Asthma and FeNO testing explained

What is FeNO testing? And how it will aid clinicians in making better clinical decisions for asthma diagnosis.
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What is FeNO testing?

FeNO stands for Fractional exhaled Nitric Oxide.
It is a simple breath test that measures the level of Nitric Oxide Concentration that the body naturally produces in the airways.
In an asthmatic person, the bronchial tubes in the airways become inflamed. Nitric Oxide is produced at a higher than normal concentration. This can be used as a non-invasive biomarker of airway inflammation in asthma sufferers.1,2

The test is very simple to perform and easy to interpret with a single value reading in parts per billion (ppb).1,2 Unlike spirometry, it requires minimal patient compliance and respiratory effort.

A positive FeNO reading in conjunction with spirometry and peak flow tests can help confirm a diagnosis of asthma in a patient and/or support ongoing treatment for the disease.1

Infographic of Normal and Asthmatic airways

Why FeNO testing?

FeNO testing is shown to be superior to the majority of convential tests of lung function3
Whilst Spirometry and provocation testing methods are widely used in asthma assessments, these tests only really show a partial picture as they are indirectly associated with airway inflammation.4
Spirometry and peak flow measurement also rely heavily on patient compliance and the need for a good quality spirometry test. Results from spirometry and peak flow tests can effectively show that a patient has an obstruction, however conditions such as COPD or chronic cough may present symptoms that are similar to that found in asthma.5 Consequently this could lead to patients either being mis-diagnosed or not receiving the right treatment for the disease.6

The benefits of FeNO testing

Minimal respiratory effort required

This is ideal for testing young children and adults who can’t perform Spirometry in order to identify airway inflammation

A FeNO test is easy to interpret

A FeNO reading gives a single value in parts per billion (ppb) with an interpretation guide to aid clinical decision making


Provides a complementary tool to other ways of assessing airways disease, including asthma.

Manage mis-diagnosis of asthma

FeNO testing aids in identifying patients who do/do not require ongoing treatment.7


1. Ricciardolo F. Multiple roles of nitric oxide in the airways [Internet]. Multiple roles of nitric oxide in the airways. 2017 [cited 27 March 2017]. Available from:

2. Feng X. Fractional Exhaled Nitric Oxide in Relation to Asthma, Allergic Rhinitis, and Atopic Dermatitis in Chinese Children: Journal of Asthma: Vol 48, No 10 [Internet]. 2017 [cited 27 March 2017]. Available from:

3. Andrew D. Smith, Jan O. Cowan, Sue Filsell, Chris MacLachlan, Gabrielle Monti-Sheehan, Pamela Jackson and D. Robin Taylor. Diagnosing Asthma: Comparisons between Exhaled Nitric Oxide Measurements and Conventional Tests. Am J Respir Crit Care Med Vol 169. pp 473-478, 2004.

4. An Official ATS Clinical Practice Guideline: Interpretation of Exhaled Nitric Oxide Levels (FeNO) for Clinical Applications. Am J Respir Crit Care Med Vol 184. pp 602-615, 2011.

5. P R. Airway inflammation in patients with symptoms suggesting asthma but with normal lung function [Internet]. ERS Journal. 2017 [cited 27 March 2017]. Available from:

6. David T. Misdiagnosis of COPD and Asthma in Primary Care Patients 40 Years of Age and Over: Journal of Asthma: Vol 43, No 1 [Internet]. 2017 [cited 27 March 2017]. Available from:

7. D R Taylor, MW Pinenburg, A D Smith and J C D Jongste. Exhaled nitric oxide measurements: clinical application and interpretation. Thorax 2006;61:817-827.