FeNO Testing for Asthma: What Fractional Exhaled Nitric Oxide Reveals About Your Airways

A single breath into a handheld device. Ten seconds. And you have a measurement that can tell your physician whether your asthma is driven by eosinophilic inflammation — information that guides diagnosis, monitors steroid response, and determines which biologic therapy is most likely to work for you.

Fractional exhaled nitric oxide (FeNO) testing is one of the most clinically actionable advances in asthma diagnostics over the past two decades. At Advanced Asthma Clinic in Plantation, Florida, Dr. Frank Hull uses FeNO as a routine component of the biomarker evaluation for patients with difficult-to-control asthma and for those being considered for biologic therapy.

This guide explains the science behind FeNO, how the test is performed, what your result means, and how FeNO fits into the broader picture of asthma management in South Florida.

What Is Fractional Exhaled Nitric Oxide (FeNO)?

Nitric oxide (NO) is a small molecule produced by cells throughout the body as part of normal signaling. In the airways, nitric oxide is produced in elevated quantities when eosinophils — a type of white blood cell central to allergic and type 2 inflammation — are present and active.

FeNO measures the concentration of nitric oxide in exhaled breath, expressed in parts per billion (ppb). Because NO production in the airways scales with eosinophilic inflammation, FeNO serves as a real-time, non-invasive marker of the inflammatory process driving many cases of asthma.

The test does not measure airflow obstruction (that is spirometry's job). Instead, it measures the type and intensity of airway inflammation. This distinction is clinically significant: a patient can have normal spirometry but highly elevated FeNO, indicating active eosinophilic inflammation that predicts poor asthma control and future exacerbations without appropriate treatment.

The Enzyme Behind the Signal: iNOS and Eosinophilic Inflammation

Airway epithelial cells express an enzyme called inducible nitric oxide synthase (iNOS). When type 2 cytokines — particularly interleukin-4 (IL-4) and interleukin-13 (IL-13), released by eosinophils, mast cells, and innate lymphoid cells — are present, iNOS expression is upregulated dramatically. The resulting surge in nitric oxide production is measurable in exhaled breath within hours of inflammation onset.

This is why FeNO responds so quickly to treatment. When inhaled corticosteroids (ICS) are taken correctly, they suppress the IL-4/IL-13 signaling cascade, iNOS expression drops, and FeNO falls — often within days. A patient whose FeNO does not fall with adequate ICS use is signaling either poor adherence or steroid-refractory inflammation, both clinically important findings.

How the FeNO Test Is Performed

The test is simple and non-invasive. No needles, no bronchodilators, no exercise — just a controlled breath.

1. Preparation: Avoid smoking, vigorous exercise, and large meals for at least one hour before the test. These can transiently affect FeNO readings. No fasting is required.
2. The breath maneuver: The patient inhales deeply through the device (which filters incoming air to remove ambient NO), then exhales slowly and steadily at a controlled flow rate of 50 mL per second for approximately 10 seconds.
3. Measurement: The device uses electrochemical sensing to analyze the NO concentration in the exhaled air in real time. Most devices require two or three reproducible maneuvers.
4. Results: FeNO values are displayed immediately in ppb and are available for clinical interpretation during the same visit.

The entire process typically takes less than 10 minutes from setup to result. At Advanced Asthma Clinic, FeNO testing is commonly performed alongside spirometry and blood eosinophil measurement as part of a comprehensive asthma workup.

Interpreting Your FeNO Result

The American Thoracic Society (ATS) and Global Initiative for Asthma (GINA) guidelines use the following reference thresholds for FeNO interpretation in adults:

FeNO Level	Classification	Clinical Implication
< 25 ppb	Low	Eosinophilic inflammation unlikely to be the primary driver; consider non-type-2 phenotype, poor ICS adherence, or alternative diagnosis
25-50 ppb	Intermediate	Possible eosinophilic inflammation; clinical significance depends on context -- correlate with symptoms, ICS use, and blood eosinophils
> 50 ppb	High	Significant eosinophilic airway inflammation; ICS therapy likely beneficial; strong predictor of biologic response

These thresholds are guidelines, not absolute cutoffs. Clinical interpretation always requires context: current medication use, recent infections, smoking status, and symptom burden all affect the FeNO value and its meaning. Always consult your physician for interpretation of your individual result.

