The fractional exhaled nitric oxide (FeNO) helps measure the inflammation of your lungs, diagnose asthma, and monitor how well your current asthma treatments are working.

FeNO (pronounced “fee-no”) stands for fractional exhaled nitric oxide. While this gas is found in the atmosphere, the body also produces nitric oxide when inflammation is present in the airways.

FeNO testing isn’t new — it’s been used in the diagnosis and management of asthma since the 1990s. The first commercial device was manufactured about 20 years ago. Today, devices that measure FeNO include NIOX VERO, Fenom Pro, and the NObreath FeNO Monitor.

Here’s more about what these tests measure, how they’re performed, and their accuracy.

FeNO tests measure the amount of fractional exhaled nitric oxide present in the airways. A doctor can use this information to help diagnose certain types of asthma, but it’s important to note that a FeNO test by itself cannot diagnose asthma.

Instead, a FeNO test helps to confirm an asthma diagnosis. The test can also help rule out similar conditions and predict how well your body may respond to the use of corticosteroids as asthma treatments.

How does the FeNO test help those with asthma?

If you already know you have asthma, FeNO tests can help your doctor track if your current treatment plan is working.

For example, the test can show whether your medications need to be increased or decreased in order to better manage your inflammation. It may also help indicate how regularly you’ve been taking your prescribed treatments.

FeNO tests may even help your doctor pinpoint what type of asthma you have, like allergic asthma or eosinophilic asthma. They can also give valuable information about your inflammation markers over time.

As with any medical test, there are certain pros and cons to FeNO testing. If you have concerns, consider discussing these points with your doctor before undergoing testing.

The biggest possible advantage is fewer asthma exacerbations. Research suggests that having FeNO testing may make it less likely to have asthma attacks. That said, there’s an inconclusive correlation between FeNO levels and asthma severity overall.

Other advantages of the FeNO test are that it:

  • takes only a few minutes
  • is noninvasive
  • has low or no side effects
  • requires very little preparation
  • provides results right away

Some disadvantages of the FeNO test are that it:

  • needs to be performed in a doctor’s office
  • cannot be used for kids under 5 years old
  • may cause short-term lightheadedness
  • can be expensive without insurance
  • doesn’t always definitively diagnose asthma since conditions other than asthma can lead to an elevated FeNO. These include allergic rhinitis, obstructive sleep apnea, and chronic obstructive pulmonary disease.

Preparation for the test is simple. For best results, according to Asthma+ Lung UK, make sure to avoid the following in the hour before your test:

  • eating foods that are rich in nitrates, like leafy greens
  • consuming any alcohol or caffeine
  • using your steroid or rescue inhaler

A FeNO test involves breathing slowly into a tube to measure your levels. It’s quick and painless, and it provides results immediately. Here are the steps for taking the test:

  1. Place clips onto your nose.
  2. Empty your lungs by breathing out completely.
  3. Place the device’s mouthpiece into your mouth and inhale slowly to fill your lungs.
  4. Exhale again slowly until your device beeps. Then repeat the slow inhale and exhale pattern as indicated by your device or doctor’s instructions.

You may feel momentarily lightheaded after breathing in slowly and deeply, but the test is safe. Tell your doctor if you don’t feel well. Sitting down and allowing your breathing to return to usual can help that feeling pass.

The cost of a FeNO test is usually somewhere between $2,000–3,000 without insurance, according to a 2019 study.

If you have insurance, the test may or may not be covered by your carrier. Aetna, for instance, has designated FeNO testing as medically necessary for its subscribers and covers it in part or in full, depending on your healthcare plan. Call your insurance carrier to find out if FeNO testing is covered under your plan as well as to find out your associated copay or deductible.

FeNO NIOX test manufacturer Circassia explains that people with Medicare and Medicare Advantage plans may also find that the costs are covered or reimbursed. This may happen if the test is deemed medically necessary by your healthcare professional.

Nitric oxide is measured in parts per billion (ppb). The following ranges are from the American Thoracic Society guidelines. Other countries or organizations’ ranges may vary.

Adult levelsunder 2525–50greater than 50
Children levelsunder 2020–35greater than 35
Airway inflammation present?unlikelypossiblelikely

Once they record a baseline value, your doctor can use this information to track your condition. Your follow-up readings may also help indicate how well your treatment is working.

For example, a significant decrease in your reading may be a good indication that your course of treatment is working well.

FeNO tests do have limitations.

Airway inflammation isn’t always directly associated with increased FeNO levels. If a person has been recently treated with inhaled steroids, they may receive a false negative or testing. It can be a useful tool for supporting an asthma diagnosis but may be more useful with “ruling in” asthma than with “ruling out” the condition.

Remember that other factors, like diet, can affect FeNO results as well. For the best accuracy, Asthma+ Lung UK recommends avoiding foods and beverages the hour before your test that are rich in nitrates, like beetroot and green leafy vegetables, as well as alcohol and caffeine.

Your doctor may recommend a FeNO test to help diagnose asthma, evaluate how well your asthma treatment is working, or do a routine follow-up. According to the NHLBI, this test is appropriate for adults and children ages 5 and up.

You might consider requesting a FeNO test if:

  • you don’t have an asthma diagnosis but do have regular respiratory symptoms
  • you have an asthma diagnosis, and your current treatments aren’t helping or aren’t helping enough
  • you have an asthma diagnosis, and your doctor wants to track your body’s response to inhaled corticosteroids or similar treatments
  • you have a family history of other allergy-related conditions, like eczema or allergic rhinitis

You can repeat the FeNO test as often as every 2–3 months. It should be used along with other breathing tests, like spirometry, to give your doctor a full view of your lung function.

What is the difference between spirometry and FeNO test?

A spirometry examines your lung function by measuring your ability to inhale and exhale, including the speed and ease of your exhalation. On the other hand, the FeNO test measures the amount of nitric oxide you release when exhaling.

Does asthma cause a high FeNO?

Based on a 2021 study, the recommended cut-off point for diagnosing asthma based on the FeNO test is 39.5 ppb, which falls into the intermediate range in terms of possible airway inflammation.

That said, different medical professionals may use a different cut-off, and may base it also on your overall health and the results of other tests.

What causes high nitric oxide levels in the lungs?

If you have a high amount of this gas in your breath when you exhale, it may mean that you have airway inflammation. This is common for people with asthma but can also be caused by allergies or COPD.

Ask your doctor about FeNO testing if you’re having unexplained breathing issues or other symptoms that point to asthma.

If you have health insurance, your provider may cover the cost of the FeNO test. But it’s a good idea to call ahead to make sure you have coverage.

FeNO isn’t the only type of breathing test, so keep in mind that your doctor may order other tests to get a better understanding of your respiratory health and asthma symptoms.