FeNO test (exhaled nitric oxide)
A simple breathing test that measures a gas in your breath to look for a type of inflammation in the airways linked to asthma.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- A FeNO test measures a gas in your breath that can rise when a certain type of airway inflammation is present, often linked to asthma.
- A normal result does not rule out asthma, and a raised result does not prove it on its own — it is read alongside your symptoms and other tests.
- It is quick and comfortable: you breathe out steadily into a small handheld device for several seconds, and results are often available straight away.
- Some foods, smoking, caffeine and recent exercise can change the reading, so follow the preparation advice and ask what your number means for you.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Adds objective information to help decide whether asthma is likely, rather than relying on symptoms alone.
Children under about five or anyone unable to follow the steady breathing manoeuvre may not get a reliable reading.
You breathe out steadily for several seconds; the number appears on the machine within moments.
Your actual ppb number, explained in plain language alongside your symptoms.
You breathe out steadily for several seconds; the number appears on the machine within moments.
No restrictions — you can eat, drink, drive and return to work or school as normal.
Results are often discussed with you immediately, or at the same appointment as your other tests.
If the picture is unclear, your clinician may arrange other tests (such as spirometry, reversibility testing or...
What is a FeNO test?
A FeNO test measures the amount of nitric oxide gas in the air you breathe out. Nitric oxide is made in larger amounts when there is a particular kind of inflammation in the airways. This inflammation is often, but not always, linked to asthma.
The test is mainly used to help work out whether breathing symptoms such as cough, wheeze or breathlessness might be caused by asthma. It is one of several tests, and UK guidance suggests using it alongside your history and other tests rather than on its own.
A FeNO test cannot, by itself, prove or rule out asthma. A high reading makes one type of asthma more likely, but a normal reading does not mean your airways are healthy, because some kinds of asthma do not raise nitric oxide. Your clinician puts the result together with your story and other tests.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Options at a glance
These are the main approaches described in this guide. The right option depends on the diagnosis, your goals and what your clinician thinks is safe.
Standard handheld FeNO test
The usual test, where you breathe out steadily into a small portable machine that gives a reading in parts per billion (ppb).
FeNO to help diagnose asthma
Used when asthma is suspected, to add information alongside your symptoms, spirometry, reversibility testing or peak flow.
FeNO to help guide treatment
Sometimes repeated over time to help judge airway inflammation and how well inhaled steroid treatment is working, or whether you are using it as prescribed.
FeNO in children
Most children over about five can do the test. A slightly lower reading is used as the cut-off in children than in adults.
Preparing for your test
- Do not smoke or vape on the day of the test, as this changes the reading.
- Avoid strenuous exercise for at least an hour beforehand.
- Avoid caffeine, alcohol and hot drinks for at least an hour before the test.
- Avoid nitrate-rich foods such as green leafy vegetables and beetroot for at least three hours beforehand.
- Keep taking your usual medicines and inhalers unless your clinician tells you otherwise — but mention everything you take, including steroid inhalers.
- Tell the team if you have a cold or chest infection, as this can affect the result.
- Allow a little extra time in case the test needs repeating to get a reliable reading.
What happens
The test is done sitting down and takes only a few minutes. You take a deep breath in with your mouth open, then breathe out slowly and steadily into a filtered mouthpiece for several seconds, keeping a constant gentle flow. A small machine measures the nitric oxide in your breath and shows a number in parts per billion (ppb).
You may be asked to repeat the breath once or twice so the team can be sure the reading is reliable. There are no needles, no radiation and nothing uncomfortable beyond having to breathe out at a steady pace, which some people find takes a try or two to get right. A physiologist, nurse or doctor usually runs the test.
Is this test right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- Children under about five or anyone unable to follow the steady breathing manoeuvre may not get a reliable reading.
- It is the wrong test if the main question is airway narrowing rather than inflammation — spirometry or reversibility testing is needed for that.
- It should not be relied on to rule out asthma in someone with a convincing history but a normal result.
- It cannot assess how severe an asthma attack is and must not delay urgent treatment of acute breathlessness.
Delay or rearrange if…
- You have a current cold or chest infection, which can change the reading.
- You have just smoked or vaped, exercised hard, or had caffeine, alcohol or nitrate-rich food.
- You are acutely breathless and need urgent treatment rather than testing.
- You cannot yet provide a list of the inhalers and medicines you take, which affect interpretation.
Alternatives to discuss
- Spirometry and bronchodilator reversibility testing to look at airway narrowing.
