Asthma is not always straightforward to diagnose, and it is not always easy to know whether it is well controlled. Symptoms like coughing, chest tightness, and shortness of breath can overlap with other respiratory conditions, and standard breathing tests alone don’t always reveal what is happening at the inflammatory level. The FENO test fills that gap.
At Gwinnett Pulmonary & Sleep, our board-certified pulmonologists have been caring for patients with asthma and airway disease across Gwinnett County since 1983. We offer fractional exhaled nitric oxide testing as part of a comprehensive approach to asthma diagnosis and management, giving our team an objective, real-time window into the inflammatory activity driving your symptoms.
What Is a FENO Test?
A FENO test, fractional exhaled nitric oxide test, is a simple, non-invasive pulmonary function test that measures the concentration of nitric oxide in the air you exhale. Nitric oxide is produced by the cells lining the airways in response to eosinophilic inflammation, the type of airway inflammation most closely associated with allergic asthma and steroid-responsive disease.
When the airways are inflamed, nitric oxide levels in exhaled breath rise. The FENO test detects and quantifies that elevation, giving your pulmonologist a direct, measurable indicator of active airway inflammation, something that spirometry and peak flow measurements alone cannot provide.
The test takes only a few minutes, requires no needles or medication, and produces an immediate result.
What Does a FENO Test Measure?
The FENO test measures exhaled nitric oxide in parts per billion (ppb). These values are interpreted according to established clinical thresholds:
- Below 25 ppb in adults — Low likelihood of eosinophilic airway inflammation. Asthma driven by this mechanism is less likely; other causes of the symptoms should be investigated.
- 25 to 50 ppb in adults — Intermediate range. Results are interpreted alongside symptoms, spirometry findings, and clinical history to guide next steps.
- Above 50 ppb in adults — Elevated nitric oxide indicating significant eosinophilic airway inflammation, strongly associated with steroid-responsive asthma. This level supports both a diagnosis of asthma and a targeted treatment approach.
Different thresholds apply to children, and results are always interpreted in the context of your full clinical picture. A single FENO value does not stand alone; it is one piece of a diagnostic and monitoring framework.
How Does the FENO Test Work?
The procedure is straightforward and completed entirely in our office.
You will be asked to inhale deeply, then exhale slowly and steadily into a handheld device at a controlled flow rate, typically for approximately 10 seconds. The device analyzes the nitric oxide concentration in your exhaled breath in real time and produces an immediate readout.
No breath holding, no uncomfortable maneuvers, and no recovery time is needed. Most patients find the test simple and easy to complete. In some cases, the maneuver is repeated twice to confirm a consistent result.
Before your FENO test, you should:
- Avoid eating or drinking for at least one hour beforehand, as food and some beverages can temporarily affect nitric oxide levels
- Avoid smoking or vigorous exercise within one hour of the test
- Continue your asthma medications unless your provider specifically advises otherwise, unlike bronchodilator response testing, FENO testing does not typically require you to withhold inhalers
If you have questions about preparation, contact our office before your appointment.
When Is FENO Testing Used?
Diagnosing Asthma
When a patient presents with chronic cough, episodic wheeze, chest tightness, or exertional breathlessness, distinguishing asthma from other causes requires more than symptom history alone. An elevated FENO result supports a diagnosis of eosinophilic airway inflammation and, in combination with spirometry and bronchodilator response testing, strengthens the case for an asthma diagnosis or points away from it when levels are low.
Identifying Asthma Subtype
Not all asthma is the same. Eosinophilic asthma, driven by type 2 airway inflammation and typically associated with allergic triggers, responds well to inhaled corticosteroids and biologic therapies. Non-eosinophilic asthma involves different inflammatory pathways and may require a different treatment approach. FENO testing helps distinguish between these subtypes, allowing your pulmonologist to target treatment more precisely from the outset rather than through trial and error.
Monitoring Asthma Control and Treatment Response
For patients already diagnosed with asthma, FENO testing provides an ongoing measure of whether inflammation is truly controlled, not just whether symptoms have quieted. Symptoms can be suppressed with a rescue inhaler even when underlying inflammation persists, meaning a patient may feel better while their airways remain significantly inflamed. Serial FENO measurements allow your provider to detect that gap and adjust maintenance therapy accordingly.
Guiding Inhaled Corticosteroid and Biologic Therapy Decisions
Patients with persistently elevated FENO levels — particularly those above 50 ppb — are more likely to respond to inhaled corticosteroids and are stronger candidates for biologic therapies such as Xolair (omalizumab) and other anti-eosinophilic agents. FENO data supports these treatment decisions with objective evidence rather than symptom reporting alone.
