The 6-min walk test as a primary end-point in interstitial lung disease.

There is a need for clinical trial end-points to better assess how patients feel and function, so that interventions can be developed which alleviate symptoms and improve quality of life. Use of 6-min walk test (6MWT) outcomes as a primary end-point in interstitial lung disease (ILD) trials is growing, particularly for drugs targeting concurrent pulmonary hypertension. However, 6MWT outcomes may be influenced differentially by interstitial lung and pulmonary vascular components of ILD, making interpretation complicated. We propose that using 6MWT outcomes, including 6-min walk distance or oxygen desaturation, as primary end-points should depend upon the study population (how advanced the ILD is; whether vasculopathy is significant), the degree of disease progression, and, importantly, the effect of study treatment expected. We argue that the 6MWT as a single outcome measure is suitable as a primary end-point if the treatment goal is to improve functional performance or prevent disease progression within a study population of patients with advanced ILD or those with ILD and co-existent vasculopathy. In addition, we discuss the potential of composite primary end-points incorporating 6MWT outcomes, outlining important considerations to ensure that they are appropriate for the study population and treatment goals.