A FeNO test for asthma is a simple, non-invasive breathing test that measures inflammation in the airways. It helps healthcare professionals investigate:
- suspected asthma
- understand certain types of airway inflammation and
- assess how patients may respond to treatment.
Unlike conventional lung function tests that measure how effectively air moves through the lungs, FeNO testing provides information about the inflammatory processes that can contribute to asthma symptoms.
And for respiratory specialists, asthma clinics and healthcare providers, FeNO testing offers an additional objective measurement that can support more informed clinical decisions.
What Is a FeNO Test?
FeNO stands for Fractional Exhaled Nitric Oxide. Nitric oxide is a gas naturally produced in the body as well as within the respiratory system.
In some people with asthma, especially those with Type 2 airway inflammation, increased nitric oxide production is associated with inflammatory activity in the bronchial airways.
A FeNO test measures the concentration of this gas in a person’s exhaled breath and the results are expressed in parts per billion (ppb).
The test is particularly useful because airway inflammation may be present even when symptoms are mild or lung function measurements appear relatively normal.
However, FeNO is not a universal measure of all forms of airway inflammation. Some patients with asthma have low FeNO readings, especially those with non-type 2 inflammation or those already receiving corticosteroid treatment.
How Does a FeNO Test Work?
FeNO testing generally involves breathing into a handheld or desktop measuring device.
The procedure is straightforward:
- The patient prepares to breathe into the testing device, following the manufacturer’s instructions.
- The patient inhales and then exhales slowly and steadily through a mouthpiece.
- The device analyses the concentration of nitric oxide in the exhaled breath.
- A numerical FeNO result is displayed in parts per billion.
The measurement itself generally takes only a few seconds, although preparation, repeat attempts and clinical interpretation may take longer.
What Does a FeNO Test Measure in Asthma?
Asthma is a chronic respiratory condition that is characterised by variable airflow limitation, airway hyperresponsiveness and inflammation.
Symptoms can include:
- wheezing,
- coughing,
- chest tightness and
- shortness of breath.
However, these symptoms are not exclusive to asthma, which makes accurate assessment important. FeNO testing helps identify type 2 airway inflammation, a pattern often associated with allergic asthma and eosinophilic inflammatory activity.
Elevated FeNO levels may suggest:
- Increased type 2 inflammation within the airways.
- A greater likelihood of responding to inhaled corticosteroid treatment.
- Persistent inflammatory activity in some patients with established asthma.
- A need to review treatment adherence, inhaler technique or allergen exposure, depending on the clinical circumstances.
A raised FeNO result does not automatically establish an asthma diagnosis; because allergic rhinitis, atopy and other factors can also influence readings.
The American Thoracic Society’s clinical practice guideline explains how FeNO measurements can support the interpretation of eosinophilic airway inflammation and likely corticosteroid responsiveness.
What Is a Normal FeNO Level?
FeNO values must be interpreted according to age, symptoms, medical history, medication use and the clinical purpose of testing.
The American Thoracic Society provides the following commonly referenced interpretive ranges:
| FeNO result | Adults | Children |
|---|---|---|
| Low | Below 25 ppb | Below 20 ppb |
| Intermediate | 25–50 ppb | 20–35 ppb |
| High | Above 50 ppb | Above 35 ppb |
Low values make eosinophilic inflammation and corticosteroid responsiveness less likely in the relevant clinical context. High values make these findings more likely.
Intermediate results require particularly careful interpretation.
These are interpretive ranges, and not universal diagnostic thresholds. Specific asthma diagnostic guidelines may use different cut-offs for particular populations or clinical pathways.
FeNO measurements should therefore be assessed by qualified healthcare professionals rather than used independently to diagnose asthma or change medication.
When Is a FeNO Test Useful?
1. Investigating Suspected Asthma
Patients experiencing recurring coughing, wheezing, chest tightness or breathing difficulties may require several investigations, and FeNO testing can provide supporting evidence of type 2 airway inflammation, particularly when asthma is suspected but the diagnosis remains uncertain.
It should be considered alongside medical history, spirometry, bronchodilator reversibility testing and other investigations where indicated.
The NICE asthma guideline (NG245) incorporates FeNO measurement into diagnostic assessment pathways, illustrating its recognised role in contemporary respiratory care.
