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Validity and Responsiveness of the Depression Anxiety Stress Scales-21 (DASS-21) in COPD. BACKGROUND: We examined the validity and responsiveness of the Depression Anxiety and Stress Scales-21 (DASS-21) in patients with COPD following an 8-week pulmonary rehabilitation program (PRP). METHODS: Five hundred and fifty-seven patients with clinically stable COPD completed an 8-week outpatient multidisciplinary PRP, comprising 2 h (1 h of exercise and 1 h of education) per week. Anxiety, exercise capacity, quality of life, and dyspnea were measured pre- and post-PRP, utilizing the Anxiety Inventory for Respiratory Disease, Incremental Shuttle Walk Test, St. George's Respiratory Questionnaire, and modified Medical Research Council dyspnea scale, respectively. In addition, we administered the DASS-21 to assess both the validity and responsiveness of this tool compared with other, well-established metrics. RESULTS: The mean (SD) age of participants was 71.6 (9.4) years, and 49% were women. The DASS-21 reflected responsiveness to pulmonary rehabilitation. Among participants with a high depression score (> 9), the depression subscale score fell from 18.62 pre-PRP to 13.12 post-PRP (P < .001). Similarly, among participants with a high anxiety score (> 7), the anxiety subscale fell from 14.60 pre-PRP to 10.99 post-PRP (P < .001). Likewise, among participants with a high stress score (> 14), the stress subscale score fell from 23.51 pre-PRP to 16.34 post-PRP (P < .001). Among these subsamples, the effect size was medium at 0.49 for depression and 0.54 for anxiety, and large at 0.81 for stress. The change in DASS-21 subset (depression, anxiety, and stress) correlated with the change in total SGRQ score, at P < .001. CONCLUSIONS: The DASS-21 has acceptable validity and is a responsive scale for use in PRP in patients with COPD.