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What is FeNO?

FeNO stands for fractional exhaled nitric oxide and is used to measure the level of airway inflammation in patients' lungs.
FeNO testing is safe and easy to perform, even when lung function is severely reduced, and can help improve outcomes in asthma patients from the age of four.1

The most convenient biomarker of airway inflammation

By accurately measuring airway inflammation at the point-of-care, FeNO testing can transform asthma care with more accurate diagnosis and better management.

See how to perform a FeNO test >


One test, one number. Airway inflammation in a single FeNO level.


Performing a FeNO test takes approximately one minute - quicker than making a cup of coffee.


Taking a FeNO test is as simple as inhaling and exhaling: no blood, sweat or tears required.

A picture from inside the international space station of a window that looks out on Earth

The history of nitric oxide

NO was first described as endogenous in exhaled breath in 1991. Within 10 years, it was shown that FeNO levels were elevated in asthma and decreased with corticosteroids. This was a real breakthrough, offering a new biomarker with high sensitivity and specificity, helping accurately diagnose airway inflammation.2

NO was named Science magazine’s Molecule of the Year in 1992 and its discovery as a signalling molecule won the 1998 Nobel Prize in Physiology or Medicine. Recognising the importance of FeNO in asthma, in 1999, NIOX® became the first commercially available FeNO testing device. The rise in popularity of the technology since then has been truly meteoric, even being used in lung health research on the International Space Station.

The science behind FeNO

Healthy airway epithelial cells produce NO and low levels of exhaled NO are normal.2 When Type 2 inflammation (the type of inflammation responsible for up to 84% of asthma cases3) is present in the airways, interleukins such as IL-4 and IL-13 upregulate the activity of the iNOS (inducible Nitric Oxide Synthase) enzyme, which produces NO in the airway.4 Levels of exhaled NO are increased, giving healthcare professionals an objective measure of airway inflammation.2

A diagram demonstrating what normal Epithelial cells look like
A diagram demonstrating what activated Epithelial cells look like

Trusted by thousands of healthcare professionals

FeNO testing is recommended in most national and international guidelines as it helps improve asthma patient outcomes.

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1. Menzies-Gow A et al. Clinical utility of fractional exhaled nitric oxide in severe asthma management. Eur Respir J. 2020;55(3):1901633. 2. 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. 3. Heaney LG et al. Eosinophilic and noneosinophilic asthma: an expert consensus framework to characterize phenotypes in a global real-life severe asthma cohort. Chest. 2021;160(3):814-30. 4. Munakata M. Exhaled nitric oxide (FeNO) as a non-invasive marker of airway inflammation. Allergol Int. 2012;61(3):365-72.