FENO (fractional exhaled nitric oxide) is the term given to NO measurements that comply with the guidelines set out by the American Thoracic Society and the European Respiratory Society.

The scientists at Aerocrine who were responsible for the discovery of the role of NO in airway inflammation worked closely with these bodies in the development of the guidelines (ATS/ERS guidelines 2005).  

Among other things, these guidelines state that FENO, expressed in parts per billion (ppb), must be performed at a controlled and standardized exhalation flow rate of 50ml/sec as FENO levels are highly flow dependent. The ATS/ERS guidelines also recommend that the patient inhales NO-free air prior to testing, and specify the correct exhalation time and how the NO content should be analysed.

Unlike other NO monitoring devices, the Aerocrine NIOX Flex and NIOX MINO, designed and manufactured by Aerocrine, meet all the requirements of the ATS/ERS guidelines. Specifically, the NIOX Flex and NIOX MINO are the only currently available devices that include a NO scrubbing device to ensure that the patient inhales NO-free air before testing.