This study examines the effects of an applet-based Acceptance and Commitment Therapy focused on Repetitive Negative Thinking (RNT-ACT) for adults with comorbid anxiety and depression, and explores the mediating role of Repetitive Negative Thinking (RNT) in its efficacy.

In a randomized controlled trial, participants with the co-occurrence of elevated anxiety and depressive symptoms (GAD-7 and PHQ-9 scores ≥10; N  = 94) were randomly assigned to either the RNT-ACT intervention group ( N  = 49) or a wait-list control group ( N  = 45). The intervention group completed a 14-day online RNT-ACT program, while the wait-list group received no intervention. Assessments of RNT, anxiety, and depression were conducted at baseline, mid-intervention, post-intervention for both groups, with an additional one-month follow-up assessment for the intervention group.

The RNT-ACT intervention led to significant reductions in RNT ( F_{(2, 272)}  = 45.44, p   <  0.001), anxiety ( F_{(2, 174)}  = 25.43, p   <  0.001), and depression ( F_{(2, 174)}  = 25.43, p   <  0.001) symptoms post-intervention compared to the control group, with sustained improvements at follow-up. Longitudinal mediation analysis revealed that reductions in RNT mediated improvements in anxiety and depression.

These findings provide empirical support for transdiagnostic interventions and highlight the potential for optimizing treatment strategies for comorbid anxiety and depression. Future research should explore additional mediators to further elucidate the mechanisms underlying comorbidity.