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Validation and comparison of reference equations for the 6-min walk distance test

¹ Bay Pines Veterans Affairs Health Care System, Bay Pines, FL, and ⁵ St. Elizabeth's Medical Center, Boston, MA, USA. ² Nuestra Señora de la Candelaria University Hospital, Tenerife, and ³ Miguel Servet Hospital, Zaragoza, Spain. ⁴ University of Montevideo, Montevideo, Uruguay. ⁶ Caracas University Hospital, Caracas, Venezuela.

CORRESPONDENCE: C. G. Cote, 10000 Bay Pines Boulevard, Bay Pines, FL, 33744 USA. Fax: 1 7273191090. E-mail: claudia.cote{at}va.gov

Keywords: Chronic obstructive pulmonary disease, 6-min walk distance test, mortality, reference equations

Received: August 10, 2007
Accepted October 18, 2007

Exercise impairment as measured by the 6-min walk distance (6MWD) test afflicts many patients with chronic obstructive pulmonary disease (COPD) and is known to predict mortality. Reference equations for the 6MWD in adults have been published but not yet validated.

The present authors prospectively followed 1,379 COPD patients for 55±30 months and tested the predictive value of the baseline 6MWD in metres, the 6MWD work (kg·m^–1) and as a percentage of predicted values the 6MWD in meters according to two reference equations. All-cause mortality was the validating outcome. The best threshold values were identified for each of the tests using receiver operating characteristic (ROC) curves.

The threshold values obtained were: 350 m for the 6MWD, 25,000 kg·m^–1 for the 6MWD work, and 67 and 54% predicted for the two reference equations. All modalities of the testing were similar at predicting COPD mortality and correlated well with the 6MWD test.

In conclusion, all modalities of testing predict mortality in chronic obstructive pulmonary disease equally well. In the 6-min walk distance test, a value <350 m is associated with increased mortality and should be regarded as abnormal.