The difference (Δ) in bronchial responsiveness (BR), expressed as doubling doses of methacholine§, between the first FENO quartile (Q1) and the other quartiles (Q2–Q4) in all subjects, atopics and non-atopics, after stratifying for smoking.

§ Slope was the outcome of the regression model and doubling doses were obtained by multiplying the regression coefficients with 1.66, as described in the Methods.* p-value for trend represents the statistical significance for the association between bronchial responsiveness and FENO quartile (used as a qualitative variable).#p-value for interaction represents the significance of interaction of smoking status with FENO quartile on airways responsiveness. All the coefficients and p-values are adjusted for gender, study centre, FEV1(%pred), age, height, weight, atopy, current asthma.