Both exhaled nitric oxide and blood eosinophil count were associated with mild allergic asthma only in non-smokers.

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The fractional exhaled nitric oxide (FENO) and the blood eosinophil count (B-eos) are markers of eosinophilic inflammation used in the diagnosis and management of asthma. The relationships between smoking cigarette and both FENO and B-eos are complex and raise questions about the association between these markers and asthma in smokers. To determine the relationships between both FENO and B-eos on one hand and asthma and atopy on the other, according to smoking status. FENO and B-eos were measured in, respectively, 1579 and 1496 of the 1607 middle-aged adults randomly selected from the general population in the cross-sectional ELISABET survey. Allergic asthma was defined as asthma (a self-report of physician-diagnosed asthma, and wheezing in the previous 12months or the use of asthma medications) with atopy (allergic rhinitis or hayfever in the previous 12months, or a previous positive prick test or allergen desensitization therapy). Non-allergic asthma was defined as asthma without atopy. The analysis included 812 (51.4%) never, 473 (30%) former and 294 (18.6%) current smokers. A total of 490 (32%) participants were atopic, 80 (5.1%) had allergic asthma, and 31 (2%) had non-allergic asthma. Only 16.2% (18/111) of asthmatics were treated with glucocorticoid inhalants, suggesting that among them a majority of participants had mild asthma. A positive interaction between smoking status and allergic asthma was observed in multivariate models explaining FENO (P=0.003) and B-eos (P=0.001). Thus, compared to those without allergic asthma, participants with allergic asthma had higher FENO values (+63.4%, 95% CI=[39; 92]) and higher B-eos (+63.2% [38.2; 92.7]) in never and former smokers, but not in current smokers. Lastly, an analysis of receiver-operating characteristic curves showed that each of the two markers was able to discriminate moderately allergic asthma but only in non-smokers. FENO and B-eos were associated with the presence of mild allergic asthma only in non-smokers, not in current smokers. These findings raise questions about the clinical value of FENO and B-eos in smokers.