FeNO Test for Asthma: What your Results Mean

A FeNO test for asthma measures nitric oxide in your breath, helping your clinician identify a type of airway inflammation that often responds to steroid inhalers. Your result is reported in parts per billion (ppb). It needs to be interpreted alongside your symptoms, medicines and other test results.

For people having an asthma assessment in London or Surrey, the useful questions are: is the result raised, could treatment have affected it, and what should happen next?

What is a FeNO test?

FeNO stands for fractional exhaled nitric oxide. You may also see the measurement described as the fractional concentration of nitric oxide in exhaled breath, or the test called a nitric oxide breath test.

Nitric oxide is a gas your body produces naturally. Higher levels in exhaled breath can indicate inflammation associated with eosinophils, a type of white blood cell. This is often part of what clinicians call type 2 inflammation.

FeNO testing can help investigate suspected asthma and monitor inflammation in people already receiving treatment. It does not measure your blood oxygen level or how much air you can blow out.

FeNO test results: what do the numbers mean?

There are two related questions: whether your result helps diagnose asthma, and what it tells your clinician about airway inflammation.

Current UK thresholds for diagnosing asthma

The BTS, NICE and SIGN asthma guideline recommends the following thresholds when the clinical history suggests asthma:

Age group	FeNO result	What it means in the UK diagnostic pathway
Adults and young people over 16	50 ppb or more	Meets the FeNO criterion for diagnosing asthma with a suggestive clinical history.
Adults and young people over 16	Below 50 ppb	Does not confirm asthma through FeNO. Further tests may be needed.
Children aged 5 to 16	35 ppb or more	Meets the childhood FeNO criterion with a suggestive clinical history.
Children aged 5 to 16	Below 35 ppb	Does not exclude asthma. Further assessment may be needed.

These are diagnostic thresholds, not a scale of asthma severity. Children’s results require age-appropriate assessment; adult cut-offs should not be applied to them.

Why do some reports use low, intermediate and high ranges?

The American Thoracic Society’s interpretation guideline uses these adult bands to assess the likelihood of eosinophilic inflammation and a response to corticosteroids:

Adult FeNO level	General interpretation
Below 25 ppb	Eosinophilic inflammation is less likely. Asthma remains possible.
25 to 50 ppb	Intermediate: symptoms, treatment and previous readings matter.
Above 50 ppb	Eosinophilic inflammation is more likely; corticosteroid treatment may help.

These bands answer a different question from the UK diagnostic pathway. Exactly 50 ppb meets the current UK adult diagnostic threshold, provided the clinical history fits. A result should never be interpreted from a band alone.

What causes high nitric oxide levels in the lungs?

Raised FeNO can occur with eosinophilic asthma, but asthma is not the only explanation. Relevant factors include:

• Allergic conditions, including hay fever, and exposure to allergens to which you are sensitive.
• Eosinophilic bronchitis, which can cause a persistent cough without the typical variable airflow narrowing of asthma.
• Ongoing inflammation despite prescribed treatment, sometimes because doses are missed or the inhaler technique prevents enough medicine reaching the airways.

The British Thoracic Society’s statement on chronic cough also explains that smoking can lower FeNO and viral infections can affect it in different ways. Tell your clinician about smoking, allergies, recent infections and all inhalers you use.

A raised result does not prove that someone has missed their medication. The result starts a discussion about possible causes.

Can you have asthma with a normal FeNO result?

Yes. A low or apparently normal FeNO result does not rule out asthma. Some forms of asthma do not produce much of the inflammation that this test detects. Inhaled corticosteroids can also lower the reading, including when symptoms still need investigation.

If asthma remains suspected, the next steps can include spirometry before and after a bronchodilator, a peak flow diary, or specialist bronchial challenge testing. A blood eosinophil count may also help.

Persistent symptoms deserve follow-up even when one test is reassuring. The assessment should consider other causes of cough or breathlessness as well as asthma and wheeze.

FeNO versus spirometry: why might you need both?

Test	What it assesses
FeNO	A marker of a particular type of airway inflammation.
Spirometry	How much air you breathe out and how quickly, helping identify airflow obstruction.
Bronchodilator reversibility	Whether spirometry improves after medicine that opens the airways.

These tests provide different information. Normal spirometry between episodes does not automatically exclude asthma, and a raised FeNO result does not measure the degree of airway narrowing.

How is a FeNO test done, and how long does it take?

You usually sit down and breathe through a mouthpiece connected to a small FeNO monitor. After taking a deep breath in, you breathe out steadily as instructed, usually for around 10 seconds. The device or clinician helps you keep the right flow.

You may need to repeat the breath to obtain an acceptable measurement. The test is non-invasive and does not involve needles or radiation.

The FeNO measurement itself usually takes about five minutes. Allow longer for your appointment if it includes a consultation, spirometry or other lung function tests. A device can display a reading quickly, but when you receive the clinical interpretation depends on the service.

How to prepare for FeNO testing

Follow your appointment instructions, particularly if several breathing tests are booked together. NHS England’s FeNO patient guidance advises avoiding:

• Strenuous activity and smoking for at least one hour beforehand.
• Hot drinks, caffeine and alcohol for at least one hour beforehand.
• Nitrate-rich foods, such as beetroot and green leafy vegetables, for at least three hours beforehand.

For FeNO alone, regular medicines can generally be taken as usual. Do not stop a prescribed inhaler on your own to obtain an untreated reading. If spirometry or reversibility testing is also planned, the clinic may give different instructions about particular inhalers. Ask if anything is unclear.

Bring your inhalers or a medication list. Tell the testing team about any recent chest infection and anything you have taken that morning. If breathing symptoms worsen, use your prescribed treatment and seek advice rather than withholding medicine to complete a test.

What happens after your FeNO assessment?

If you are being investigated for asthma, your clinician should explain whether the result supports a diagnosis, whether further testing is needed and what treatment is appropriate.

If you already have asthma, FeNO can contribute to a treatment review. A raised reading may prompt a check of medication use and inhaler technique, followed by a treatment adjustment or specialist referral if needed.

A falling reading after treatment can suggest that airway inflammation is improving. Your symptoms, night waking, reliever use and asthma attacks still matter. A low number alone is not a reason to stop treatment.

Before leaving, ask:

• What does this result mean given the medicines I already take?
• Do I need another test or a repeat FeNO measurement?
• What is the treatment plan, and when will it be reviewed?
• What should I do if my breathing gets worse?

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