Airway calibre variation is a major determinant of exhaled nitric oxide's ability to capture asthma control

Changes in airway calibre have the potential to modify exhaled nitric oxide fraction (F_ENO) values and could hamper how F_ENO captures changes in asthma control. Here, our objective was to assess whether forced expiratory volume in 1 s (FEV₁) variations alter the ability of F_ENO to reflect asthma control.F_ENO, asthma control (Asthma Control Questionnaire (ACQ)) and FEV₁ were measured at least two times in 527 patients during 1819 pairs of visits. Determinants of F_ENO-ACQ discordance probability were evaluated through a logistic regression analysis. The effectiveness of F_ENO at capturing either asthma control worsening or improvement between two visits was then assessed by undertaking a stratified receiver operating characteristic curves analysis.When FEV₁ and F_ENO change in the same direction, the odds of F_ENO-ACQ being discordant are multiplied by 3 (p<0.001). The area under the curve values were 0.765 (95% CI 0.713-0.805) (improvement; p<0.001) and 0.769 (95% 0.706-0.810) (worsening; p<0.001) or 0.590 (95% 0.531-0.653) (improvement; p=0.001) and 0.498 (95% 0.416-0.567) (worsening; p=0.482) when FEV₁ and F_ENO changed in the opposite or same direction, respectively.The manner in which F_ENO and FEV₁ vary concomitantly when asthma control changes determines the ability of F_ENO to capture this change: parallel or opposite changes in FEV₁ and F_ENO either decrease or increase this ability to capture asthma control changes.