Background: Exhaled nitric oxide (ENO) has been proposed as a noninvasive marker of airway inflammation in asthma.
Objective: We investigated the relationships among ENO, eosinophilic airway inflammation as measured by induced sputum, and physiologic parameters of disease severity (spirometry and methacholine PC(20)). We also examined the effect of corticosteroid treatment and atopy on ENO levels and eosinophil counts in induced sputum.
Methods: Measurements were taken on one day in 22 healthy nonatopic subjects, 28 healthy atopic subjects, 38 asthmatic subjects not taking inhaled steroids, 35 asthmatic subjects taking inhaled steroids, and 8 subjects with eosinophilic bronchitis without asthma.
Results: ENO levels showed significant but weak correlations with eosinophil differential counts in the steroid-naive asthmatic and healthy atopic groups (r (s) < 0.05). ENO levels were significantly lower in the asthmatic subjects taking steroids compared with the asthmatic subjects not taking steroids, despite there being no difference in the sputum cell counts, and a tendency to increased airflow limitation. ENO levels and sputum eosinophil counts were equally good at differentiating from steroid-naive asthmatic subjects. ENO levels were consistently raised in subjects with eosinophilic bronchitis without asthma. Atopy had no effect on ENO levels in the healthy subjects.
Conclusion: We conclude that ENO is likely to have limited utility as a surrogate clinical measurement for either the presence or severity of eosinophilic airway inflammation, except in steroid-naive subjects.