Asthma diagnosis, type 2 airway inflammation and treatment monitoring
Fractional Exhaled Nitric Oxide (FeNO): A Quick Guide
Fractional exhaled nitric oxide testing is a quick, non-invasive breathing test that measures nitric oxide in exhaled air. It helps identify type 2 or eosinophilic airway inflammation, supports the diagnosis of asthma and can provide useful information about likely responsiveness to inhaled corticosteroid treatment.
- FeNO testing
- Airway inflammation
- Eosinophilic asthma
- Asthma diagnosis
- Test preparation
- Result interpretation
- Inhaled corticosteroids
- Test limitations
FeNO identifies one inflammation pathway
A raised result supports type 2 or eosinophilic airway inflammation. FeNO does not measure every form of airway inflammation and does not diagnose every type of asthma.
Results must be interpreted in context
Symptoms, inhaled corticosteroid use, allergy exposure, smoking, spirometry and other objective tests all influence what a FeNO result means for an individual patient.
What is fractional exhaled nitric oxide?
FeNO is the concentration of nitric oxide measured in a controlled exhaled breath. The result is reported in parts per billion, abbreviated to ppb.
Nitric oxide is a naturally occurring signalling molecule produced in several parts of the body. Within the respiratory tract, airway epithelial cells can produce increased nitric oxide when particular inflammatory pathways are active.
FeNO is most useful as a biomarker of type 2 airway inflammation, which frequently involves eosinophils and inflammatory signals such as interleukin-4 and interleukin-13. This pattern is common in allergic and eosinophilic asthma.
The test does not directly count eosinophils and does not provide a complete measurement of all airway inflammation. It gives one clinically useful piece of information that must be considered alongside symptoms and other lung-function tests .
A raised FeNO result suggests that a type 2 inflammatory pathway is active and that corticosteroid-responsive inflammation may be present, particularly in a symptomatic patient who has not already started inhaled corticosteroids.
The science behind FeNO
In type 2 airway inflammation, immune signals stimulate inducible nitric oxide synthase within airway epithelial cells. This increases nitric oxide production, allowing more nitric oxide to be detected in exhaled breath.
- Inflammatory trigger Allergens or other biological signals activate a type 2 immune response in a susceptible airway.
- Airway signalling Cytokines such as interleukin-4 and interleukin-13 stimulate airway epithelial cells.
- Increased nitric oxide The airway lining produces more nitric oxide through inducible nitric oxide synthase.
- Exhaled measurement A controlled breath allows the concentration of nitric oxide to be measured in parts per billion.
What are eosinophils?
Eosinophils are white blood cells involved in immune defence and allergic inflammation. In eosinophilic airway disease they can accumulate within the airways and release inflammatory substances, contributing to mucus production, airway sensitivity and variable narrowing.
What does a raised FeNO level indicate?
A raised FeNO result increases the likelihood that type 2 or eosinophilic airway inflammation is present. It does not by itself prove that a patient has asthma.
Eosinophilic asthma
Raised FeNO is commonly associated with asthma driven by type 2 inflammation and may predict a better response to inhaled corticosteroids.
Allergic airway disease
Allergic sensitisation and ongoing allergen exposure can increase FeNO, particularly when allergic asthma is also present.
Eosinophilic bronchitis
Non-asthmatic eosinophilic bronchitis can cause chronic cough and eosinophilic inflammation without the variable airflow obstruction typical of asthma.
Inadequately treated inflammation
A high result in established asthma may reflect insufficient anti-inflammatory treatment, poor inhaler technique or inconsistent adherence.
Continuing allergen exposure
Exposure to a relevant allergen can keep FeNO elevated even where symptoms are relatively mild.
Less common eosinophilic disease
Certain systemic eosinophilic disorders may be associated with raised FeNO, but FeNO is not a diagnostic test for those conditions.
A high FeNO result is not specific to one diagnosis
The result must be interpreted alongside the clinical history, examination, spirometry, blood eosinophils, allergy information and response to treatment.
How does FeNO testing work?
FeNO testing is quick, painless and non-invasive. The test uses a handheld or laboratory-based analyser connected to a disposable filtered mouthpiece.
