Exhaled nitric oxide, more commonly referred to as fractional exhaled nitric oxide (FeNO), is a noninvasive test used to assess eosinophilic airway inflammation. FeNO levels tend to be higher in some people with asthma and often decrease in response to inhaled corticosteroids, making the test useful in selected patients to help guide treatment decisions and monitor response to anti-inflammatory therapy. FeNO is not considered diagnostic on its own, but may provide supportive information when asthma is suspected.