Validity of the six-minute step test of free cadence in patients with chronic obstructive pulmonary disease

Objectives:

to evaluate the concurrent validity of the six-minute step test (6MST) inassessing exercise capacity of COPD patients using the six-minute walk test (6MWT)as a gold-standard. The predictive validity of the 6MST was assessed to determinea cut-off point for identification of low exercise capacity.

Method:

thirty-two COPD patients (50-87 years old) with mild to very severe obstructionperformed the 6MST and 6MWT twice.

Results:

Concurrent validity: a strong positive correlation (Pearson) between the number ofascents on the first (T1), second (T2) and the best of both (T1 or T2) testsduring the 6MWT was observed. Although a moderate negative correlation with BODEindex and FEV₁ was found, it was considered insufficient to test thevalidity, therefore ROC curves were not applied. The predictive validity (ROC) ofthe 6MST to identify low physical capacity (compared with the 6MWT) using theperformance of T1 or T2, or solely T1 was considered accurate, and the area underthe curve was 0.8 (IC95% 0.62-0.98) and 0.85 (IC95% 0.70-0.99), respectively. Toclassify patients, the cut-off points of 86 and 78 steps were chosen, with bothvalues showing 90% of sensitivity and specificity of 64% and 68% for T1 or T2, orsolely T1, respectively.

Conclusion:

The number of steps on the 6MST was valid to verify exercise capacity in COPDpatients and the cut-off point of 78 steps was able to identify patients with poorexercise tolerance. Values under this cut-off point are considered to identifypatients with a poorer prognosis.