Reference values of fractional excretion of exhaled nitric oxide among non-smokers and current smokers

BACKGROUND: Fractional exhaled nitric oxide (FE(NO)) is used to assess of airway inflammation; diagnose asthma and monitor adherence to advised therapy. Reliable and accurate reference values for FE(NO) are needed for both non-smoking and current smoking adults in the clinical setting. The present study was performed to establish reference adult FE(NO) values among never-smokers, former smokers and current smokers. METHODS: FE(NO) was measured in 5265 subjects aged 25–75 years in a general-population study, using a chemiluminescence (Niox ™) analyser according to the guidelines of the American Thoracic Society and the European Respiratory Society. Atopy was based on the presence of immunoglobulin E (IgE) antibodies to common inhalant allergens (measured using Phadiatop® test). Spirometry without bronchodilation was performed and forced vital capacity (FVC), forced expired volume in 1 s (FEV(1)) and the ratio of FEV(1) to FVC values were obtained. After excluding subjects with asthma, chronic bronchitis, spirometric airway obstruction and current cold, 3378 subjects remained. Equations for predictions of FE(NO) values were modelled using nonparametric regression models. RESULTS: FE(NO) levels were similar in never-smokers and former smokers, and these two groups were therefore merged into a group termed “non-smokers”. Reference equations, including the 5th and 95th percentiles, were generated for female and male non-smokers, based on age, height and atopy. Regression models for current smokers were unstable. Hence, the proposed reference values for current smokers are based on the univariate distribution of FE(NO) and fixed cut-off limits. CONCLUSIONS: Reference values for FE(NO) among respiratory healthy non-smokers should be outlined stratified for gender using individual reference values. For current smokers separate cut-off limits are proposed. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12890-017-0456-9) contains supplementary material, which is available to authorized users.