Reductions in negative repetitive thinking and metacognitive beliefs during transdiagnostic internet cognitive behavioural therapy (iCBT) for mixed anxiety and depression.

We explored whether transdiagnostic internet-delivered cognitive behavioural therapy (iCBT) for mixed anxiety and depression effectively reduces repetitive negative thinking (RNT), and whether reductions in RNT and positive metacognitive beliefs mediate symptom improvement during iCBT. Participants with generalized anxiety disorder (GAD), major depressive disorder (MDD), or mixed GAD/MDD diagnoses were randomly allocated to a 6-lesson clinician-guided iCBT anxiety and depression program (n = 46) or wait-list control (WLC, n = 53). Depression (PHQ-9), generalized anxiety (GAD-7), RNT (Repetitive Thinking Questionnaire) and positive beliefs about RNT (Positive Beliefs about Rumination Scale) were assessed at pre-, mid-, and post-treatment or matched time points for WLC. Tests of serial indirect effects explored the potential meditating role of RNT and positive belief reductions on the impact of iCBT on depression and anxiety symptoms post-treatment. Results showed that both RNT frequency and positive beliefs about the value of RNT reduced significantly following iCBT compared to WLC, with gains maintained at 3-month follow-up. Reductions between pre- and mid-treatment in positive beliefs and RNT mediated improvements in depression symptoms post-iCBT, and reductions in positive beliefs mediated improvements in GAD symptoms. These findings indicate that iCBT is an effective treatment for RNT and positive metacognitive beliefs. Future dismantling studies are needed to assess the most effective treatment components.