Nitric oxide (NO) is a molecule your body makes that helps with many processes, like widening blood vessels and fighting off germs. Your lungs also make nitric oxide. Doctors can measure the amount of nitric oxide in your breath in a test called fractional exhaled nitric oxide (FeNO) testing.
FeNO testing can tell your doctor about the level of inflammation in your airways. If it’s high, that could mean you have asthma or another condition that affects your lungs. The FeNO test is pretty simple — you just breathe into a machine, and it measures the nitric oxide in your breath.
This article will discuss the normal range for a nitric oxide test, what it might mean if your levels are high, and when your doctor might order this test. It’s especially helpful in diagnosing and managing asthma in adults and kids ages 4 and older.
Understanding nitric oxide and airway inflammation
The science behind nitric oxide
Nitric oxide (NO) is a gas that’s produced in the lungs by various cells, including epithelial cells (the cells that line the surfaces of your body) and inflammatory cells.
These cells use enzymes called nitric oxide synthase (NOS) to produce NO.
In airway inflammation, such as what happens when you have asthma, the amount of inflammatory mediators increases. These mediators stimulate NOS, which causes increased NO production.
NO contributes to vasodilation (widening of the blood vessels), mucus production, and airway hyperresponsiveness (when your airways narrow too easily).
Nitric oxide as a biomarker
A biomarker is a measurable indicator of a biological state or condition. Fractional exhaled nitric oxide (FeNO) is a useful biomarker for airway inflammation because it’s non-invasive, relatively easy to measure, and reflects the degree of inflammation in your airways.
Elevated FeNO levels are often associated with type 2 inflammation, which is a common feature of allergic asthma.
FeNO testing: procedure and devices
Let’s take a closer look at how fractional exhaled nitric oxide, or FeNO, testing is done.
How a FeNO test is performed
During a FeNO test, you’ll breathe steadily into a handheld device for a certain length of time. It’s important to keep your breath flowing out at a constant rate.
To get the most accurate results, you’ll need to use the right technique. That means breathing out slowly and steadily, without any leaks around the mouthpiece. It’s also important for the person being tested to cooperate fully, which can be a challenge with younger children.
Available FeNO devices
A few different FeNO devices are used in clinics and labs.
Examples of FeNO devices include:
- NIOX VERO®
- Vivatmo pro
Some devices are small and easy to carry around, making them good for testing in different locations. Others are bigger and designed for use in a lab.
The best device for a particular situation depends on what it will be used for. For example, a doctor who wants to test patients in their office might prefer a portable device. A lab that does a lot of FeNO testing might choose a larger, more accurate device.
Normal FeNO levels: Definitions and interpretation
What does a normal FeNO test result look like? It depends on whether you’re an adult or a child.
Defining normal ranges
- Normal FeNO: less than 25 parts per billion (ppb) for adults, less than 20 ppb for children
- Intermediate FeNO: 25-50 ppb for adults, 20-35 ppb for children
- High FeNO: more than 50 ppb for adults, more than 35 ppb for children
Factors influencing normal ranges
Many things can affect FeNO test results, including your age, gender, weight, height, diet, and whether you smoke.
Age and height are especially important for children, since FeNO levels tend to go up as children get older and taller.
Your doctor will consider these factors when interpreting your FeNO results.
High FeNO Levels: Implications and Interpretation
If your FeNO test comes back high, what does that mean? Let’s break it down.
Asthma and Elevated FeNO
High FeNO levels often point to airway inflammation, which is a hallmark of asthma. But it’s important to remember that a high FeNO alone doesn’t automatically mean you have asthma. It’s one piece of the puzzle.
FeNO testing can be a valuable tool in both diagnosing and managing asthma, but other aspects of well-being are important too, such as calmer living by shrinking anxiety. It’s particularly helpful in assessing how well inhaled corticosteroids are working to control inflammation. Studies have shown that nitric oxide testing improves the accuracy of asthma diagnoses, especially in people who aren’t already taking corticosteroids.
High FeNO values can also be a red flag for potential asthma flare-ups. Research has found that people with higher baseline FeNO levels are more likely to experience exacerbations. One study showed that patients with a FeNO level above 45 ppb had a six times higher rate of exacerbations per year compared to those with lower levels.
Other Conditions Associated with Elevated FeNO
Asthma isn’t the only culprit behind high FeNO levels. Other respiratory conditions can also cause an increase, including allergic rhinitis (hay fever), COPD, obstructive sleep apnea, and certain interstitial lung diseases.
That’s why it’s so important for your doctor to consider other possible diagnoses when interpreting high FeNO results. An elevated FeNO level on its own isn’t enough to diagnose asthma; it needs to be considered alongside your clinical symptoms, medical history, and other test results.
FeNO and Treatment Response
FeNO levels can be a useful indicator of how well you’re responding to treatment, particularly inhaled corticosteroids. FeNO reflects the level of type 2 inflammation in your airways, which can help predict whether you’ll respond well to inhaled corticosteroids and biologic therapies.
There’s also something called “FeNO suppression testing,” which can potentially help identify patients who aren’t taking their inhaled corticosteroids as prescribed. Studies have shown that when patients who weren’t adhering to their treatment regimen were directly observed taking their inhaled corticosteroids for 7 days, their FeNO levels decreased significantly (by 47% to 79% of baseline values).
Factors affecting FeNO test results
Many factors can influence the results of a FeNO test. It’s important to be aware of these factors so the results can be interpreted accurately.
Patient-specific factors
Age, gender, height, and weight can all affect FeNO levels. Age and height are especially important in children, while gender can play a role in adults. Weight can indirectly affect FeNO levels because of its association with body composition and other health conditions.
