FeNO is due to detect the concentration of nitric oxide in the exhaled breath, it is measured in ppb, which refers to how many particles of nitric oxide are present in each breath, normally it should not exceed 25 ppb, below 25 ppb is considered normal. Nitric oxide represents the level of inflammation associated with eosinophils in the airways and is especially seen in asthma or allergies, etc. It can be tested by breath to see if the patient’s value is normal or not. If this value exceeds 35 ppb or more, it is considered that it may be related to allergic asthma and appropriate anti-allergic and asthma treatment should be given in order to receive better results.