- Sit comfortably The test is generally performed while seated after the preparation instructions have been checked.
- Breathe out normally You may first be asked to empty your lungs in a relaxed and controlled way.
- Take a deep breath through the device The device provides nitric-oxide-free or filtered air for the inhalation.
- Exhale slowly and steadily You breathe out through the mouthpiece at a controlled flow, often guided by an image, sound or indicator on the screen.
- Maintain the exhalation Adults usually need a steady exhalation lasting several seconds. Some people need more than one attempt to achieve the correct flow.
- Receive the result The analyser displays the nitric oxide concentration in parts per billion almost immediately.
Do not worry if the first attempt is unsuccessful
The test depends on maintaining a specific exhalation pressure and flow. It is common to need repeated attempts while learning the technique, particularly for younger patients or people with significant breathlessness.
Preparing for a FeNO test
Preparation instructions vary between laboratories. Follow the instructions supplied by the clinic performing your test, as these take priority over general advice.
| Before testing | Why it may matter | Practical advice |
|---|---|---|
| Eating and drinking | Recent food and drink can temporarily alter the reading | Follow the laboratory’s preparation instructions and mention anything recently consumed |
| Nitrate-rich food | Beetroot and some green vegetables can temporarily increase nitric oxide measurements | Avoid them for the period specified by the testing centre |
| Caffeine and hot drinks | These may affect measurement conditions in some protocols | Avoid them for the interval stated in your appointment instructions |
| Smoking or vaping | Smoking can suppress FeNO and produce a misleadingly low result | Avoid smoking or vaping beforehand and report your usual exposure accurately |
| Exercise | Recent exertion can affect the reading | Avoid vigorous physical activity shortly before testing |
| Respiratory medication | Inhaled corticosteroids can lower FeNO and affect interpretation | Do not stop prescribed treatment unless the testing service specifically instructs you to do so |
| Recent infection | A recent respiratory illness may alter airway inflammation | Tell the clinician about recent colds, infections and antibiotic or steroid treatment |
Do not independently stop asthma medication before the test
Inhaled corticosteroids may reduce FeNO, but withholding treatment can destabilise asthma. Only alter medication when the clinician or respiratory laboratory responsible for the test has given specific instructions.
The role of FeNO in asthma diagnosis
Asthma is diagnosed using a compatible clinical history supported by objective evidence. No single symptom or test is sufficient in every patient.
Clinical history
Variable symptoms
Asthma commonly causes episodes of wheeze, cough, breathlessness or chest tightness that vary over time.
Recognisable triggers
Exercise, allergens, viral infections, cold air and occupational exposures may provoke symptoms.
Night or early-morning symptoms
Nocturnal coughing or wheezing can support the history but is not specific to asthma.
Objective respiratory testing
FeNO
Supports the presence of type 2 airway inflammation and may help predict corticosteroid responsiveness.
Spirometry
Measures airflow and can identify variable airflow obstruction when repeated after a bronchodilator.
Further tests
Peak-flow monitoring, blood eosinophils or bronchial challenge testing may be considered when uncertainty remains.
Alternative explanations
Upper-airway disorders
Rhinitis, sinus disease and inducible laryngeal obstruction can cause symptoms resembling asthma.
Other lung disease
Infection, COPD, bronchiectasis and other respiratory conditions may cause cough or breathlessness.
Non-respiratory causes
Cardiac disease, anaemia, deconditioning and dysfunctional breathing may also require consideration.
Learn more about specialist asthma diagnosis and treatment .
In adults with a history suggestive of asthma, current joint BTS/NICE/SIGN guidance recommends diagnosing asthma when FeNO is 50 ppb or more. In children aged 5 to 16, the diagnostic threshold is 35 ppb or more. A result below these thresholds does not independently exclude asthma.
Understanding FeNO test results
In clinical practice, adult FeNO results can be divided into practical interpretation bands of below 25 ppb, 25–40 ppb and above 40 ppb. These bands describe the likelihood of type 2 or eosinophilic airway inflammation. They should not be confused with the formal BTS/NICE/SIGN adult diagnostic threshold of 50 ppb.
