The minimal important difference of the pulmonary functional status and dyspnea questionnaire in patients with severe chronic obstructive pulmonary disease

Background: The modified version of the Pulmonary Functional Status and Dyspnea Questionnaire (PFSDQ-M) is used in patients with COPD to obtain information about their functional status. It consists of 3 components (change in activities, dyspnea and fatigue) ranging from 0 to 100 and has been shown to be responsive following pulmonary rehabilitation (PR). The interpretation of changes in PFSDQ-M score after an intervention is difficult in the absence of the minimal important difference (MID) of the PFSDQ-M. This study aims at investigating the MID of the PFSDQ-M. Methods: We enrolled 301 patients with COPD (FEV1 42 +/- 15%pred) that completed the PFSDQ-M before and after a 3-month PR program (Delta Chronic Respiratory Disease Questionnaire (CRDQ) + 16 +/- 12 points, Delta Six-minute walking distance (6MWD) + 47 +/- 89 m, both p < 0.001). An anchor-based approach consisted of calculating the correlation between the Delta PFSDQ-M and anchors with an established MID (Delta CRDQ and Delta 6MWD). Linear regression analyses were performed to predict the MID from these anchors. Secondly several distribution-based approaches (Cohen's effect size, empirical rule effect size and standard error of measurement method) were used. Results: Anchor-based estimates for the different PFSDQ-M-components were between -3 and -5 points based on CRDQ score and -6 (only calculated for change in activities) based on 6MWD. Using the distribution-based methods, the estimates of MID ranged from -3 to -5 points for the different components. Conclusions: We concluded that the estimate of MID of the PFSDQ-M after pulmonary rehabilitation corresponds to a change of 5 points (range -3 to -6) in each component in patients with severe COPD.