Both allergic and nonallergic asthma are associated with increased FENO levels, but only in never-smokers

Background:  Allergic asthma is consistently associated with increased FE_NO levels whereas divergence exists regarding the use of exhaled nitric oxide (NO) as marker of inflammation in nonallergic asthma and in asthmatic smokers. The aim of this study is to analyze the effect of having allergic or nonallergic asthma on exhaled nitric oxide levels, with special regard to smoking history.
Methods:  Exhaled NO measurements were performed in 695 subjects from Turin (Italy), Gothenburg and Uppsala (both Sweden). Current asthma was defined as self-reported physician-diagnosed asthma with at least one asthma symptom or attack recorded during the last year. Allergic status was defined by using measurements of specific immunoglobulin E (IgE). Smoking history was questionnaire-assessed.
Results:  Allergic asthma was associated with 91 (60, 128) % [mean (95% CI)] increase of FE_NO while no significant association was found for nonallergic asthma [6 (–17, 35) %] in univariate analysis, when compared to nonatopic healthy subjects. In a multivariate analysis for never-smokers, subjects with allergic asthma had 77 (27, 145) % higher FE_NO levels than atopic healthy subjects while subjects with nonallergic asthma had 97 (46, 166) % higher FE_NO levels than nonatopic healthy subjects. No significant asthma-related FE_NO increases were noted for ex- and current smokers in multivariate analysis.
Conclusions:  Both allergic and nonallergic asthma are related to increased FE_NO levels, but only in never-smoking subjects. The limited value of FE_NO to detect subjects with asthma among ex- and current smokers suggests the predominance of a noneosinophilic inflammatory phenotype of asthma among ever-smokers.