FeNO in clinical practice

Fractional exhaled nitric oxide (FeNO) testing is a non-invasive, point-of-care method for assessing eosinophilic airway inflammation, particularly useful alongside clinical history in diagnosing and managing asthma.^[1]

The joint guideline from the British Thoracic Society (BTS), Scottish Intercollegiate Guidelines Network (SIGN), and National Institute for Health and Care Excellence (NICE) highlights FeNO as a core objective test, especially valuable in primary care settings where rapid, accessible diagnostics are essential.^[1]

This article outlines practical guidance on how to perform FeNO testing in line with the updated national recommendations.

Clinical Role of FeNO Testing

According to the 2024 BTS/SIGN/NICE guideline, FeNO testing is recommended in the following clinical scenarios:^[2]

1. Diagnosis of asthma

FeNO or blood eosinophils is now listed as the first line test to carry out when the history suggests asthma to confirm the diagnosis:

• In adults (≥17 years) a FeNO level of ≥50 parts per billion (ppb) supports an asthma diagnosis when aligned with the clinical history
• In children and young people (5–16 years): a FeNO level of ≥35 ppb is considered positive.

If the result is normal, spirometry and reversibility should be carried out as the next test, if this is normal, proceed to a peak flow diary, followed by referral for a bronchial challenge if this is normal.

2. Monitoring Asthma Control^[2]

Consider fractional exhaled nitric oxide (FeNO) monitoring for adults with asthma:

• At their regular review, and
• Before and after changing their asthma therapy.

• When checking the FeNO level, if it is raised this may indicate poor adherence to treatment or the need for an increased dose of inhaled corticosteroid.

Assessing Medication Adherence^[3]

• Persistently raised FeNO despite inhaled corticosteroid (ICS) treatment may indicate poor adherence or continued exposure to environmental triggers.

Patient Preparation^[4]

Effective patient preparation is critical to obtaining accurate FeNO results. The NHS patient leaflet provides the following practical instructions.

• Avoid physical activity for at least one hour before the test
• Do not smoke within one hour of testing
• Avoid hot drinks, alcohol, and caffeine for at least one hour before testing
• Do not eat nitrate-rich foods (e.g. celery, beetroot, spinach, lettuce, leeks) for at least 3 hours before the test
• Continue taking regular medication but inform the clinician about any recent inhaler use or anything that might affect the result.

Performing the FeNO Test^[5][6]

The following steps outline how to perform FeNO testing in practice, based on manufacturer guidance and widely adopted clinical protocols:

1. Explain the Procedure:

• Let the patient know that the test is simple and non-invasive. It involves breathing into a handheld device at a controlled pace for approximately 6 seconds in a child (5-16) and 10 seconds in adults (17+)

2. Breathing Technique:
   The patient should-

• Inhale fully through the mouth (not the nose)
• Hold their breath briefly, and
• Exhale slowly and steadily into the FeNO device for 6 seconds in children, 10 seconds in adults

3. Device Use:

• Ensure correct flow using on-screen feedback
• If the first attempt is successful and meets the quality standards, a single reading may be sufficient. However, a second attempt is recommended in most cases to confirm reproducibility (two within 10% of each other) – particularly if the result is borderline or the patient is being assessed for initial diagnosis

4. Documentation:

• Record the FeNO level (in ppb), date/time, and any relevant clinical notes

Interpreting FeNO Results⁷

Interpretation should always consider the patient’s history, symptoms, and other investigations.⁷

Adults (≥17 years):

• <25 ppb: low- eosinophilic inflammation unlikely
• 25–50 ppb: intermediate- consider in clinical context
• >50 ppb: high- supports eosinophilic asthma

Children (<12 years):

• <20 ppb: low
• 20–35 ppb: intermediate
• >35 ppb: high — eosinophilic inflammation likely

Conclusion

FeNO testing is a valuable, evidence-based tool for diagnosing and managing asthma, especially in detecting eosinophilic airway inflammation and evaluating steroid responsiveness.

The 2024 BTS/SIGN/NICE guidelines strongly support its use in primary and secondary care, especially for patients aged 5 years and older.

When combined with clinical judgement and objective tests, FeNO measurement, I believe, enhances diagnostic confidence, supports adherence monitoring, and contributes to better asthma outcomes.

Any advice given and opinions expressed in this article are those of the author and do not reflect the view of Chiesi Limited (Chiesi).  All content in this article is for informational and educational purposes only.  Although Chiesi strives to always provide accurate information, it is not responsible for and does not verify for accuracy any of the information contained within.

[1] Wang K, Stonham C, Rutherford C, Pavord ID. Fractional exhaled nitric oxide (FeNO): the future of asthma care? Br J Gen Pract. 2023 Nov 30;73(737):565-568.

[2] Asthma: diagnosis, monitoring and management. Available at: https://www.nice.org.uk/guidance/ng245/resources/asthma-diagnosis-monitoring-and-chronic-asthma-management-bts-nice-sign-pdf-66143958279109

[3] Scenario: Follow-up of asthma. Available at: https://cks.nice.org.uk/topics/asthma/management/follow-up-of-asthma/

[4] Fractional exhaled nitric oxide (FeNO) testing – Patient information leaflet. Available at: https://www.england.nhs.uk/aac/wp-content/uploads/sites/50/2021/08/fractional-exhaled-nitric-oxide-feno-patient-leaflet.pdf

[5] Asthma and lung. FeNO test. Available at: https://www.asthmaandlung.org.uk/symptoms-tests-treatments/tests/feno-test

[6] Measuring FeNO. Available at: https://www.niox.com/en-us/feno-asthma/measuring-feno/

[7] Interpreting FeNO test results. Available at: https://www.niox.com/en-gb/digital-platform/interpreting-feno