A systematic review including 72 studies (the total number of subjects was not reported nor calculable) was abstracted in DARE. Studies that compared motivational interviewing (defined according to Miller and Rollnick) with traditional advice were eligible for inclusion. 19 studies provided sufficient data for inclusion in the meta-analysis, which showed a significant effect for motivational interviewing for combined effect estimates for body mass index (effect size 0.72, 95% CI 0.33 to 1.11, P=0.0001; 6 studies, n=1140), total blood cholesterol (effect size 0.27 mmol/L, 95% CI 0.20 to 0.34, P=0.0001; 3 studies, n=1 358), systolic blood pressure (effect size 4.22 mmHg, 95% CI 0.23 to 8.99, P=0.038; 2 studies, n=316), blood alcohol concentration (effect size 72.92 mg%, 95% CI 46.80 to 99.04, P=0.0001; 6 studies, n=278), and standard ethanol content (effect size 14.64 standard units, 95% CI 13.73 to 15.55, P=0.0001; 7 studies, n=648), while combined effect estimates for cigarettes per day (3 studies, n=190) and for HbA1c (4 studies, n=243) were not significant.
The narrative synthesis revealed that motivational interviewing had a significant and clinically relevant effect in 74% (53/72) of the studies, with an equal effect on physiological and psychological diseases (included fields were alcohol abuse, psychiatric/addiction, weight loss/physical activity, smoking cessation and diabetes/asthma). The median duration of an individual counselling encounter was 60 minutes (range 10 to 120 minutes). Among the studies using encounters of 60 minutes, 81% (26/32) showed an effect. Out of 11 studies using less than 20 minutes per encounter 64% (7/11) showed an effect. The median duration of follow-up was 12 months (range 2 months to 4 years). A prolonged follow-up period increased the percentage of studies showing an effect. Psychologists and physicians obtained an effect in approximately 80% of the studies, while other healthcare providers obtained an effect in 46% of the studies. More than one encounter with the patient ensured the effectiveness of motivational interviewing.
Comment: The quality of evidence is downgraded by review limitations (limited reporting of review methods and the lack of adequate information on individual studies). Studies are needed to prove that motivational interviewing can be implemented into daily clinical work in health care.