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In clinical practice, assessment of expiratory nitric oxide (F-ENO) may reveal eosinophilic airway inflammation in asthmatic and other pulmonary diseases. Currently, measuring of F-ENO is standardized to exhaled flow level of 50 ml s(-1), since the expiratory flow rate affects the F-ENO results. To enable the comparison of F-ENO measured with different expiratory flows, we firstly aimed to establish a conversion model to estimate F-ENO at the standard flow level, and secondly, validate it in five external populations. F-ENO measurements were obtained from 30 volunteers (mixed adult population) at the following multiple expiratory flow rates: 50, 30, 100 and 300 ml s(-1), after different mouthwash settings, and a conversion model was developed. We tested the conversion model in five populations: healthy adults, healthy children, and patients with COPD, asthma and alveolitis. F-ENO conversions in the mixed adult population, in healthy adults and in children, showed the lowest deviation between estimated FENO from 100 ml s(-1) and measured F-ENO at 50 mL s(-1): -0 center dot 28 ppb, -0 center dot 44 ppb and 0 center dot 27 ppb, respectively. In patients with COPD, asthma and alveolitis, the deviation was -1 center dot 16 ppb, -1 center dot 68 ppb and 1 center dot 47 ppb, respectively. We proposed a valid model to convert F-ENO in healthy or mixed populations, as well as in subjects with obstructive pulmonary diseases and found it suitable for converting F-ENO measured with different expiratory flows to the standard flow in large epidemiological data, but not on individual level. In conclusion, a model to convert F-ENO from different flows to the standard flow was established and validated.