The minimal important difference for the St George's Respiratory Questionnaire in patients with severe COPD

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Abstract
The St George's Respiratory Questionnaire (SGRQ) is a validated, commonly used questionnaire for measuring quality of life in patients with chronic obstructive pulmonary disease (COPD). The current established minimal important difference (MID) for SGRQ scores in an average COPD population is −4 units. However, for patients with severe COPD, the MID has not been thoroughly validated. We re-determined the SGRQ MID for this patient group.115 severe COPD patients (forced expiratory volume in 1 s (FEV1) 26±9% of predicted, SGRQ score 62±11 units; mean±sd,) who participated in seven different bronchoscopic lung volume reduction clinical trials were included in the analysis. Anchor- and distribution-based methods were used to define the MID for SGRQ scores. FEV1, 6-min walk distance and residual volume were used as anchors.Combining both anchor- and distribution-based methods, we identified a SGRQ MID of −8.3 units at 1 month and −7.1 units at 6 months.This study proposes an alternative SGRQ MID for patients with severe COPD of −8.3 units at 1 month and −7.1 units at 6 months follow-up after intervention. Our new MID estimates could be applied for both interpreting SGRQ outcomes as well as sample size determination in future clinical trials investigating interventions in severe COPD patients.

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