Below 25 ppb
Type 2 or eosinophilic airway inflammation is less likely at the time of testing. Asthma is not excluded, particularly in people already using inhaled corticosteroids, current smokers or those with non-eosinophilic asthma.
25–40 ppb
This is an intermediate result. It should be interpreted alongside symptoms, allergy exposure, inhaled corticosteroid use, blood eosinophils, spirometry and bronchodilator reversibility.
Above 40 ppb
Type 2 or eosinophilic airway inflammation is more likely. Corticosteroid-responsive airway disease should be considered in the appropriate clinical setting.
Above 40 ppb is elevated, but 50 ppb is the adult diagnostic threshold
A FeNO result above 40 ppb is clinically elevated and increases the likelihood of type 2 airway inflammation. However, current BTS/NICE/SIGN guidance uses 50 ppb or more as the threshold that supports diagnosing asthma in an adult with a suggestive clinical history.
Values between 41 and 49 ppb remain clinically relevant but do not independently meet that particular diagnostic threshold. Spirometry with bronchodilator reversibility and other objective testing may therefore still be required.
Children aged 5 to 16
Current BTS/NICE/SIGN guidance uses a FeNO level of 35 ppb or more to support diagnosing asthma in a child aged 5 to 16 with a suggestive clinical history. Adult interpretation bands should not simply be applied to children.
A lower FeNO result does not rule out asthma
FeNO can be low in non-eosinophilic asthma, in current smokers, after inhaled or oral corticosteroid treatment, or when airway inflammation is controlled on the day of testing. Current BTS/NICE/SIGN guidance advises further objective testing when asthma remains clinically suspected.
FeNO for monitoring asthma treatment
FeNO can sometimes help monitor type 2 airway inflammation after asthma has been diagnosed. Serial results are generally more useful when interpreted against the patient’s own previous values and treatment history.
Response to corticosteroids
A fall after starting or improving inhaled corticosteroid treatment can support a reduction in type 2 inflammation.
Inhaler adherence
Persistently high FeNO can prompt a supportive discussion about whether preventer treatment is being taken consistently.
Inhaler technique
Poor technique may prevent adequate corticosteroid delivery and contribute to continuing inflammation.
Allergen exposure
FeNO may remain raised when a relevant allergen continues to drive inflammation despite prescribed treatment.
Treatment adjustment
FeNO may contribute to a treatment review, but medication should not be increased or reduced solely from one reading.
Severe asthma phenotyping
FeNO may form part of the biomarker assessment used when evaluating difficult or severe type 2 asthma.
FeNO should be interpreted alongside asthma symptoms, attacks, reliever use, adherence, inhaler technique, spirometry, blood eosinophils and exposure to relevant allergens.
Conditions and factors associated with altered FeNO
| Condition or factor | Possible FeNO pattern | Interpretation |
|---|---|---|
| Type 2 or eosinophilic asthma | Frequently raised | Supports eosinophilic airway inflammation but still requires clinical correlation |
| Non-asthmatic eosinophilic bronchitis | May be raised | Can cause chronic cough without the variable airflow obstruction typical of asthma |
| Allergic rhinitis or atopy | May be raised | Allergy can influence FeNO even when lower-airway symptoms are limited |
| COPD | Variable | A raised result may identify type 2 inflammatory features in a subset of patients but does not independently diagnose asthma |
| Smoking | Often lower | Smoking may suppress FeNO and complicate interpretation |
| Inhaled or oral corticosteroids | Often lower | Treatment can normalise FeNO despite an underlying history of eosinophilic asthma |
| Bronchiectasis or infection | Variable | FeNO is not a primary diagnostic test for bronchiectasis or bacterial chest infection |
| Cystic fibrosis | Often normal or low | FeNO is not routinely used to diagnose or monitor cystic fibrosis lung disease |
| Systemic eosinophilic disorders | May be raised | FeNO is not sufficiently specific to diagnose a systemic eosinophilic condition |
FeNO testing in chronic cough
FeNO may be useful when assessing chronic cough because asthma and non-asthmatic eosinophilic bronchitis can both involve corticosteroid-responsive airway inflammation.
