Background: The measurement of fractional exhaled nitric oxide (FENO) is used as a marker of airway inflammation. The aim of this study was to establish reference values of FENO for adults.
Methods: FENO at a flow rate of 50 mL/s was analyzed in 3,376 adults using a chemiluminescence analyzer according to American Thoracic Society/European Respiratory Society guidelines. Blood samples were analyzed, and atopy was defined as the presence of specific IgE. All subjects responded to a respiratory questionnaire. Those who had never smoked (n = 1,803) were selected for this study. After the exclusion of subjects with physician-diagnosed asthma, asthma symptoms, ever wheezing, dry cough, or use of inhaled steroids, 1,131 healthy never-smokers remained, including 845 nonatopic and 286 atopic subjects.
Results: Based on multiple regression modeling, we propose the following reference equation for healthy never-smoking adults: Ln(FENO) = 0.057 + 0.013 × height (in centimeters) + 0.0088 × age (in years). The residual SD was 0.51, and the explanatory value was 9%. In a model, based on nonatopic subjects alone, the reference equation obtained was slightly different, as follows: Ln(FENO) = −0.0026 + 0.013 × height (in centimeters) + 0.010 × age (in years). The residual SD for this equation was 0.48, and the explanatory value was 11%.
Conclusions: Normal values of FENO for adults may be predicted on the basis of age and height. However, as the reference equations only account for about 9 to 11% of the variation, the most important information that could be extracted from the study is that the upper limits of FENO range from 24.0 to 54.0 parts per billion, depending on age and height.