Rationale: The response characteristics of the six-minute walk test (6MWT) in studies of idiopathic pulmonary fibrosis (IPF) are only poorly understood, and the change in walk distance that constitutes the minimum important difference (MID) over time is unknown.
Objectives: To examine changes over time in distance walked (i.e., 6MWD) during the 6MWT and to estimate the change in distance that constitutes the MID in patients with IPF. Methods: We used data from a recently completed trial in IPF that included subjects who completed the 6MWT, Baseline/Transition Dyspnea Index (BDI/TDI), and forced vital capacity (FVC) at six and twelve months to examine longitudinal changes in 6MWD over. We used both anchor- and distribution-based approaches as well as linear regression analysis to determine the MID for 6MWD. The BDI/TDI's Magnitude of Task (MTA) domain score and FVC were used as clinical anchors.
Main results: Among 123 subjects alive and able to complete the 6MWT at both follow-up time points, 6MWD did not to change significantly over time (378.1m at baseline vs. 376.8m at six months vs. 361.3m at twelve months, p = NS). The point estimate for the 6MWD MID was 30m with a range of 14.6-58.5m.
Conclusion: In a group of IPF patients with moderate physiologic impairment, for those alive and able to complete a 6MWT at six and twelve months, 6MWD does not change significantly over time. At the population level, the MID for 6MWD appears to be around 30 meters. Further investigation using other anchors and derivation methods is required to refine estimates of the MID for 6MWD in this patient population.