Healthcare providers should consider all of these factors when they interpret FeNO results. This can help avoid misinterpretations and ensure the most accurate diagnosis and treatment plan.
Environmental and lifestyle factors
Smoking, diet, and alcohol consumption can also affect FeNO levels. Smoking can lower FeNO levels because of oxidative stress. Certain foods and beverages may temporarily change FeNO levels.
To improve the accuracy of FeNO testing, patients should avoid smoking, vaping, alcohol, caffeine, carbonated beverages, and highly processed foods for at least one hour before the test.
Medication use
Inhaled corticosteroids and other medications can influence FeNO levels. Inhaled corticosteroids are known to lower FeNO levels by reducing inflammation in the airways.
It’s very important to document all medications a person is taking, including inhaled corticosteroids, to accurately interpret FeNO results.
FeNO vs. spirometry and other diagnostic tools
So, how does a FeNO test compare to other tests for asthma and lung function?
Complementary roles
FeNO testing and spirometry can both be used to assess respiratory function, but they look at different things. Spirometry measures how much air you can breathe in and out, and how quickly. FeNO testing, on the other hand, measures the amount of nitric oxide in your exhaled breath, which indicates inflammation in your airways.
Measuring nitric oxide levels along with spirometry testing can help confirm a diagnosis of asthma.
FeNO in the context of a comprehensive asthma evaluation
There are other diagnostic tools that doctors use to assess asthma, too. One of the most common is allergy testing. Allergy testing can reveal whether allergies are contributing to a person’s asthma.
Doctors may also use bronchial provocation testing to assess airway hyperresponsiveness. This test involves inhaling increasing doses of a substance that can cause the airways to narrow. If your airways narrow more easily than normal, it suggests that you have airway hyperresponsiveness.
It’s important to remember that FeNO testing is just one tool for diagnosing asthma and assessing a management and treatment plan. No single tool confirms a diagnosis or measures the effectiveness of your treatment plan.
Clinical Applications of FeNO Testing
The FeNO test isn’t just about knowing the numbers. It’s a tool doctors use in several ways to understand and manage respiratory conditions, especially asthma.
Diagnosing Asthma
FeNO plays a role in diagnosing asthma, particularly when symptoms are unusual or unclear. Keep in mind that the FeNO test isn’t a stand-alone diagnosis. It’s an extra piece of information that doctors use alongside other tests and evaluations.
Nitric oxide testing can improve the accuracy of an asthma diagnosis. It’s most reliable in people who aren’t taking corticosteroids, which can affect FeNO levels.
The FeNO test can also help doctors understand the specific type of asthma a person has. FeNO levels often show how much “type 2 inflammation” is present, which helps predict how well someone will respond to inhaled corticosteroids and newer biologic therapies.
Managing Asthma
FeNO levels can help doctors monitor how well a treatment plan is working and adjust medication doses accordingly. While FeNO can be helpful in tweaking inhaled corticosteroid doses, there’s conflicting evidence about whether it should be the only factor considered.
High FeNO values have been linked to a higher chance of asthma flare-ups, giving doctors a heads-up to take preventive measures.
Identifying Non-Adherence
Sometimes, people with severe asthma don’t get better because they aren’t using their inhaled corticosteroids as prescribed. A FeNO suppression test can potentially identify these situations.
Frequently Asked Questions
What are the results of a nitric oxide test?
The results of a nitric oxide test typically come in the form of a numerical value, measured in parts per billion (ppb). This value indicates the concentration of nitric oxide in the exhaled breath. Your doctor will interpret these results in the context of your overall health and symptoms to determine if your nitric oxide levels are within the normal range.
What should my nitric oxide level be?
The ideal nitric oxide level varies depending on the specific test being used and the lab’s reference range. However, a generally accepted normal range for fractional exhaled nitric oxide (FeNO) tests is below 25 ppb for adults and below 20 ppb for children. Elevated levels may suggest airway inflammation, often associated with asthma.
When should nitric oxide levels be checked?
Nitric oxide levels are typically checked when a doctor suspects airway inflammation, particularly in cases of asthma. The test helps diagnose asthma, assess its severity, and monitor the effectiveness of asthma treatments. It may also be used to differentiate asthma from other respiratory conditions.
What are the symptoms of low nitric oxide?
Symptoms of low nitric oxide can be varied and sometimes subtle. They might include high blood pressure, erectile dysfunction, fatigue, decreased exercise tolerance, and impaired cognitive function. However, these symptoms can also be indicative of other conditions, so it’s important to consult with a healthcare professional for proper diagnosis.
Is nitric oxide bad for your kidneys?
Nitric oxide plays a complex role in kidney health. While it can have protective effects on the kidneys under certain circumstances, excessive production of nitric oxide has been linked to kidney damage in some studies. The effects of nitric oxide on the kidneys depend on various factors, including the specific kidney condition and the concentration of nitric oxide.
Closing Thoughts
FeNO testing is a useful way to check for airway inflammation, especially when healthcare providers are trying to understand and manage asthma. What’s considered a “normal” FeNO level depends on your age, and other things can affect the results, too.
It’s important to remember that a FeNO test is just one piece of the puzzle. Doctors should always consider your medical history, do a physical exam, and use other tests like spirometry to get the full picture.
FeNO testing can be really helpful in guiding asthma treatment. It can help doctors decide on the right dose of inhaled corticosteroids and even predict how well you’ll respond to the treatment.
As we continue to study and improve FeNO testing, it will likely become even more useful in the future for managing all kinds of breathing problems.