When used with well established techniques such as spirometry, FENO measurements can help in the differential diagnosis of eosinophilic bronchial asthma. Although the mechanisms are poorly understood, there is a large body of evidence to suggest that eosinophilic inflammation is a good predictor of corticosteroid success. As a result, measurements with FENO can help avoid inappropriate or ineffective therapy.
• Exhaled air from asthmatic subjects contains levels of NO 2–4 times higher than those in normal subjects
• FENO measurement has been shown to be superior to the majority of conventional tests recommended in international guidelines for diagnosing asthma in symptomatic patients.
Although the processes involved are poorly understood, patients with ongoing eosinophilic inflammation can be expected to respond to steroid treatment. Since FENO measurements are strongly linked to the level of eosinophilic inflammation, they can be used for the immediate and non-invasive identification of steroid responsiveness.
In fact, FENO measurements correctly identify steroid responders better than spirometry or PEF variability. Even if patients have non-specific asthma symptoms, one can predict with confidence that steroid treatment will work if their exhaled NO levels are high. [Smith 2004]. Read more here.
The converse is also true: normal FENO measurements in patients with symptoms indicate there is probably no underlying eosinophilic inflammation. For these patients other reasons for the symptoms need to be investigated and other forms of therapy than steroids considered.