2. Assessing Likely Response to Corticosteroids
Inhaled corticosteroids are central to the management of many forms of asthma because they reduce airway inflammation.
Patients with elevated FeNO measurements may be more likely to respond to corticosteroid therapy, although treatment decisions require consideration of the wider clinical picture.
FeNO therefore provides useful information when clinicians are evaluating inflammatory activity and possible treatment response.
3. Monitoring Airway Inflammation Over Time
Asthma symptoms and airway inflammation do not always change together.
A patient may report relatively few symptoms while inflammatory activity remains elevated. Conversely, breathing difficulties may occur without elevated FeNO.
Repeated measurements can help clinicians identify trends, assess treatment response and investigate persistent inflammation.
A change in FeNO is not, by itself, sufficient reason to increase or reduce medication. Results should be interpreted alongside symptoms, exacerbation history, lung function and adherence.
4. Supporting Difficult-to-Control Asthma Reviews
When asthma remains poorly controlled, clinicians may need to investigate several possible explanations, including incorrect inhaler technique, inconsistent treatment use, allergen exposure, comorbidities or alternative diagnoses.
FeNO testing can contribute to these assessments by providing information about type 2 inflammatory activity.
For specialist respiratory services, this may be particularly useful when determining whether symptoms are associated with ongoing inflammation or whether additional investigation is required.
FeNO Testing vs Spirometry: What Is the Difference?
FeNO and spirometry answer different clinical questions.
| FeNO testing | Spirometry |
|---|---|
| Measures nitric oxide in exhaled breath | Measures airflow and lung volumes |
| Provides information about type 2 airway inflammation | Helps assess airflow obstruction |
| May support assessment of corticosteroid responsiveness | Helps identify variable expiratory airflow limitation |
| Can support inflammatory monitoring | Can support diagnosis and monitoring of lung function |
Neither test replaces the other.
Using both, where clinically appropriate, can provide a more complete understanding of asthma than relying on symptoms or a single measurement alone.
Bosch Vivatmo – FeNO Measurement in Clinical Practice
Bosch Healthcare Solutions developed the Vivatmo range of FeNO measurement devices to support assessment and monitoring of airway inflammation.
Two relevant solutions include:
- Vivatmo Pro: A professional FeNO measurement system intended for use in healthcare settings, including respiratory clinics and medical practices.
- Vivatmo ME: A personal FeNO measurement device designed to support home monitoring, with readings that patients can discuss with their healthcare professionals.
These technologies demonstrate how exhaled-breath analysis can extend beyond specialist laboratory investigations into more accessible respiratory monitoring environments.
For healthcare organisations that are evaluating FeNO equipment, considerations include patient suitability, clinical workflow, measurement procedures, consumables, staff training, device maintenance and local regulatory availability.
Further information about the technology is available from Bosch Healthcare Solutions’ official Vivatmo website.
Is FeNO Testing Suitable for Every Asthma Patient?
Not necessarily.
FeNO testing is most informative when the clinical question concerns Type 2 airway inflammation or potential corticosteroid responsiveness.
Readings can be affected by:
- smoking,
- allergic conditions,
- respiratory infections,
- recent food intake,
- medication and other factors.
Furthermore, a normal FeNO reading does not exclude asthma, and a high reading does not prove that asthma is responsible for a patient’s symptoms.
For these reasons, FeNO should be viewed as a complementary clinical measurement and not a standalone diagnostic solution.
Looking for FeNO Testing for Your Healthcare Facility?
For respiratory clinics, hospitals and healthcare providers, incorporating FeNO testing may provide an additional way to assess airway inflammation and support asthma management.
At MediDiagAfrica, we explore diagnostic technologies that can help healthcare professionals make more informed decisions, including molecular diagnostics and respiratory assessment solutions.
If your organisation is considering FeNO measurement technology, we welcome enquiries about Bosch Vivatmo and its potential suitability for your clinical environment.
Interested in Vivatmo FeNO testing equipment?
Our aim is to help you explore the technology, understand relevant considerations and identify appropriate next steps.
Product availability, intended use and regulatory status may vary by country. FeNO testing should be used in accordance with applicable clinical guidelines and manufacturer instructions.