What is FeNO in Asthma?


Photo of the Niox-Vero device used to measure FeNO in a person. One of the tests used for people with asthma is a fractional exhaled nitric oxide (FeNO) test. Your body makes nitric oxide (a gas), which gets released when you breathe out or exhale. A FeNO test measures the level of nitric oxide in your breath. Elevated levels of exhaled nitric oxide indicate airway inflammation.

Why is FeNO elevated in asthma?

Asthma begins with inflammation. The lining of the airways becomes swollen, inflamed and clogged with mucus and fluid.

Asthma inflammation is the quiet part of asthma. When we talk about quiet asthma, that’s when the inflammation and swelling takes place. You can’t feel or see what’s going on, but each time your airways are exposed to your asthma triggers, the inflammation may increase, and your symptoms may get worse.

High FeNO levels indicate your airways are inflamed or irritated. This is why FeNO testing is a valuable tool in testing for asthma.

While an elevated FeNO test does not confirm a diagnosis of asthma, it does indicate that you may be experiencing airway inflammation or irritation. This, along with your medical history, physical exam and other tests, should help your doctor determine if you have asthma.

Illustration of nurse testing man for asthma using a medical device.

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What is a normal FeNO level?

In clinical practice, a normal FeNO test is any number less than 25 parts per billion (ppb) in adults. For children, anything less than 20 ppb is considered normal.

What does a high FeNO test mean?

You may be asking, what causes a high FeNO? Just because you have an elevated FeNO does not necessarily mean you have asthma. What it does suggest is that you may be experiencing some level of airway inflammation. And airway inflammation is often an indicator of asthma.

Your healthcare provider will want to look at your symptoms, history, and other tests for asthma to make a diagnosis.

What is FeNO in asthma patients?

A FeNO of 25-50 ppb in adults (20-35 ppb in children) is considered an intermediate range. Numbers in this range should be matched with the medical history and testing. FeNO numbers of over 50 ppb in adults (over 35 ppb in children) are considered high and indicate airway inflammation.

FeNO testing is also a helpful tool in ongoing asthma management. Patients with well-controlled asthma typically have lower levels of exhaled nitric oxide. While patients with uncontrolled asthma may have a number of over 30 ppb, FeNO levels may also be elevated during asthma exacerbations.

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FeNO vs. spirometry

Many patients with asthma are familiar with spirometry testing for asthma. You may be asking what is the difference between FeNO and spirometry?

Spirometry tests your lung function. It measures your inhalation, exhalation, and how fast and easy you can exhale. Lung function tests are helpful in measuring how well your lungs are functioning. In comparison, FeNO looks at what amount of nitric oxide you are exhaling and can indicate levels of airway inflammation.

Both are useful tools in diagnosing asthma and determining asthma control.

How is a FeNO test done?

FeNO tests are performed at a doctor’s office. The FeNO measurement device consists of a mouthpiece with a tube attached to a measurement device. You breathe through a mouthpiece and exhale slow and steady. The device then measures the amount of FeNO you exhale. You often repeat the test a few times, but it should take less than 5 minutes.

Are there things that can affect FeNO test results?

Like most other tests, results can vary depending on several factors. Age, gender, weight, and height may impact FeNO results.

One thing to consider is that diet and smoking are factors that can impact results. You should avoid smoking, vaping, alcohol, caffeine, carbonated drinks and highly processed foods at least an hour prior to your test. If you know you will be undergoing a FeNO test, check with your doctor’s office ahead of time to get specific instructions.

Illustration concept of a pulmonary team working on lungs. The doctors are very small and testing different parts of the lungs. 

Are there other diseases that cause elevated FeNO levels?

FeNO tests are still primarily associated with asthma testing and monitoring, but there are other conditions that may cause elevated levels of exhaled nitric oxide. These include:

Who should undergo FeNO testing?

FeNO testing is used routinely in asthma diagnosis and management and guidelines are frequently updated. FeNO should be used in children over the age of 4 and adults. The latest guidelines indicate that FeNO testing should be used in people with asthma who may need treatment. This is in addition to the usual tests performed prior to treatment. FeNO testing should not replace those tests.

It is important to remember that FeNO testing is just one tool in diagnosing and assessing asthma management. No single tool confirms a diagnosis or measures the effectiveness of your asthma management plan.

Work closely with your healthcare provider and follow up regularly to assess how well your treatment is improving your asthma symptoms.

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Are there other conditions that may look like asthma or complicate asthma?

There are other types of respiratory conditions that are different than asthma.  The symptoms, diagnosis and treatment can vary depending upon the condition. Here are some of them.

Reviewed by:
Bradley Chipps, MD, FACAAI, is a board-certified allergist and pediatric pulmonologist with Capital Allergy and Respiratory Disease Center in Sacramento, California. He earned his medical degree from University of Texas Medical Branch in Galveston in 1972. He is Past President of the American College of Allergy, Asthma and Immunology (ACAAI).