Understanding Fractional Exhaled Nitric Oxide (FeNO)

Ever pondered what your breath could reveal about your health? Measurement of FeNO is a useful and non-invasive way to understand if you have a specific type of inflammation in your airways. In essence, FeNO measures a specific type of gas in your exhaled breath, shedding light on the inflammation within your airways.

The Science Behind FeNO

At the heart of FeNO testing is the measurment of nitric oxide (NO); the higher nitric oxide levels are in your breath the more airway inflammation exists.

The Biological Significance of FeNO

Fractional Exhaled Nitric Oxide serves as a biomarker for eosinophilic airway inflammation, a type of inflammation predominantly seen in asthma and some other allergic conditions. Nitric oxide is a gaseous molecule produced by various cells in the body, including those lining the respiratory tract. In the airways, NO is primarily generated by the endothelial cells and epithelial cells as a response to inflammatory stimuli.

Eosinophilic Inflammation: A Closer Look

Eosinophilic inflammation is characterized by the accumulation of eosinophils, a type of white blood cell, in the airway tissues. Eosinophils are part of the body’s immune response and play a crucial role in fighting off certain infections and in the development of allergic reactions. When the airways become inflamed due to an allergic reaction, eosinophils release inflammatory mediators that can cause airway constriction, mucus production, and further inflammation.

In asthma and other atopic diseases, eosinophilic inflammation is a key pathological feature. The detection of elevated FeNO levels indicates an active eosinophilic inflammation within the airways. This is particularly relevant for asthma management, as the presence of eosinophilic inflammation suggests a specific pathway of the disease that may respond well to corticosteroids or other anti-inflammatory treatments.

How Does FeNO Testing Work?

Preparing for Your Test

Prepping for a FeNO test is straightforward. You might need to avoid certain foods, drinks, or activities that could skew the results, ensuring the most accurate measurement of your FeNO levels. For example, you mustn’t drink coffee or smoke before your test.

During Your Test

You’ll breathe into a mouthpiece connected to the FeNO device for a few minutes. It’s quick and completely non-invasive. It requires sustained exhalation which some people struggle with, so don’t get despondent if you don’t get it right first time.

After Your Test

Right after the test, you’ll chat with your doctor about the results. Depending on what they find, you might discuss further tests or tweak your treatment plan.

FeNO’s Role in Respiratory Medicine

FeNO testing has revolutionised how doctors diagnose and manage asthma and other respiratory issues. It offers a peek into your airways’ inflammation, helping customise your treatment for better outcomes.

Clinical Applications of FeNO Measurement

The clinical utility of FeNO measurement extends beyond diagnosis. It also plays a crucial role in monitoring the effectiveness of anti-inflammatory treatment, particularly inhaled corticosteroids. A decrease in FeNO levels after the initiation of steroid therapy can indicate a successful reduction in airway inflammation. Conversely, persistently high FeNO levels may signal the need for a reassessment of the treatment regimen.

Who Should Consider a FeNO Test?

If coughing, wheezing, or shortness of breath troubles you, a FeNO test can be helpful. It’s also invaluable for those with known asthma to keep tabs on their condition.

Conditions Linked with High FeNO Levels


Asthma tops the list when it comes to conditions associated with elevated FeNO levels. This isn’t surprising, given that asthma involves eosinophilic inflammation of the airways. If you’re asthmatic, monitoring your FeNO can help manage your condition, especially in tailoring your treatment with inhaled corticosteroids.

Eosinophilic Bronchitis

This condition mimics asthma with one key difference: it doesn’t lead to the same level of airway restriction. However, like asthma, it features eosinophilic inflammation, which ups your FeNO levels.

Atopic Diseases

Conditions like allergic rhinitis fall into this category. They’re driven by an allergic response, often involving eosinophilic inflammation, thus elevating FeNO levels.


Chronic Obstructive Pulmonary Disease (COPD) usually brings to mind smoking-related lung damage. But some people with COPD also have asthmatic components, known as the asthma-COPD overlap syndrome (ACOS), which can raise FeNO levels.

Cystic Fibrosis:

Cystic fibrosis is primarily known for its impact on lung mucus, but it can also show elevated FeNO levels, potentially reflecting inflammation.

The Autoimmune Angle

Certain autoimmune diseases that target the lungs, such as Churg-Strauss syndrome, can lead to high FeNO levels due to the eosinophilic inflammation they cause.

What This Means for You

If you’ve got elevated FeNO levels, it’s a signal to take a closer look at your respiratory health, particularly if you have symptoms like wheezing, coughing, or shortness of breath. However, it’s crucial to remember that high FeNO levels aren’t a standalone diagnosis. They’re a piece of the puzzle, valuable for guiding treatment decisions, especially regarding anti-inflammatory medications.

Normal vs. Elevated FeNO Levels

What counts as “normal” for FeNO levels? Typically, anything below 25 parts per billion (ppb) in adults and 20 ppb in kids. Anything above these numbers could signal trouble and necessitate a deeper dive, particularly if greater than 40 ppb.

Limitations to Keep in Mind

Despite its benefits, FeNO testing isn’t foolproof. Diet, smoking, and certain medicines can affect your FeNO levels. Plus, it’s one piece of the asthma diagnosis puzzle, not the whole picture.

Frequently Asked Questions (FAQs)

  1. What’s the purpose of a FeNO test?
    • It measures nitric oxide to help diagnose and manage asthma and other respiratory conditions.
  2. How long does the test take?
    • Just a few minutes. It’s quick and painless.
  3. Can I eat before the test?
    • Yes, but avoid nitrates to ensure accuracy.
  4. Is the test uncomfortable?
    • Not at all. You simply breathe into a mouthpiece.
  5. How often should I get tested?
    • It depends on your health status. Your doctor will guide you.


Fractional exhaled nitric oxide testing is a powerful tool in the field of respiratory medicine, offering insights into airway inflammation. Whether you’re living with asthma or experiencing symptoms of a respiratory condition, a FeNO test can play a crucial role in diagnosing and managing your health. By understanding what FeNO is, how the test is performed, and how to interpret the results, you’re better equipped to navigate your respiratory health journey. If you have respiratory health concerns please get in touch with Dr Ricardo Jose.


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Disclaimer: The information provided in this article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your healthcare provider with any questions you may have regarding a medical condition or treatment

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