FeNO Testing

What FeNO Testing Measures

FeNO stands for fractional exhaled nitric oxide. It is a simple breath test that measures the level of nitric oxide gas present in the air you exhale. Nitric oxide is a naturally occurring gas in the body, and its concentration in exhaled breath tends to rise with a specific type of airway inflammation (often referred to as “type 2” or eosinophilic inflammation) that is common in many, though not all, people with asthma.

In plain terms, FeNO gives physicians a window into a particular kind of inflammatory activity in the airways, without the need for more invasive testing. It does not measure airflow, lung capacity, or lung damage directly; those are assessed by other tools, such as pulmonary function testing.

FeNO vs. Spirometry

FeNO and spirometry are often used together during an asthma evaluation, but they measure fundamentally different things. The table below summarizes how each one contributes to the picture.

Comparison table

Dimension	FeNO	Spirometry
What It Measures	Nitric oxide gas, a marker of a type of airway inflammation	Airflow and lung function
What It Doesn't Show	Airflow, lung capacity, or lung damage	The type of inflammation driving symptoms
Role in an Asthma Evaluation	One piece of supporting evidence, alongside symptoms and history	A core objective measure, typically interpreted together with FeNO and clinical history

The Type 2 Inflammation Connection, in Plain Terms

Asthma is not a single, uniform condition: different people can have airway inflammation driven by different underlying processes. One common pattern, often described as type 2 or eosinophilic inflammation, involves a set of immune signals that, among other effects, increase the amount of nitric oxide produced in the airway lining. This is why measuring exhaled nitric oxide can serve as an indirect marker for this type of inflammation.

This explanation is intentionally kept at a conceptual level; the detailed immunology of type 2 inflammation is a matter for your physician to discuss as it applies to your specific situation. The key practical point is that elevated FeNO can reflect a pattern of airway inflammation relevant to asthma care for many patients.

FeNO’s Role in Asthma Evaluation

When a physician is evaluating possible asthma, FeNO adds one piece of supporting evidence to a picture built from your symptoms (such as wheeze, cough, or shortness of breath), a physical exam, medical history, and spirometry. It is best understood as an adjunct tool that adds useful context to a thorough evaluation, not a shortcut that replaces one.

FeNO Is Not a Standalone Diagnostic Test

This is one of the most important things to understand about FeNO testing: it is not a standalone diagnostic test for asthma. A normal FeNO result does not rule out asthma. Some people with asthma have forms of airway inflammation that FeNO does not capture well. Likewise, an elevated FeNO result does not, by itself, confirm an asthma diagnosis, since other conditions and factors can also raise nitric oxide levels.

Because of this, FeNO results are always interpreted together with your symptoms, medical history, other testing, and clinical judgment. If you have questions about what your specific FeNO result means, that conversation belongs with the physician who ordered the test.

Using FeNO to Help Guide Treatment

Beyond diagnosis, FeNO measurements can sometimes help inform whether inhaled corticosteroid therapy is likely to be helpful for a given patient, since this type of medication targets the inflammation that FeNO reflects. Some clinicians also use FeNO over time as one way to help monitor how well a patient is responding to anti-inflammatory treatment.

This use is appropriately hedged rather than universal: not every clinician uses FeNO this way, and it is one tool among several that may inform treatment decisions, rather than a fixed protocol applied the same way to every patient.

Limitations and Factors That Affect Results

FeNO levels can be affected by a number of factors that are unrelated to how well-controlled a person’s asthma is, which is why results always need clinical context.

Smoking

Known to lower FeNO levels, sometimes making it harder to interpret results in people who currently smoke.

Recent Corticosteroid Use

Inhaled or oral corticosteroids also tend to lower FeNO, since these medications reduce the underlying inflammation being measured.

Allergic or Atopic Inflammation

Can raise FeNO even in people who do not have asthma, since the type of inflammation FeNO reflects is not exclusive to asthma.

Because of these influences, a FeNO result is never read in isolation: your physician will factor in your smoking status, current medications, and any allergic conditions when interpreting what a given number means for you.

What the Test Feels Like

FeNO testing is simple and non-invasive. You breathe out steadily into a small handheld device or mouthpiece at a controlled, even rate for about 10 seconds, similar in feel to a breathalyzer test. There are no needles, no discomfort, and no recovery time needed. The entire breathing portion typically takes less than a minute, and the full visit, including setup and instructions from your care team, usually takes just a few minutes.

How Results Are Interpreted

A FeNO result is one data point that your physician weighs together with your full clinical picture: your reported symptoms, physical exam findings, medical history, and results from other tests such as spirometry. It is not a number meant to be interpreted on its own, and it should not be used to self-diagnose or self-manage asthma. If FeNO testing is part of your evaluation, your physician will explain how your result fits into the broader assessment and what, if anything, it means for next steps in your care.