The 6-minute walk test in scleroderma--how measuring everything measures nothing.

The 6-min walk test (6MWT) is a standardized measure of submaximal exercise capacity that has served as the primary measure of outcome in studies of pulmonary arterial hypertension. Never fully validated in scleroderma, a variety of data suggest that many non-pulmonary aspects of scleroderma contribute to its results thereby blunting the ability of the 6MWT to measure change in lung function. Although reproducible, the lack of correlation with measures of parenchymal lung disease demonstrates an insurmountable lack of both construct and content validity. The 6MWT should be abandoned as an outcome measure in lung disease complicating scleroderma.