A fractional exhaled nitric oxide (FeNO) test is a type of lung function test that can help doctors diagnose certain inflammatory lung conditions, such as asthma.

The test measures levels of nitric oxide gas in exhaled air. Because cells produce nitric oxide in response to inflammation, high levels can indicate inflammation in the airways.

Medical professionals may use FeNO testing to diagnose and monitor asthma and to guide decisions about its management and treatment.

This article describes the FeNO test, including what it involves and whether it can help treat asthma. It also discusses the benefits of FeNO testing, how accurate it is, and what the results mean.

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A FeNO test is a type of lung function test that measures the level of nitric oxide gas present in the air a person exhales from their lungs.

Cells produce nitric oxide gas in response to inflammation relating to certain types of asthma, such as allergic or eosinophilic asthma.

As such, an allergist may request a FeNO test to diagnose or monitor asthma or determine the effectiveness of anti-inflammatory steroid medications.

According to the American Academy of Allergy Asthma and Immunology, a FeNO test involves exhaling slowly and steadily into a handheld FeNO device.

The device will then display the level of nitric oxide gas in the exhaled air, using a measurement of parts per billion (ppb).

As the American Lung Association (ALA) explains, the FeNO test takes around 5 minutes and involves the following steps:

  • Step 1: The person places a clip on their nose.
  • Step 2: They exhale completely to empty their lungs of air.
  • Step 3: The person places the mouthpiece of the FeNO device into their mouth and inhales slowly and deeply to fill their lungs with air.
  • Step 4: They then exhale slowly and steadily until they hear a beep or see a light come on to indicate that the test is complete.

According to the ALA, a person may need to repeat the FeNO test several times to confirm the results.

Although the FeNO test itself does not treat asthma, it can help medical professionals monitor the condition and decide on the right treatment.

According to the Asthma and Allergy Foundation of America, a FeNO test can help medical professionals track levels of airway inflammation over time.

This helps them determine whether anti-inflammatory steroid medications are working effectively and whether medication dosages need increasing or reducing.

According to the AAFA, a FeNO test is quick, noninvasive, and offers immediate results. It can support a diagnosis of asthma and helps doctors rule out conditions that may cause similar symptoms.

A 2017 review article notes that a FeNO test can also predict which individuals will respond favorably to inhaled corticosteroid therapy.

The United Kingdom’s National Health Service (NHS) notes some additional benefits of the FeNO test. These include:

  • improving accuracy in diagnosing asthma
  • improving people’s understanding of their diagnosis and its treatment
  • reducing the risk of worsening asthma and hospital admissions
  • reducing the inappropriate prescribing of corticosteroid treatments

A 2017 review and meta-analysis investigated the diagnostic accuracy of FeNO devices among individuals aged 5 years or older with suspected asthma.

The review concluded that the test has moderate accuracy in diagnosing asthma among children of this age.

The researchers added that FeNO testing may offer slightly more accurate results for those belonging to one or more of the following groups:

  • children
  • nonsmokers
  • those who have not previously taken corticosteroids

The guidelines for FeNO test results differ for adults and children.

Guidelines for adults

A 2019 study provides information on FeNO test results and the levels of exhaled nitric oxide that medical experts deem normal, intermediate, and high among adults.

These are as follows:

  • Normal: Below 25 ppb
  • Intermediate: Between 25 and 50 ppb
  • High: More than 50 ppb

According to the researchers, individuals with asthma typically produce a high FeNO reading.

Guidelines for children

The Allergy and Asthma Network (AAN) also provides the typical guideline FeNO readings for children. These are as follows:

  • Normal: Below 20 ppb
  • Intermediate: 20-35 ppb
  • High: More than 35 ppb

As with adults, high readings in children indicate airway inflammation that may be due to underlying asthma.

Are there things that can affect FeNO test results?

As the AAN explains, the following can affect FeNO test results:

  • age
  • gender
  • weight
  • height
  • diet
  • smoking and vaping

The AAN adds that people should avoid the following at least 1 hour before their FeNO test:

  • smoking
  • vaping
  • drinking alcohol or carbonated beverages
  • consuming caffeine
  • consuming highly processed foods

A doctor may use the results of FeNO testing to guide treatments for asthma.

According to the National Institutes of Health (NIH), doctors may recommend a FeNO test if a person:

  • has uncontrolled and persistent asthma and is also taking a corticosteroid alone or in combination with other medications
  • has symptoms that may require additional anti-inflammatory treatment
  • has a genetic predisposition to allergic diseases, such as:
    • asthma
    • allergic rhinitis
    • eczema
  • has a doctor who recommends monitoring the person’s asthma every 2–3 months

A fractional exhaled nitric oxide (FeNO) test assesses lung inflammation. Doctors may recommend it to help diagnose inflammatory lung conditions such as asthma.

They may also recommend the test to help monitor asthma or to guide decisions about asthma treatments.

The FeNO test is a noninvasive test that provides rapid results. Research suggests the test is moderately accurate in diagnosing asthma in individuals aged 5 years or older.

Certain factors can affect the results, including smoking and vaping.

Anyone needing further information on FeNO testing or asthma management can ask a doctor for further advice.