The fractional exhaled nitric oxide measured at an expiratory flow of 50mL/s (FE 50) is a marker of airway inflammation, and high levels are associated with greater response to steroid treatment. In asthma, FE 50 increases with bronchodilation and decreases with bronchoconstriction, the latter potentially causing an underestimate of the degree of airway inflammation when asthma worsens. It is unknown whether the same effect occurs in chronic obstructive lung disease (COPD).