Evaluating Unexplained Chronic Cough
Eosinophilic airway inflammation can cause chronic cough in the absence of classic asthma symptoms. When cough has persisted despite standard treatment and no cause has been identified, FENO testing can help determine whether steroid-responsive airway inflammation is contributing, a finding that significantly changes the treatment approach.
Indications and Contraindications
FENO testing is indicated for patients who:
- Have suspected asthma and require objective confirmation of airway inflammation
- Have a confirmed asthma diagnosis but symptoms that are poorly controlled despite treatment
- Are being evaluated for biologic therapy candidacy
- Have chronic cough of unexplained origin
- Require serial monitoring of airway inflammation during treatment adjustment
Your provider will assess whether FENO testing is appropriate for your specific situation and how results should be weighted alongside other clinical findings.
What Happens If Your FENO Is Elevated?
An elevated FENO result — particularly above 50 ppb — is a clinically meaningful finding. It indicates active eosinophilic airway inflammation and, in the right clinical context, supports:
- Initiating or escalating inhaled corticosteroid therapy
- Evaluating candidacy for biologic medications targeting type 2 inflammation
- Revisiting adherence to existing controller medications if levels remain high despite treatment
- Ruling out alternative diagnoses when FENO is low despite ongoing symptoms
An elevated result is not a cause for alarm — it is actionable information. Our team will review your FENO result alongside your spirometry, symptom history, and medication use to determine the most appropriate next step in your care.
Schedule Your FENO Asthma Test in Gwinnett County
If you have been struggling with asthma symptoms, a persistent cough, or feel that your current treatment isn’t doing enough, a FENO test can provide the clinical clarity your care plan needs. At Gwinnett Pulmonary & Sleep, we combine precise diagnostic testing with decades of pulmonary expertise to give you answers — and a treatment approach built around them.
Call our office to schedule an appointment, or request one online.
FENO testing is available at all five of our offices: Lawrenceville, Duluth, Snellville, Dacula, and Suwanee, with appointments typically available within a short wait.
Frequently Asked Questions
How does the FENO test work step by step?
You inhale deeply, then exhale slowly and steadily into a handheld device at a controlled flow rate for approximately 10 seconds. The device measures nitric oxide concentration in real time and produces an immediate result. The maneuver may be repeated once to confirm consistency. The entire test takes only a few minutes.
Is the FENO test the same as a spirometry test?
No. Spirometry measures airflow and lung volumes to detect obstructive or restrictive lung patterns. The FENO test measures airway inflammation directly. The two tests provide different types of information and are often used together as part of a comprehensive asthma evaluation.
What is a normal FENO level?
In adults, FENO values below 25 ppb are generally considered low, suggesting eosinophilic inflammation is not a significant driver of symptoms. Values between 25 and 50 ppb are intermediate and require clinical interpretation. Values above 50 ppb indicate elevated inflammation strongly associated with steroid-responsive asthma. Different reference ranges apply to children.
What does an elevated FENO result mean?
An elevated FENO, particularly above 50 ppb, indicates significant eosinophilic airway inflammation. In clinical context, this supports an asthma diagnosis, suggests the condition may respond well to inhaled corticosteroids, and may indicate candidacy for biologic therapy. Your pulmonologist will interpret the result alongside your symptoms, spirometry, and medication history.
Are there contraindications to FENO testing?
FENO testing may be less informative in current smokers, as smoking suppresses nitric oxide and can produce falsely low results. Active respiratory infection at the time of testing can also affect results. Your provider will assess whether testing is appropriate and how to interpret results in light of any factors that may influence nitric oxide levels.
How should I prepare for a FENO test?
Avoid eating, drinking, or smoking for at least one hour before the test. Refrain from vigorous exercise within one hour of your appointment. Continue your asthma medications unless your provider specifically advises otherwise. If you have questions about preparation, contact our office before your visit.
Is the FENO test used in clinical trials?
Yes. FENO testing has been extensively studied in clinical research and is used in trials evaluating asthma therapies, particularly biologic agents targeting eosinophilic inflammation. Elevated baseline FENO is often used as an enrollment criterion in trials of medications such as anti-IL-5 and anti-IgE therapies, as it identifies patients most likely to respond.
Is FENO testing covered by insurance?
FENO testing is covered by Medicare and many private insurance plans when ordered with a supporting clinical indication. Our team can verify your coverage before your appointment.