The objective of the study was to evaluate the best method for interpreting the bronchodilator test (BDT). Five formulas for expressing the BDT results were analyzed and compared: changes experienced by maximum expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) measured in milliliters, in percentage with respect to the baseline, in percentage with respect to the predicted, in percentage with respect to the possible, and in standardized residuals. Ninety-eight chronic obstructive pulmonary disease (COPD) patients were submitted to a respiratory function test on two different days. On each occasion three spirometries were conducted: basal, post-placebo and post bronchodilator. As a gold standard, a normality interval was defined using the variability experienced with the placebo between the two days of the study. The best formulas according to their sensitivity, specivity and area under receiver operating characteristic (ROC) curve were the ‘‘standardized residuals’’, with a cut point of .3, and the ‘‘percentage with respect to the predicted’’ with a cut point of 6%.