Minimal clinically important difference in physical activity in people with asthma.

Background: Previous studies have shown that increasing physical activity in daily life (PADL) improves asthma clinical control and quality of life. However, the minimal clinically important difference (MCID) to promote those improvements remains unclear. The aim of this study was to estimate the MCID for PADL in people with moderate-to-severe asthma.

Methods: Data from consecutive individuals with moderate-to-severe asthma who completed an 8-week (once weekly) face-to-face behavioural intervention to increase PADL were included to compute the MCID. The MCID was estimated based on the number of steps·day^-1 and time in moderate-to-vigorous physical activity (MVPA) (min·day^-1), which included analyses via anchor-based methods (linear regression and receiver operating characteristic curves) and distribution-based methods (half of the standard deviation and the standard error of the mean). The pooled MCID was estimated by calculating the weighted arithmetic mean of the results from the anchor-based (Asthma Control Questionnaire and Asthma Quality of Life Questionnaire) and distribution-based methods.

Results: The MCID estimates for steps·day^-1 ranged from 446 to 1995, which were derived from anchor-based and distribution-based methods. With respect to MVPA, MCID estimates ranged from 2 to 13 min·day^-1, which were also derived from both methods. The pooled MCIDs were 1413 for steps·day^-1 and 8 min·day^-1 for MVPA.

Conclusion: The MCIDs proposed in this study are 1413 steps for steps·day^-1 and 8 min of MVPA per day in people with asthma. These values can be used to interpret the efficacy of intervention programmes to improve health outcomes.