- Peak flow monitoring at home to look for variability over time.
- Blood eosinophil count as another marker that can support an asthma diagnosis.
- A bronchial challenge test in specialist settings when other tests are inconclusive.
- Watchful waiting with symptom review if the picture is mild or unclear.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Adds objective information to help decide whether asthma is likely, rather than relying on symptoms alone.
- Quick, usually not painful and non-invasive, with no needles or radiation.
- Results are often available immediately, helping decisions to be made sooner.
- Can support a decision to start, continue or step up inhaled steroid treatment.
- Repeating it over time can help show how airway inflammation is responding to treatment.
Risks & complications
- No physical harm from the test itself; at most, light-headedness from the breathing manoeuvre that quickly passes.
- A reading that is hard to interpret on its own and needs other tests to make sense of it.
- Needing to repeat the breath because the first attempt was not steady enough.
- A falsely reassuring (low) result in someone who does have asthma — particularly non-allergic types that do not raise nitric oxide.
- A raised result from causes other than asthma, such as allergic rhinitis (hay fever).
- A result lowered by smoking, recent food or drink, or taking inhaled steroids, which can mislead if not accounted for.
- Inability to complete the test because of a severe cough or breathlessness on the day.
- Spread of a respiratory infection if equipment and filters are not properly cleaned (modern single-use filters make this very unlikely).
The main pitfall with FeNO is reading it in isolation. A normal number does not rule out asthma, and a high number does not prove it. Smoking, hay fever, recent food, caffeine and inhaled steroids can all shift the reading. Ask your clinician what your specific number means in the context of your symptoms and other tests.
Published figures to discuss
FeNO is a low-risk, non-invasive test, so meaningful complication rates are not the issue — accuracy is. Readings are affected by smoking, hay fever, recent food and drink, inhaled steroids, infections and the way the breath is performed. The test is more useful for supporting a diagnosis than excluding one, because some asthma does not raise nitric oxide, so false-reassurance is the key limitation rather than a numerical adverse-event rate.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Physical harm from the test | Very low | FeNO is a breathing test with no radiation, needle, medicine or sedation. | Guide sourcesClinical context |
| Threshold supporting asthma diagnosis | NICE uses FeNO 35 ppb or more as a positive test in children and young people in the right clinical context | Adult pathways and combined testing differ; FeNO should be interpreted with symptoms, spirometry and bronchodilator reversibility. | NICE NG245 — Asthma: diagnosis, monitoring and management (recommendations)nice.org.ukSource-linked context |
| False low result | Recognised | Smoking, recent steroid treatment and some asthma phenotypes can lower FeNO, so a normal result does not exclude asthma. | Guide sourcesClinical context |
| False high result | Recognised | Allergic rhinitis, eczema, eosinophilic inflammation and recent exposures can raise FeNO without proving asthma on their own. | Guide sourcesClinical context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is no physical recovery. You can return to normal activities, work, eating and driving straight away. The main thing that 'happens afterwards' is getting and understanding your result.
- No symptoms or after-effects from the test itself.
- Feeling you needed a practice attempt to breathe out at the right steady pace.
- Waiting to put the result together with other tests before anything changes.
- No change to how you feel — the test measures inflammation, it does not treat it.
Aftercare
- Ask for your actual number in parts per billion (ppb), not just 'normal' or 'high'.
- Ask how the result fits with your symptoms and any other tests.
- Keep taking your inhalers and medicines as prescribed.
- Keep a note of the result so it can be compared if the test is repeated later.
- Ask what would happen next if the result is high, normal or borderline.
- Mention if you smoke, vape or have hay fever, as these affect interpretation.
- Follow any plan made for further tests or a treatment trial.
- A list of all your inhalers, medicines and supplements
- A note of recent smoking, vaping or hay fever symptoms
- Avoided caffeine, alcohol and hot drinks for an hour beforehand
- Avoided green leafy vegetables and beetroot for three hours beforehand
- A note of your main symptoms and when they happen
- Any previous FeNO, spirometry or peak flow results
- Questions about what the number means for you
⚠ Get urgent help if…
- Worsening breathlessness, wheeze or chest tightness that is not relieved by your usual reliever inhaler — seek urgent medical help.
- A reliever inhaler that is needed far more often than usual or is not lasting.
- Being too breathless to speak in full sentences, eat or sleep — this is an emergency.
- Lips or fingertips turning blue, or feeling drowsy or confused — call 999.
- A high FeNO result that is being ignored without any plan to review your symptoms or treatment.
