Minimal important difference of the transition dyspnoea index in a multinational clinical trial

Dyspnoea is a primary symptom of chronic obstructive pulmonary disease (COPD). The baseline (BDI) and transition (TDI) dyspnoea indices are commonly used instruments to assess breathlessness and the impact of intervention. Its validity and pattern of response in multinational clinical trials has not been established. In a retrospective analysis of a cohort of 997 COPD patients who received tiotropium, salmeterol or placebo, in addition to usual care, the validity and pattern of response of the BDI and TDI were examined. The BDI was significantly correlated with the dyspnoea diary (DD) score and the symptom and activity components of the St. George's respiratory questionnaire (SGRQ), establishing concurrent validity. Furthermore, the TDI was also correlated with the changes in DD, SGRQ symptom and activity scores. Construct validity was established by the association between baseline forced expiratory volume in one second (FEV1) and BDI and ΔFEV1 with TDI. Physician's global evaluation (PGE) was significantly associated with BDI as well as ΔPGE with TDI. Significant correlations have also been observed when the cohorts were classified according to native English and native nonEnglish speaking countries. A change in PGE of 1 category (i.e. 2 units on an 8‐point scale) was associated with a mean TDI of ∼1 unit (0.9–1.3 mean focal score), lending further support to the clinical significance of this change inherent in the instrument's descriptors. TDI responders (i.e. focal score ≥1 unit) used less supplemental salbutamol, had fewer exacerbations and had significantly improved health status as measured by impacts and total SGRQ scores compared with nonresponders. In conclusion, the transition dyspnoea index is a valid instrument when used in a multinational clinical trial and the patterns of response confirm a 1‐unit change in the transition dyspnoea index focal score as being clinically important.

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