The minimal important difference for residual volume in patients with severe emphysema.

Residual volume (RV) measured by body plethysmography is a routine measurement in clinical pulmonary practice and is often an important outcome variable in clinical trials. However, it is not known what size of improvement can be regarded as being important in severe emphysema patients. Therefore, the aim of the study is to establish the minimal important difference (MID) for RV in severe emphysema patients undergoing bronchoscopic lung volume reduction. 91 severe emphysema patients were included. RV and total lung capacity (TLC) were measured by body plethysmography. MID estimates were calculated by anchor-based and distribution-based methods. Forced expiratory volume in 1 s, 6-min walk distance and St George's Respiratory Questionnaire total score were used as anchors and Cohen's effect size was used as distribution-based method. The calculated MID estimates according to the different anchors and methods ranged between -0.31 and -0.43 L for RV, -6.1 and -8.6% for percentage change in RV (RV%) from baseline, and -2.8 and -4.0% for RV/TLC. These MID estimates are useful for sample size determination in new studies on interventions aimed at reducing RV and for interpreting the results from clinical trials in severe emphysema patients.