Effects of Transdiagnostic Cognitive Behavioural Therapy on Long-Term Quality of Life: A Causal Mediation Analysis Across Anxiety and Depressive Symptoms.

Introduction: Anxiety and depression significantly impair quality of life (QoL) in primary care (PC) patients. While transdiagnostic cognitive behavioural therapy (TD-CBT) is effective, the mechanisms underlying its long-term impact on QoL remain unclear. This study examined whether changes in anxiety and depressive symptoms mediate the effect of TD-CBT on QoL at 12-month follow-up. Methods: Data were used from the "Psychology in Primary Care" trial (PsicAP, from its Spanish acronym), which included 1061 PC patients with anxiety and depression, randomised to TD-CBT plus treatment-as-usual (TAU) or TAU alone. Anxiety and depression were assessed at baseline, post-treatment and 6-month follow-up. QoL was measured at baseline and 12-month follow-up. Path analyses using structural equation modelling (SEM) were used to study direct and indirect effects, controlling for baseline scores and gender. Results: TD-CBT significantly reduced anxiety and depression immediately post-treatment compared to TAU. The only significant indirect effect on 12-month QoL across all dimensions operated sequentially through sustained reductions in depressive symptoms. No significant mediation was found via anxiety symptoms. No specific temporal sequence of symptom improvement mediating QoL was identified. Discussion: TD-CBT improves anxiety and depressive symptoms at post-treatment; nevertheless, long-term QoL improvement occurs primarily through sustained reduction in depressive symptoms and through the direct effects of treatment itself. These findings support the implementation of TD-CBT in PC to achieve lasting functional recovery, highlighting the crucial role of addressing and sustaining improvements in depression.