Cough-variant asthma
Some people have cough as the predominant asthma symptom with little obvious wheeze.
Eosinophilic bronchitis
This can cause persistent cough with eosinophilic inflammation but without classic variable airflow obstruction.
Other cough mechanisms
Reflux, rhinitis, infection, medication, structural lung disease and cough hypersensitivity may produce cough with a normal FeNO result.
Combined testing
Spirometry, chest imaging and other investigations may be required according to the duration, severity and associated symptoms.
Read more about specialist investigation of chronic cough .
Limitations of FeNO testing
It is not a standalone diagnosis
A raised result supports one inflammatory mechanism but does not independently prove asthma.
A low result does not exclude asthma
Non-eosinophilic asthma and treated asthma may produce a normal or low FeNO result.
Results vary with treatment
Inhaled and oral corticosteroids can reduce FeNO, sometimes before diagnostic testing takes place.
Smoking may suppress results
Current smoking can lower FeNO and make eosinophilic inflammation harder to detect.
Allergens can raise results
Ongoing exposure to a relevant allergen can increase FeNO and must be considered during interpretation.
Technique must be controlled
The result depends on exhaling at the correct flow, so testing should follow a standardised procedure.
Do not adjust treatment from a FeNO number alone
Clinical interpretation bands and formal diagnostic thresholds have different purposes. Treatment decisions should consider the patient’s symptoms, attacks, inhaler technique, adherence, spirometry and other objective results.
Who may benefit from FeNO testing?
Suspected asthma
Patients with variable wheeze, cough, breathlessness or chest tightness may benefit as part of an objective diagnostic pathway.
Persistent unexplained cough
FeNO may help identify an eosinophilic airway component within a wider chronic-cough assessment.
Uncertain asthma control
Testing may help determine whether continuing type 2 inflammation is contributing to symptoms or attacks.
Apparent treatment failure
Persistently high results can prompt review of adherence, inhaler technique, allergen exposure and the diagnosis.
Difficult or severe asthma
FeNO can contribute to inflammatory phenotyping and specialist treatment assessment.
Children able to perform the manoeuvre
FeNO is non-invasive and can often be completed by children from school age with appropriate coaching.
What happens after the test?
The result is available immediately, but the appropriate next step depends on why testing was performed.
- Review the clinical history The clinician relates the result to cough, wheeze, breathlessness, triggers and symptom variability.
- Check current treatment Any inhaled corticosteroid, oral steroid or other respiratory medicine is considered when interpreting the value.
- Review other objective tests Spirometry, bronchodilator reversibility, peak-flow records, blood eosinophils or allergy testing may be required.
- Confirm or reconsider the diagnosis A raised result may support asthma, while discordant results may lead to further respiratory investigation.
- Agree the treatment plan Treatment is selected according to the overall diagnosis and clinical risk, not the FeNO value alone.
- Plan follow-up Repeat testing may be helpful in selected patients when assessing treatment response or persistent inflammation.
When symptoms need urgent assessment
Do not wait for a FeNO test during a serious asthma deterioration
- severe or rapidly worsening breathlessness;
- difficulty speaking in complete sentences;
- blue or grey lips;
- exhaustion, confusion or collapse;
- rapidly worsening wheeze or chest tightness;
- poor response to the reliever treatment in your action plan;
- a markedly reduced peak-flow reading;
- severe chest pain or coughing blood.
Follow your written asthma action plan. Call 999 for severe breathing difficulty, collapse or another immediately life-threatening symptom.
Conclusion
Fractional exhaled nitric oxide is a quick, painless and non-invasive test that measures nitric oxide in a controlled exhaled breath.
A raised FeNO level supports the presence of type 2 or eosinophilic airway inflammation and may indicate a greater likelihood of responding to inhaled corticosteroid treatment.
In adult clinical practice, results may be interpreted using bands of below 25 ppb, 25–40 ppb and above 40 ppb. A level above 40 ppb is clinically elevated and makes eosinophilic inflammation more likely.
