such as HIV-positive drug misusers who were making changes to their lifestyle.
Strength and limitations of this study
Publication bias is a well-known problem. However, a funnel plot (Figure 2) of all the studies within the research area of motivational interviewing indicates publication bias to be non-significant. Furthermore, a methodological quality rating79-81 shows that, except for one study, all the randomised controlled trials in this review have a high methodological quality. All studies in the meta-analysis demonstrate a positive effect or tendency, although not all studies show a significant effect of motivational interviewing. However, the meta-analysis was only performed on the 19 studies (out of 42) that stated objective measures and statistical data needed for the meta-analysis in the article or delivered these data afterwards on request. Of the remaining 23 studies, 17 concluded that there was a significant effect of motivational interviewing and 6 showed a non-significant effect (Supplementary Table 1). These remaining studies did not deliver the statistical data needed for meta-analysis either in the article or afterwards. However, in view of the fact that a vast majority (33/42) of the randomised controlled trials with objective measures concluded that there was a significant effect of motivational interviewing, we believe a potential selection bias to be non-significant and the results of the meta-analysis to be valid.
Implications for future research
This review underscores the crucial importance of the motivational interviewing setting and study design for obtaining an effect. A follow-up period shorter than 3 months increases the risk of counselling failure, probably due to lack of intervention.13,19,20,24,26,32,60
Another important aspect was the use of indirect measures versus direct measures. This review showed that an effect of motivational interviewing can be demonstrated by indirect measures such as questionnaires, but also by direct objective measures such as blood pressure, blood glucose, weight and length of hospital stay. When it is possible to measure effect by epidemiological as well as clinical direct measures and to capture effect by clinical endpoints, this should be done to ensure the reliability of the results. The optimal design would match the specificity and reliability of direct measures with the in-depth qualitative perspective of indirect measures; for example, using questionnaires. Furthermore, in the optimal design it is imperative that an effort is made in future studies to describe precisely how motivational interviewing education is performed and how to use the methods in client counselling, allowing us all to learn more about how to increase and maximise its effect.
Implications for clinical practice
This review shows that motivational interviewing has been used in the treatment of various lifestyle problems and diseases, psychological as well as physiological. The review shows that approximately 75% of the studies do obtain an effect, regardless of whether the problems are psychological or physiological, which is supported by the meta-analysis. We can therefore argue that motivational interviewing is not limited in any way to counselling of a small group of selected clients, but can be used in the treatment of a broader area of diseases that to some extent are influenced by behaviour. When viewed in combination with the fact that there are no apparent harmful effects or adverse effects of motivational interviewing, it suggests that this is a method with an important potential effect, from which patients very well may benefit.
The review and meta-analysis leads to the conclusion that motivational interviewing in a scientific setting outperforms traditional advice giving in the treatment of a broad range of behavioural problems and diseases. However, evaluation of exact methods of motivational interviewing in a clinical setting is lacking. We now need large-scale studies of randomised controlled trials and qualitative studies on how to implement the methods of motivational interviewing to prove that it can be implemented into daily clinical work for healthcare providers and yield effects for the benefit of patients.