Agreement of different reference equations to classify patients with COPD as having reduced or preserved 6MWD.

Background: Interpretation of the six-minute walk distance (6MWD) is enhanced by using recommended reference equations. Whenever possible, the choice of equation should be region-specific. A potential problem is that different equations for the 6MWD may have been developed for the same population, and it may be complicated to choose the most suitable.

Objective: To verify the agreement of different reference equations in classifying patients with Chronic Obstructive Pulmonary Disease (COPD) as having reduced or preserved 6MWD.

Methods: 159 patients with COPD performed the six-minute walk test according to international standardization. They were classified as having reduced 6MWD if it was below the lower limit of normal. Five Brazilian equations (Iwama; Britto1; Britto2; Dourado; Soares) and the two non-Brazilian equations most cited worldwide (Troosters; Enright) were used. The agreement for patients classified as reduced or preserved 6MWD was verified by Cohen's Kappa (pair-to-pair) analysis. The proportion of patients classified as having reduced walked distance was compared by the Chi-squared test.

Results: Agreement between equations varied largely in classifying subjects as having reduced or preserved 6MWD (Kappa: 0.10-0.82). Brazilian equations with the highest agreement were Iwama, Britto1 and Britto2 (Kappa>0.75). The proportion of patients classified as having reduced 6MWD was statistically similar only between equations in which the agreement was higher than 0.70.

Conclusion: Even reference equations from the same country vary considerably in the classification of reduced or preserved 6MWD, and it is recommended that the region-specific ones be used as they give with higher agreement for similar and comparable interpretation of the patients' functional exercise capacity.