Objective: To improve the effectiveness of treatment in patients with chronic fatigue syndrome it is worthwhile studying factors influencing outcomes. The aims of this study were (1) to assess the association of expectancy and credibility on treatment outcomes, and (2) to identify baseline variables associated with treatment expectancy and credibility. Methods: 122 patients were included in a randomized controlled trial of whom 60 received cognitive behavioural therapy (CBT) and 62 multidisciplinary rehabilitation treatment (MRT). Expectancy and credibility were measured with the credibility and expectancy questionnaire. Outcomes of treatment, fatigue, and quality of life (QoL), were measured at baseline and post-treatment. Multiple linear regressions were performed to analyse associations. Results: In explaining fatigue and the physical component of the QoL, the effect of expectancy was significant for MRT, whereas in CBT no such associations were found. The main effect of expectancy on the mental component of QoL was not significant. For credibility, the overall effect on fatigue and the physical component of QoL was not significant. In explaining the mental component of QoL, the interaction between treatment and credibility was significant. However, the effects within each group were not significant. In the regression model with expectancy as dependent variable, only treatment centre appeared significantly associated. In explaining credibility, treatment centre, treatment allocation and depression contributed significantly. Conclusions: For clinical practice it seems important to check the expectations of the patient, since expectations influence the outcome after MRT.
- Quality of life