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 (FENO) values and could hamper how FENO captures changes in asthma control. Here, our objective was to assess whether forced expiratory volume in 1 s (FEV1) variations alter the ability of FENO to reflect asthma control.FENO, asthma control (Asthma Control Questionnaire (ACQ)) and FEV1 were measured at least two times in 527 patients during 1819 pairs of visits. Determinants of FENO-ACQ discordance probability were evaluated through a logistic regression analysis. The effectiveness of FENO 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 FENO change in the same direction, the odds of FENO-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 FENO changed in the opposite or same direction, respectively.The manner in which FENO and FEV1 vary concomitantly when asthma control changes determines the ability of FENO to capture this change: parallel or opposite changes in FEV1 and FENO either decrease or increase this ability to capture asthma control changes.