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

Rationale: The optimal method of physical activity measurement has not been determined in patients with fibrotic interstitial lung disease (ILD).

Objectives: 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.

Methods: 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.

Results: 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.

Conclusions: 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.