Association of spirometry findings with AHR
FEV1 reflects mainly the functioning of the large and medium airways(30), whereas FEF represents the functioning of the medium-sized and small airways(30-32). There are several studies observing the important role of FEV1 in predicting AHR(12, 14, 33). After excluding children with an obstructive pattern, FEF becomes more important, and FEV1 has been reported to lose its significance(7, 32). Thus, FEF might have a possible role in the prediction of AHR, particularly in patients with a normal FEV1 and FEV1/FVC ratio(31),(32). However, the predictive value of FEF for AHR is controversial. While some authors found a significant association of FEF values, especially of a forced mid-expiratory flow between 25% and 75% of vital capacity (FEF25-75%), with AHR in patients with normal FEV1(31, 32, 34), others did not(35, 36). This inconsistency may be due, at least, to the fact that some studies also included other diseases like cystic fibrosis(35, 36). In our population, none of the spirometry variables recorded at baseline were significantly associated with AHR. However, after dichotomizing the FEF values at a cutoff of 65%, as proposed by others(31),,(32), the role of FEF75< 65% in predicting AHR became significant.