Minimal Important Difference for Physical Activity and Validity of the International Physical Activity Questionnaire in Interstitial Lung Disease.

The optimal method of physical activity measurement has not been determined in patients with fibrotic interstitial lung disease (ILD). To assess the validity, internal consistency, and responsiveness of the International Physical Activity Questionnaire long form (IPAQ-LF) and to estimate the minimal important difference (MID) for moderate to vigorous physical activity (MVPA) in patients with fibrotic ILD. This two-center prospective cohort study included a convenience sample of 111 outpatients with fibrotic ILD who wore waist and wrist ActiGraph accelerometers for 7 consecutive days, followed by self-administration of the IPAQ-LF. Both measurements were performed at baseline and 6-month follow-up. Spearman rank correlations were used to evaluate the IPAQ-LF validity in comparison to activity monitor data, lung function, step count, and quality of life. The internal consistency of the IPAQ-LF was determined using Cronbach's α. Self-reported changes in IPAQ-LF parameters were compared among tertiles of change measured by the waist activity monitor to assess the responsiveness of the IPAQ-LF after 6 months. Anchor- and distribution-based methods were used to estimate the MID for MVPA. Self-reported MVPA minutes, activity-related energy expenditure, sedentary time, and inactive time of the IPAQ-LF generally showed moderate to strong correlations with corresponding waist activity monitor data, step count, lung function, and quality of life. The Cronbach's α of the IPAQ-LF was 0.78. The IPAQ-LF was responsive at detecting increases in weekly MVPA and energy expenditure. The MIDs for MVPA were 8 to 26 and 13 to 58 min/wk using the anchor-based method for waist activity monitor and IPAQ-LF data, respectively. The distribution-based MID estimate for MVPA was 104 to 242 min/wk. The IPAQ-LF has acceptable validity and internal consistency for measuring daily physical activity in patients with fibrotic ILD. The IPAQ-LF was responsive at detecting increases in physical activity but limited in its ability to detect declines or changes in inactivity. Using an anchor-based approach, the MID for MVPA measured by a waist activity monitor is approximately 26 min/wk.