Exhaled Nitric Oxide (FeNO) Test

Patient plan

Exhaled Nitric Oxide (FeNO) Testing: what to expect

Quick answer

For exhaled nitric oxide (feno) testing patients: An exhaled nitric oxide test measures one marker of airway inflammation and adds context to asthma evaluation, spirometry, and treatment decisions.

Who should consider this evaluation

Exhaled Nitric Oxide (FeNO) Testing is for patients whose main question is what feno measures in the airways. A focused visit is most useful when symptoms, timing, or a prior result remain unclear.

What the visit or test can answer

During a exhaled nitric oxide (feno) testing evaluation, the clinician can use the history and examination to decide whether what happens during a feno breath test should be part of the visit, and whether how food, exercise, smoking, illness, and medication can affect results would change the plan.

How to prepare

To prepare for exhaled nitric oxide (feno) testing, bring the timeline, medicines, prior reports, photographs, and the specific exposure or symptom question that led you here. Review when feno adds context to spirometry and asthma evaluation before the appointment.

Limits and safety

Ask what exhaled nitric oxide (feno) testing can and cannot establish, including why feno does not diagnose or rule out asthma by itself. A test result should not be used alone to make a major diet, medicine, or exposure change.

Red flags

If exhaled nitric oxide (feno) testing symptoms match when breathing symptoms need immediate care, seek urgent care rather than waiting for a scheduled test. Emergency treatment takes priority over diagnostic clarification.

Alternatives when this test is not enough

If exhaled nitric oxide (feno) testing does not answer the question, the next discussion may include Allergies and Testing, Pulmonary Function Testing, Asthma Evaluation or coordination with another clinician.

Your next step

Before a exhaled nitric oxide (feno) testing visit: Review the related breathing tests, then find a clinic to confirm preparation and availability.

What Is a FeNO Test?

FeNO means fractional exhaled nitric oxide. The breath test measures nitric oxide in exhaled air and may add information about airway inflammation in an appropriate clinical assessment. It does not diagnose or exclude asthma on its own. American Thoracic Society guidance discusses its use alongside clinical judgment.

What the test measures or addresses	Question for the clinician
Nitric oxide in exhaled air	Why is this measurement relevant to my assessment?
One part of an airway inflammation discussion	How does it fit with symptoms and other findings?
A result influenced by clinical context	What information needs to be considered when interpreting it?
A different measurement from spirometry	Would another respiratory test answer a separate question?

Is FeNO the same as an allergy test?

It does not identify a specific pollen, food or other allergen. The clinician can explain why it is being considered in the respiratory assessment.

Can I interpret a high or low number myself?

The result requires clinical context. This page gives no cutoff for diagnosing asthma, grading its severity or changing medicine.

Who provides preparation instructions?

The testing team supplies instructions for the actual appointment. Questions about medicines or other preparation should be directed there rather than answered from a general web page.

Patient undergoing a respiratory test with medical equipment.

The Breathing Maneuver

What Happens During a FeNO Breath Test

You will usually sit upright, breathe through a mouthpiece, take in a full breath, and then exhale slowly at a controlled rate. Visual or audio cues from the device help you maintain the correct airflow.

The measurement may be repeated to obtain a reliable result. There are no needles and no radiation. Most patients can return to their usual activities immediately after the test.

Protect Test Quality

How Food, Exercise, Smoking, Illness, and Medication Can Affect Results

Follow the appointment instructions

Follow the instructions provided for your appointment. Do not stop an inhaler or other asthma medication unless the ordering clinician specifically tells you to do so.

The testing team can explain which preparation details matter for the ordered FeNO measurement. Ask the clinic for the instructions that apply to your appointment and discuss any questions about your current care with the ordering clinician.

Report recent changes before testing

Tell the clinical team if you recently had a cold, asthma flare, unusual allergen exposure, or change in medication.

The Number Needs Context

Why FeNO Does Not Diagnose or Rule Out Asthma by Itself

A FeNO number should never be interpreted in isolation. Your clinician considers your age, symptoms, examination, allergy history, current corticosteroid treatment, tobacco exposure, recent infection, test technique, and spirometry results.

A change over time may be more informative than a single measurement. FeNO can help guide a treatment discussion, but medication decisions should also reflect symptom control, exacerbation history, lung function, adherence, and your individual treatment goals.

A Supporting Test

When FeNO Adds Context to Spirometry and Asthma Evaluation

FeNO may be useful when:

an asthma diagnosis remains uncertain after the initial evaluation;
symptoms and spirometry do not tell the same story;
type 2 airway inflammation is suspected;
anti-inflammatory asthma treatment is being considered;
asthma remains poorly controlled despite prescribed treatment; or
a clinician wants to monitor airway inflammation over time.

FeNO is not intended to evaluate a medical emergency or replace an asthma action plan.

Not an Emergency Test

When Breathing Symptoms Need Immediate Care

FeNO is not an emergency test. Follow your asthma action plan for an acute flare. Call 911 for severe breathing difficulty, blue or gray lips, difficulty speaking, confusion, fainting, or symptoms that do not improve with prescribed rescue treatment.

Patient Questions

Frequently Asked Questions

Can FeNO diagnose asthma by itself?

No. Asthma is a clinical diagnosis. FeNO can provide supporting evidence of type 2 airway inflammation, but some people with asthma have a low result and some elevated results occur in other clinical settings.

Does a high FeNO result mean my asthma is severe?

Not by itself. The number reflects one aspect of airway inflammation. Severity and control depend on symptoms, exacerbations, lung function, medication needs, and other clinical information.

Is FeNO the same as spirometry?

No. FeNO measures nitric oxide associated with airway inflammation. Spirometry measures how much air you can exhale and how quickly you can exhale it. The tests answer different questions and may be used together.

Should I stop my inhaler before the test?

Only if your clinician gives you specific instructions. Stopping corticosteroids or bronchodilators without guidance can affect both your symptoms and the interpretation of the test.

Can a cold or allergy flare change the result?

Yes. Respiratory infections and recent allergen exposure may affect airway nitric oxide. Tell the clinical team about recent symptoms before testing.

Related Care Guides

Continue With the Most Relevant Next Step

Allergies and Testing

See where FeNO fits within the broader diagnostic service category.

View guide

Pulmonary Function Testing

Compare an inflammation marker with tests that measure airflow, volume, and gas transfer.

View guide

Asthma Evaluation

Review how symptoms, lung function, and supporting tests are interpreted together.

View guide

Use FeNO as one part of the asthma evaluation

Review the related breathing tests, then find a clinic to confirm preparation and availability.

If you are deciding when to see an asthma and allergy specialist, review the available care paths and bring a focused question to the visit.

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