[Bronchodilator test: is 9% of the predicted value the same as 12% of the initial value]

The bronchodilator test (BDT) is an important tool used in pulmonary function. Changes in forced expiratory volume in one second (FEV1) can be expressed as absolute change, or per cent of initial or predicted value. When the initial value is used, there may be a bias, as the smaller this value is, the greater the response will be. The main objective of this study is to establish whether there is any difference in using per cent of the initial spirometry value or per cent of the predicted value in order to consider a bronchodilator test positive, and if the initial obstruction of the patient influences such differences.

A retrospective analysis of the BDT made between October 1997 and February 2008. The results using an increase of 9% from the predicted FEV1 were compared with using 12% from the initial FEV1. The patients were divided into three groups depending on initial obstruction: no obstruction (FEV1>80% of predicted), mild (FEV1=60-80% of predicted) and moderate-severe (FEV1<60% of predicted). The kappa index of agreement between both methods was calculated.

A total of 4352 BDT were analysed. The agreement between both methods was high (k=0.832). In the group without initial obstruction (N=3007) the kappa index was 0.781, in the mild obstruction group (N=1067) the kappa index was 0.966 and in the moderate-severe group (N=278) it was 0.788.

This study demonstrates that, although there is a good agreement between both methods, in patients with initial moderate-severe obstruction and in patients without initial obstruction this agreement tends to be lower.