Factors That Affect FeNO Readings

Several factors can raise or lower FeNO independently of eosinophilic inflammation:

• Raises FeNO: allergen exposure, respiratory infections, nitrate-rich foods (beets, leafy greens), smoking cessation, atopic dermatitis
• Lowers FeNO: current smoking, alcohol consumption, long-term systemic corticosteroid use, excellent ICS adherence
• Variable effects: exercise (transient reduction), pregnancy (modest reduction in some studies)

Your physician will account for these factors when interpreting your result. A single FeNO measurement is rarely definitive — the trend over serial measurements is often more informative than any single value.

FeNO as a Clinical Decision Tool

Diagnosing Asthma

FeNO supports asthma diagnosis when spirometry is normal or borderline. In patients with episodic respiratory symptoms and negative bronchoprovocation testing, an elevated FeNO strengthens the case for eosinophilic airway disease. It is particularly useful in:

• Patients who cannot perform reliable spirometry
• Distinguishing asthma from COPD in overlap presentations
• Evaluating suspected occupational asthma where workplace allergen sensitization drives type 2 inflammation
• Pediatric patients where asthma symptoms can be difficult to distinguish from other childhood respiratory conditions

Monitoring Treatment Response

Serial FeNO measurements are an objective way to assess whether asthma treatment is working. Within two to four weeks of starting or optimizing inhaled corticosteroid therapy, FeNO should fall by at least 20% if the inflammation is ICS-responsive.

Failure of FeNO to decline despite confirmed inhaler adherence indicates steroid-refractory inflammation — a red flag that should prompt specialist referral and consideration of biologic therapy.

Conversely, a patient who achieves excellent asthma control on low-dose ICS but has persistently elevated FeNO is at increased risk of exacerbations if ICS is stepped down. FeNO monitoring can prevent premature de-escalation of therapy.

Predicting Inhaled Corticosteroid Response

A landmark study published in The Lancet (Petsky et al.) demonstrated that using FeNO to guide ICS dosing reduces exacerbation rates compared with symptom-guided therapy alone. The ATS clinical practice guidelines recommend FeNO-guided management as an adjunct to clinical assessment in patients with persistent asthma.

FeNO and Biologic Therapy Selection

This is where FeNO testing becomes most impactful for patients with severe or difficult-to-control asthma. Biologic therapies approved for asthma each target specific inflammatory pathways, and FeNO helps identify which pathways are active in a given patient.

Biologic (Brand)	Target	FeNO Relevance	Primary Eligibility Marker
Dupilumab (Dupixent)	IL-4Rα (blocks IL-4 and IL-13)	High FeNO is a primary eligibility marker; FeNO ≥ 25 ppb predicts strongest response	FeNO ≥ 25 ppb or blood eos ≥ 150 cells/µL
Tezepelumab (Tezspire)	TSLP (upstream epithelial alarmin)	Effective across all FeNO levels; no minimum FeNO requirement -- only biologic approved for low-eosinophil asthma	No minimum biomarker threshold
Mepolizumab (Nucala)	IL-5	Blood eosinophils are the primary marker; elevated FeNO supports type 2 phenotype	Blood eos ≥ 150 cells/µL
Benralizumab (Fasenra)	IL-5Rα	Blood eosinophils are the primary marker; elevated FeNO corroborates type 2 phenotype	Blood eos ≥ 300 cells/µL
Omalizumab (Xolair)	IgE	FeNO confirms type 2 inflammatory background; primary eligibility based on total IgE and allergen sensitization	Total IgE + allergen sensitization

In practice, Dr. Hull uses FeNO alongside blood eosinophil counts, total IgE, and allergen sensitization testing to build a complete biomarker profile for each patient. No single test determines biologic eligibility alone — the combination of biomarkers, clinical history, and exacerbation frequency guides the final recommendation.

The Low-FeNO Patient: What It Means

A FeNO below 25 ppb in a symptomatic asthma patient is diagnostically important. It suggests that eosinophilic (type 2) inflammation is not the dominant driver and raises consideration of:

• Non-eosinophilic asthma phenotypes -- neutrophilic or paucigranulocytic inflammation, which responds differently to treatment
• Poor ICS adherence -- active eosinophilic inflammation masked by sporadic steroid exposure
• Obesity-driven airway dysfunction -- particularly relevant in South Florida, where obesity-associated asthma is common in Broward County
• Alternative diagnoses -- vocal cord dysfunction, COPD, cardiac asthma, or hyperventilation syndrome mimicking asthma
• Tezepelumab candidacy -- as the only biologic effective across all phenotypes including low-eosinophil asthma, refractory patients with low FeNO may still be biologic candidates

FeNO Testing in South Florida: Why It Matters Here

South Florida's climate creates a unique asthma environment. Year-round high humidity, subtropical mold spore counts, persistent pollen exposure from invasive tropical plant species, and poor air quality days related to Saharan dust intrusion combine to drive chronic airway inflammation in many patients.

