A FeNO test is performed to understand what's happening in the airway in terms of inflammation. Spirometry, on the other hand, is a test that evaluates lung function. Where a FeNO test tells us how much nitric oxide is in your exhalation, spirometry tells us how fast and easy you exhale. In the diagnosis and treatment of asthma, it can be advantageous for your doctor to perform both of these tests. This achieves a more comprehensive view of the potential cause of respiratory symptoms.