An inflammatory marker in exhaled breath

What is FenO?

The abbreviation FeNO stands for fractional exhaled nitric oxide (NO).

NO molecules
Schematic illustration of molecules in exhaled breath (blue = NO)

Nitric oxide, or NO, is a colorless gas that is produced in different parts of the body, such as the blood vessels, the nervous system, and the lungs. As a bioactive molecule, NO can have a variety of effects. It plays an important role in the immune defense system and in various inflammatory processes. In the airways, NO is mainly produced by the epithelial cells of the mucous membrane. The highest concentrations occur in the paranasal sinuses. In the bronchial system, by contrast, the concentrations are significantly lower. NO produced in the respiratory tract is released into the atmosphere with exhaled breath. The NO concentration in the air is measured in “parts per billion” (ppb).1

  • NO develops during allergic inflammation in the respiratory tract.
  • The NO concentration can be measured in exhaled breath.
  • There is a proven relationship between the NO concentration in exhaled breath and the extent of airway inflammation– the higher the NO content, the more severe the inflammation!

FeNO measurement

To detect NO produced in the lower airways, the exhaled breath has to be fractionated. The measurement of fractional exhaled NO – known as FeNO – has been standardized for clinical application.

Quick and stress-free FeNO testing thus permits a reliable statement to be made about the degree of airway inflammation in asthmatics.

FeNO concentration is therefore an important parameter for determining the extent of airway inflammation in allergic asthma. Since measurement is relatively easy to perform and does not stress the patients, it is increasingly used as an additional diagnostic and progression parameter.

An uncomplicated, standardized, and reliable method of diagnosing airway inflammation

Benefits of FeNO measurement

The many benefits of FeNO measurement in the doctor’s practice have already been demonstrated in numerous scientific studies:


Diagnosing allergic asthma

FeNO testing allows your physician to precisely determine the extent of airway inflammation. The FeNO result is obtained using a simple breath test, allowing your doctor to get to the bottom of the cause of your asthma.


Optimized therapy management

FeNO measurement helps you to optimally adjust your therapy to your current state of health in consultation with your doctor, ensuring you only take as much medicine as you actually need.


Better overview of disease progression

Regular FeNO measurement allows you to monitor the extent of respiratory inflammation. Managing the treatment of asthma with the help of regular FeNO testing can help to reduce acute asthma episodes. 2,3 A worsening of the inflammation is indicated by increased FeNO values, allowing your doctor to adjust your therapy at an early stage.4

What do the FeNO values mean?

NO concentration in exhaled breath is measured in “parts per billion” (ppb). That is roughly equivalent to five brown blades of grass on an entire soccer field.

According to scientific findings, normal FeNO values depend, among other things, on height and gender.

  • Women usually have lower values than men
  • Children lower values than adults

As a guide:

  • Values below 25 ppb are considered normal (children below 20 ppb) and
  • Values above 50 ppb (children above 35 ppb) indicate active inflammation of the airways.

The American Thoracic Society (ATS), based in New York, is a leading international society in the field of respiratory disease, critical illness, and sleep disorders. Their guideline states that values between 0 and 25 ppb are low, between 25 and 50 ppb are in the mid-range and above 50 ppb are high.

The renowned British National Institute for Health and Care Excellence (NICE) also publishes guidelines for the application of treatments and procedures. The current Asthma Directive states that FeNO values above 40 ppb in adults (above 35 ppb in children) are high.

In general, your doctor makes the diagnosis and decides on a therapy plan based on reference values.

FeNO interpretation table
Interpretation table according to recommendation of the American Thoracic Society (ATS)

1 Informationsblatt der Deutschen Atemwegsliga e.V. FeNO-Messung https://www.atemwegsliga.de/informationsmaterial-asthma.html

2 Petsky et al. Cochrane Database of Systematic Reviews 2016;9:CD011440

3 Essat et al. Eur Respir J 2016;47:751-68

4 Bodini A et al. Chest 2017;132:1520-25