Six-minute-walk test in idiopathic pulmonary fibrosis: test validation and minimal clinically important difference.
BACKGROUND
The 6-minute-walk test (6MWT) is a practical and clinically meaningful measure of exercise tolerance with favorable performance characteristics in various cardiac and pulmonary diseases. Performance characteristics in patients with idiopathic pulmonary fibrosis (IPF) have not been systematically evaluated.


OBJECTIVE
To assess the reliability, validity, and responsiveness of the 6MWT and estimate the minimal clinically important difference (MCID) in patients with IPF.


METHODS
The study population included all subjects completing a 6MWT in a clinical trial evaluating interferon gamma-1b (n = 822). Six-minute walk distance (6MWD) and other parameters were measured at baseline and at 24-week intervals using a standardized protocol. Parametric and distribution-independent correlation coefficients were used to assess the strength of the relationships between 6MWD and measures of pulmonary function, dyspnea, and health-related quality of life. Both distribution-based and anchor-based methods were used to estimate the MCID.


RESULTS
Comparison of two proximal measures of 6MWD (mean interval, 24 d) demonstrated good reliability (coefficient = 0.83; P < 0.001). 6MWD was weakly correlated with measures of physiologic function and health-related quality of life; however, values were consistently and significantly lower for patients with the poorest functional status, suggesting good construct validity. Importantly, change in 6MWD was highly predictive of mortality; a 24-week decline of greater than 50 m was associated with a fourfold increase in risk of death at 1 year (hazard ratio, 4.27; 95% confidence interval, 2.57- 7.10; P < 0.001). The estimated MCID was 24-45 m.


CONCLUSIONS
The 6MWT is a reliable, valid, and responsive measure of disease status and a valid endpoint for clinical trials in IPF.
National Heart & Lung Institute, Imperial College, London, United Kingdom. r.dubois@imperial.ac.uk
Roland M
RM
du Bois
Derek
D
Weycker
Carlo
C
Albera
Williamson Z
WZ
Bradford
Ulrich
U
Costabel
Alex
A
Kartashov
Lisa
L
Lancaster
Paul W
PW
Noble
Steven A
SA
Sahn
Javier
J
Szwarcberg
Michiel
M
Thomeer
Dominique
D
Valeyre
Talmadge E
TE
King
D000368:Aged; D004417:Dyspnea / Q000209:etiology / Q000503:physiopathology; D005080:Exercise Test / Q000379:methods*; D005260:Female; D006801:Humans; D054990:Idiopathic Pulmonary Fibrosis / Q000150:complications / Q000175:diagnosis* / Q000503:physiopathology; D008168:Lung / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011788:Quality of Life; D015203:Reproducibility of Results; D016138:Walking