Fractional Exhaled Nitric Oxide (FeNO) Testing in Asthma: A Primary Care Guide

Fractional exhaled nitric oxide (FeNO) testing is a non-invasive, point-of-care breath test that measures eosinophilic airway inflammation. It has become an important objective tool in asthma diagnosis and management in primary care. The 2024 joint BTS/NICE/SIGN asthma guideline recommends FeNO as a first-line test to support asthma diagnosis in both adults and children, marking a major step forward in routine respiratory care.

FeNO provides an immediate result and can help GPs to:

• Support diagnosis of asthma alongside history, examination, and spirometry

• Predict likelihood of response to inhaled corticosteroids (ICS)

• Monitor steroid responsiveness and treatment adherence over time

• Guide treatment step-up or step-down decisions in eosinophilic asthma

Measuring FeNO gives you real-time information about airway inflammation, not just airflow, and helps you decide:

• Is there likely type-2 inflammation here?

• How likely is this patient to respond to inhaled corticosteroids (ICS)?

• Is current anti-inflammatory treatment working / being taken?

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• FeNO is reported in parts per billion (ppb) and acts as a surrogate marker of eosinophilic inflammation in the lower airways.

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However, FeNO should not be used in isolation. Results are influenced by factors such as atopy, smoking, infections, and recent ICS use. Understanding these limitations and integrating FeNO into a broader...