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Interpreting FeNO

Use our online FeNO interpretation tool to understand your patient's FeNO results.

Diagnosis       Management

Patient age:

The world's most established cut-off points

In 2011, the American Thoracic Society (ATS) produced a guideline recommending the cut-off points still used every day to interpret FeNO today.1 The cut-off points were derived from a large number of studies, where 83% of participants used NIOX® devices.2 NIOX® technology remains the FeNO device of choice in clinical studies to this day.

Cut-off points for diagnosis

FeNO testing can accurately assess the level of airway inflammation, now recognised as a key characteristic of asthma. Consequently, absolute FeNO values matters when suspecting an asthma diagnosis.

A FeNO value of >50 ppb in an adult or >35 ppb in a child shows that airway inflammation is likely and the patient should respond well to treatment with inhaled corticosteroids (ICS).1 A diagnosis of asthma is 7x more likely when FeNO >40 ppb.3

If the FeNO reading is low (<25 ppb in adults and <20 ppb in children), asthma is unlikely and other diagnoses may be considered, such as COPD, GERD or cardiac disease. In addition, the patient is unlikely to respond to ICS.1

Personal FeNO baseline for management

Once a diagnosis of asthma has been confirmed, it is more relevant to consider the variation in FeNO results between visits, instead of strict thresholds. This variation should be included as part of ongoing asthma management.1

Each patient has an individual baseline FeNO value. It is important to note that the baseline can vary according to whether the patient is currently receiving inhaled corticosteroids (ICS) as ICS reduce FeNO levels.

At follow-up tests, a variation in FeNO >20% for baseline values over 50 ppb or >10 ppb for values under 50 ppb is considered significant. A significant decrease shows a meaningful response to therapy.1

Access our guide for the interpretation of FeNO levels with NIOX VERO®.


FeNO testing, a great educational tool

FeNO results can be used to educate patients about the importance of taking prescribed medication regularly. Patients who track their FeNO levels with their physician over time have shown great interest in reducing the value to a healthier point and, with a clear number on the test device screen, are able to visualise the benefit of taking their medication correctly.

Listen to Dr Russell, Clinical Director of the West Hampshire Integrated Respiratory Service in the UK, as he describes his experience of using FeNO and the benefits in patient education.

Factors influencing FeNO

Some external factors may influence FeNO levels. The effects are usually modest and transient. However, to get the most accurate results, FeNO testing should be performed prior to spirometry and it is recommended that patients refrain from eating (especially nitrate-rich foods), drinking, smoking and exercising one to two hours before a FeNO test.4

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1. Dweik RA et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FeNO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602-15. 2. Data on file MKT-DOF-001. Circassia Group Plc. 3. Wang Z et al. Agency for Healthcare Research and Quality (AHRQ). The clinical utility of fractional exhaled nitric oxide (FeNO) in asthma management. Comparative Effectiveness Reviews, 197. 2017. 4. NIOX VERO User Manual 000191-14. 5. Mansur AH et al. Disconnect of type 2 biomarkers in severe asthma; dominated by FeNO as a predictor of exacerbations and periostin as predictor of reduced lung function. Respir Med. 2018;143:31-8. 6. Wang K et al. Using fractional exhaled nitric oxide to guide step-down treatment decisions in patients with asthma: a systematic review and individual patient data meta-analysis. Eur Respir J. 2020;55(5):1902150. 7. Petsky HL et al. Tailoring asthma treatment on eosinophilic markers (exhaled nitric oxide or sputum eosinophils): a systematic review and meta-analysis. Thorax. 2018;73(12):1110-9.

* Hypothetical patients