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 (FEV(1) 42 ± 15%pred) that completed the PFSDQ-M before and after a 3-month PR program (∆Chronic Respiratory Disease Questionnaire (CRDQ) +16 ± 12 points, ∆Six-minute walking distance (6MWD) +47 ± 89 m, both p < 0.001). An anchor-based approach consisted of calculating the correlation between the ∆PFSDQ-M and anchors with an established MID (∆CRDQ and ∆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.