BACKGROUND AND AIM: Measurement of exhaled nitric oxide (NO) is useful in the diagnosis and management of asthma. The aims of this study were to investigate the effects of physiologic confounders on levels of fractional exhaled NO (FE(NO)) in healthy adults and to establish reference values of FE(NO) measured according to American Thoracic Society (ATS) guidelines. METHODS: FE(NO) was measured in 122 healthy nonsmoking subjects of 20 to 65 years with a chemiluminescence analyser using the single breath online technique and an exhalation flow of 50 mL/s. RESULTS: The geometric mean [SE] FE(NO) was 21.6[1.06] ppb in males and 16.3[1.07] ppb in females (p < 0.01). FE(NO) increased significantly with body size and spirometric indices. In a stepwise regression analysis, body weight was the only variable included in the model (r = 0.36, p < 0.0001) and explained gender differences in FE(NO). When weight-related variables, including BMI, BSA and dead space volume, were analysed in a stepwise regression model, dead space volume gave the best correlation with FE(NO) (R = 0.39; p < 0.0001). CONCLUSION: The present study estimated that mean FE(NO) in healthy Caucasian subjects of 20 to 65 years, measured according to ATS guidelines with the online single breath technique, ranges from 15 to 24 ppb depending on the body weight. We suggest that the volume of dead space may explain the effect of weight on exhaled NO. However, a substantial part of FE(NO) variability in normal subjects remains unexplained