These interpretation bands are separate from the current BTS/NICE/SIGN asthma diagnostic threshold. In an adult with a suggestive history, FeNO of 50 ppb or more supports diagnosing asthma. In children aged 5 to 16, the threshold is 35 ppb or more.
A lower FeNO result does not exclude asthma. Results may be reduced by inhaled corticosteroids and smoking, while allergy exposure and eosinophilic airway disease can raise them.
FeNO should therefore be interpreted alongside symptoms, spirometry, bronchodilator reversibility, blood eosinophils, treatment history and other relevant investigations.
Where cough, wheeze, breathlessness or chest tightness persists, a specialist asthma assessment can establish whether FeNO and other lung-function tests are appropriate.
Frequently asked questions
What is the purpose of a FeNO test?
It measures nitric oxide in exhaled breath to help identify type 2 or eosinophilic airway inflammation. It can support asthma diagnosis and treatment monitoring.
Does a high FeNO result prove that I have asthma?
No. It supports eosinophilic airway inflammation and may strengthen an asthma diagnosis when the clinical history is compatible, but other conditions and allergy exposure can also raise FeNO.
Can I have asthma with a normal FeNO result?
Yes. FeNO may be normal in non-eosinophilic asthma, after corticosteroid treatment, in smokers or when inflammation is well controlled.
What does a FeNO result below 25 ppb mean?
It makes active type 2 or eosinophilic airway inflammation less likely at the time of testing, but it does not rule out asthma.
What does a FeNO result between 25 and 40 ppb mean?
It is an intermediate result. Interpretation depends on symptoms, inhaled corticosteroid treatment, smoking, allergy exposure, blood eosinophils and other objective tests.
What does a FeNO result above 40 ppb mean?
It is clinically elevated and makes type 2 or eosinophilic airway inflammation more likely. It does not independently confirm asthma.
Why does NICE use 50 ppb for adults if above 40 ppb is elevated?
Above 40 ppb is a practical clinical band indicating elevated FeNO. The joint BTS/NICE/SIGN guideline uses the more specific threshold of 50 ppb or more to support formally diagnosing asthma in an adult with a suggestive history.
What FeNO level supports asthma diagnosis in a child?
Current BTS/NICE/SIGN guidance uses 35 ppb or more in children aged 5 to 16 with a clinical history suggestive of asthma.
How long does the test take?
The breathing manoeuvre takes only seconds and the overall test commonly takes a few minutes, although repeat attempts may be required.
Is FeNO testing painful?
No. It involves breathing through a filtered mouthpiece and requires no needles, radiation or internal instruments.
Should I stop my inhalers before the test?
Do not stop prescribed inhalers unless the respiratory laboratory or clinician arranging the test specifically tells you to do so.
Can smoking affect a FeNO result?
Yes. Smoking can suppress FeNO and lead to a lower reading. Tell the clinician about smoking, vaping and recent exposure.
Can FeNO show whether my preventer inhaler is working?
A fall in FeNO may support reduced type 2 inflammation after corticosteroid treatment, but symptoms, attacks, inhaler technique and other measurements must also be reviewed.
References and further information
- National Institute for Health and Care Excellence, British Thoracic Society and Scottish Intercollegiate Guidelines Network. Asthma: diagnosis, monitoring and chronic asthma management. NICE guideline NG245. View the joint BTS/NICE/SIGN asthma guideline
- British Thoracic Society. BTS/NICE/SIGN joint guideline on asthma: diagnosis, monitoring and chronic asthma management. View the BTS guideline page
- Dweik RA, Boggs PB, Erzurum SC, et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels for clinical applications. American Journal of Respiratory and Critical Care Medicine. 2011;184:602–615. View the ATS interpretation guideline
- NHS England. Fractional exhaled nitric oxide: your asthma inflammation test explained. View the NHS patient leaflet
- London Chest Specialist. Asthma diagnosis and treatment. Read about specialist asthma care
- London Chest Specialist. Lung-function tests. Read about respiratory diagnostic testing
- London Chest Specialist. Chronic cough specialist assessment. Read about chronic-cough investigation
