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. If you have a high amount of this gas in your breath when you exhale, it may mean that you have inflammation. This is common for people with asthma, allergies, or eczema.

A doctor can use this information to help them diagnose certain types of asthma, but it’s important to note that a FeNO test by itself cannot diagnose asthma. Instead, the National Heart, Lung, and Blood Institute (NHLBI) says 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 control your inflammation, according to the Asthma and Allergy Foundation of America (AAFA). It may also help indicate how regularly you have 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. A 2016 study showed that people who had FeNO testing were less likely to have asthma attacks than people who did not have testing. Overall, the FeNO group had 41 percent fewer asthma attacks over the course of a year than the control group.

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
  • does not diagnose all types of asthma
  • cannot be used for kids under 5 years old
  • may cause short-term lightheadedness
  • can be expensive without insurance

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. The AAFA says it’s quick and painless and provides results immediately.

  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 and $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 American Thoracic Society defines the ranges as follows:

LowIntermediateHigh
Adult levelsunder 2525–50greater than 50
Children’s 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. The American Thoracic Society explains that 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.

In a 2017 review, other researchers explain that FeNO can be a useful tool for supporting an asthma diagnosis. They specify that it’s more useful with “ruling in” asthma than it is 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 in the hour before your test that are rich in nitrates, like beetroot and green leafy vegetables, as well as alcohol and caffeine.

When should you have a FeNO test?

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 to 3 months. It should be used along with other breathing tests, like spirometry, to give your doctor a full view of your lung function.

FeNO stands for fractional exhaled nitric oxide. The FeNO test is a common test to help measure the inflammation of your lungs, diagnose asthma, and monitor how well your current asthma treatments are working.

Ask your doctor about FeNO testing if you are having unexplained breathing issues or other symptoms that point to asthma. If you have health insurance, your provider may cover the cost of the 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 fuller understanding of your respiratory health and asthma symptoms.