- Being reassured by a normal FeNO despite ongoing significant breathing symptoms — ask for further assessment.
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
A FeNO result is given as a number in parts per billion (ppb). In adults, UK guidance treats a level of 50 ppb or more as supporting a diagnosis of asthma when the history fits; in children aged 5–16, a level of 35 ppb or more is used. Lower or borderline numbers do not rule asthma out.
A 'good' result is one your clinician can interpret clearly alongside your symptoms and other tests. The test cannot tell you how severe asthma is on a given day, cannot replace spirometry or peak flow, and cannot guarantee that your airways are healthy if the number is low.
A FeNO reading reflects airway inflammation around the time of the test, so it is a snapshot rather than a permanent result. It can change with treatment, infections, allergen exposure and the seasons. If it is used to guide treatment, it is often repeated over time, and your clinician will advise how often retesting is useful for you.
Related tests, treatments or support
FeNO is rarely used alone. It is commonly combined with spirometry, bronchodilator reversibility testing, peak flow monitoring and sometimes blood tests (such as eosinophil counts) or allergy testing, so that a diagnosis rests on more than one piece of evidence.
Follow-up & long-term care
Your result is usually discussed at the same visit or a follow-up appointment. Depending on the number and your symptoms, the next step may be further breathing tests, a trial of inhaled treatment, or a plan to repeat FeNO later to track inflammation.
- Repeat FeNO testing may be arranged to monitor inflammation or check response to inhaled steroids.
- Continue inhaled treatment as prescribed, as stopping it can raise the reading.
- Bring up-to-date results to reviews so trends can be seen over time.
Repeat, follow-on and what comes next
- Readings may need repeating if the breath is not steady enough or the first result is borderline.
- A result may be reinterpreted once smoking, hay fever, infection or inhaler use is taken into account.
- FeNO is often repeated over time to track inflammation rather than treated as a one-off answer.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- Your actual ppb number, explained in plain language alongside your symptoms.
- A clear plan for what each possible result means and what happens next.
- Combination with other tests rather than a diagnosis from FeNO alone.
- A named route to ask questions and arrange any repeat testing or follow-up.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- Whether FeNO is done alone or as part of a fuller breathing-test session (with spirometry, reversibility or peak flow).
- Who performs and interprets the test — for example a respiratory physiologist, nurse or consultant.
- Whether a consultation and explanation of results are included in the price.
- Whether further tests or a follow-up appointment are needed.
- The clinic or hospital setting and location.
- Whether the FeNO test, the appointment and the interpretation of results are all included.
- Whether any other breathing tests are bundled in or charged separately.
- Who will perform the test and who will explain the result.
- What happens, and what it costs, if the test needs repeating.
- Whether a follow-up appointment or onward referral is included.
- The cancellation and rescheduling policy.
On the NHS? FeNO testing is recommended in UK asthma guidance and is increasingly available on the NHS; private testing may be chosen for speed, convenience or as part of a fuller respiratory assessment.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- Being told a normal result rules out asthma when it does not.
- A raised result triggering treatment without checking for hay fever or other causes.
- No explanation of how food, smoking, caffeine or inhalers may have affected the reading.
- Using FeNO alone, without the symptoms and other tests needed to make sense of it.
Marketing red flags
- Claims that a single breath test can definitively diagnose or exclude asthma.
- Promising an instant asthma diagnosis from one number with no clinical assessment.
- Selling FeNO as a standalone 'lung health check' that catches all breathing problems.
- Quoting a normal result as proof of healthy lungs.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- What was my exact FeNO number in ppb, and how does it compare with the cut-off for my age?
- How does this result fit with my symptoms and my other breathing tests?
- What happens next if my result is high, normal or borderline?
- Could smoking, hay fever or my inhaler have affected my reading?
- Will repeating this test help track my treatment, and how often should it be done?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my test, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this test not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
Is the FeNO test available on the NHS?
Does the test hurt?
Can a normal FeNO rule out asthma?
Why was I told not to eat certain foods first?
Can children have a FeNO test?
Will my inhaler affect the result?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: Asthma + Lung UK — FeNO test NICE NG245 — Asthma: diagnosis, monitoring and management (recommendations) NHS England — Fractional exhaled nitric oxide (FeNO) FeNO in asthma care — review (PMC)
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
Related guides: Reversibility testing · Peak flow monitoring · Bronchial challenge (provocation) test · Body plethysmography (lung volume test) · Bronchial and transbronchial biopsy