For patients in Broward County and the greater Fort Lauderdale area, serial FeNO monitoring provides an objective measure of how environmental exposures are affecting airway inflammation over time. A patient who reports stable symptoms in winter but worsens each summer may show a predictable FeNO pattern that guides preemptive treatment adjustment.

Dr. Frank Hull has used FeNO as a routine clinical tool in his Plantation practice for over a decade. Correlating FeNO trends with local pollen seasons, hurricane-related mold surges, and wildfire smoke events gives patients a measurable, objective connection between their environment and their inflammation levels.

Insurance Coverage for FeNO Testing

FeNO testing is billed under CPT code 95012. Coverage policies include:

• Medicare: Covered under Part B when medically necessary for asthma diagnosis or management. A physician order is required.
• Medicaid (Florida): Coverage varies by managed care plan. Pre-authorization may be required.
• Commercial insurance: Most major carriers cover FeNO testing for asthma indications. Coverage specifics depend on your plan, deductible status, and in-network designation.

Our clinic's billing team can verify your coverage prior to testing. Patients without insurance coverage can inquire about self-pay rates at the time of scheduling.

What to Expect at Your Visit

FeNO testing is typically performed at the same visit as spirometry. Before your appointment:

• Avoid eating a large meal within two hours of the test
• Do not smoke for at least four hours beforehand
• Avoid strenuous exercise for at least one hour before the test
• Continue all medications as prescribed unless specifically instructed otherwise
• Bring a list of all current asthma medications, including dose and frequency

Results are reviewed with you during the same visit. Dr. Hull will explain what your FeNO level means in the context of your other test results and symptoms, and how it informs the next step in your treatment plan.

Frequently Asked Questions

What is a normal FeNO level?

FeNO below 25 ppb is considered low. Between 25 and 50 ppb is intermediate. Above 50 ppb is high and indicates significant eosinophilic airway inflammation. Your physician will interpret your specific result in clinical context. Always consult your physician regarding treatment decisions.

How long does a FeNO test take?

Less than 10 minutes. No fasting required. Results are immediate and reviewed at the same visit.

Can FeNO testing help choose an asthma biologic?

Yes. Elevated FeNO above 25 ppb is one qualifying criterion for dupilumab (Dupixent) and predicts strong response to dupilumab and tezepelumab (Tezspire). It is used alongside blood eosinophil counts and IgE levels to build a complete biomarker profile. Your physician combines all biomarkers with your clinical history to make the final recommendation.

Is FeNO testing covered by insurance?

FeNO testing (CPT 95012) is covered by Medicare and most commercial insurance plans for asthma management. Our team can verify your benefits before your appointment. Always confirm with your own insurance provider.

Does FeNO testing work for all types of asthma?

FeNO is most informative for eosinophilic (type 2) asthma. A normal FeNO in a symptomatic patient is also diagnostically useful -- it helps identify non-eosinophilic phenotypes that may respond differently to treatment.

Summary: What FeNO Testing Adds to Your Asthma Care

Fractional exhaled nitric oxide testing gives patients and physicians something that symptoms alone cannot provide: an objective, reproducible measurement of airway inflammation type and severity. In the more than 20 years that Dr. Frank Hull has treated asthma patients in South Florida, FeNO has moved from a research tool to a routine clinical instrument -- one that has changed treatment decisions for patients who were previously under- or over-treated based on symptoms alone.

For patients exploring biologic therapy, FeNO is a key piece of the biomarker puzzle. For newly diagnosed patients, it confirms the inflammatory mechanism. For patients who feel their asthma is controlled, it can reveal silent ongoing inflammation that predicts future exacerbations -- the kind of early warning that prevents emergency department visits and hospitalizations.

If you live in Broward County and have not had a FeNO test as part of your asthma workup, ask your physician whether it is appropriate for your situation. Always consult your healthcare provider before making changes to your asthma treatment plan.