Don’t Rely on Spirometry Alone

Up to 30% of asthma patients present with normal spirometry at the time of clinical assessment — particularly those tested outside of symptomatic episodes or following bronchodilator use. FeNO detects persistent underlying eosinophilic inflammation independent of airflow, providing diagnostic evidence where spirometry fails. This is especially critical in patients with atypical presentations such as cough-variant asthma, where obstruction is absent but airway inflammation is active.

Ruling Out Asthma Mimics

Conditions such as vocal cord dysfunction, inducible laryngeal obstruction (ILO), dysfunctional breathing, and COPD can mimic asthma symptomatically. A low FeNO (<25 ppb in adults) in a symptomatic patient with normal spirometry strongly suggests the symptoms are not driven by eosinophilic airway inflammation, redirecting the diagnostic pathway toward these alternatives and avoiding unnecessary ICS prescribing.

Serial FeNO in Workplace Surveillance

In occupational asthma surveillance, serial FeNO measured at work and away from work can help identify work-related eosinophilic sensitisation. A pattern of elevated FeNO on working days that normalises over weekends or annual leave provides objective biological evidence of occupational exposure driving airway inflammation, supporting medico-legal documentation and workplace risk assessments.

Eosinophilic vs. Non-Eosinophilic Asthma

FeNO is specifically elevated in Type 2 (eosinophilic/atopic) asthma driven by IL-4 and IL-13 cytokine signalling. Low FeNO in a symptomatic patient points toward non-eosinophilic phenotypes — including neutrophilic or paucigranulocytic asthma — which respond poorly to ICS and may require alternative anti-inflammatory strategies such as macrolide antibiotics or targeted therapies. Accurate phenotyping prevents ICS overuse and its systemic side effects.

Combining FeNO with Blood Eosinophils

FeNO and peripheral blood eosinophil counts (BEC) reflect complementary aspects of Type 2 inflammation. FeNO captures local airway epithelial inflammation driven by IL-13, while BEC reflects systemic eosinophilia. Using both together — sometimes referred to as the “T2 high” signature — provides a more complete inflammatory picture. Patients with high FeNO and high BEC (>300 cells/µL) represent the most ICS-responsive and biologic-eligible phenotype.

FeNO as a Proxy for Atopic Sensitisation

Elevated FeNO strongly correlates with atopic sensitisation — particularly to aeroallergens such as house dust mite, grass pollen, and pet dander. In patients where allergy testing is not immediately available, a high FeNO can prompt earlier investigation and consideration of allergen immunotherapy (AIT) as a disease-modifying treatment. FeNO may also help predict which patients with allergic rhinitis are at risk of developing asthma.

Predict Who Will Respond to Inhaled Steroids

High FeNO (>40 ppb in adults) is the strongest available predictor of ICS responsiveness, outperforming bronchodilator reversibility testing in multiple prospective trials. In patients newly presenting with respiratory symptoms, a high FeNO justifies an ICS trial with greater confidence than spirometry alone. Conversely, initiating ICS in a patient with low FeNO and non-eosinophilic features is unlikely to confer benefit and exposes them to unnecessary side effects.

Supporting Biologic Therapy Selection

In severe, treatment-refractory asthma, FeNO is a key eligibility and monitoring biomarker for targeted biological therapies. High FeNO supports eligibility for dupilumab (anti-IL-4Rα), which targets the IL-4/IL-13 axis most directly reflected by FeNO. Elevated FeNO alongside high BEC supports mepolizumab or benralizumab (anti-IL-5 pathway). Tezepelumab, which targets TSLP upstream of all Type 2 pathways, may benefit even patients with lower FeNO when other T2 markers are present.

Safe ICS Dose Reduction Using FeNO Guidance

Guideline-recommended asthma step-down is often deferred due to clinician uncertainty about relapse risk. FeNO-guided step-down protocols have demonstrated that patients with consistently low FeNO (<25 ppb) during clinical stability can reduce ICS doses with a significantly lower rate of exacerbation compared to symptom-guided step-down alone. This approach reduces cumulative steroid exposure — important for minimising long-term risks including adrenal suppression, osteoporosis, and cataracts.

Unmasking Non-Adherence Before Escalation

Persistently high FeNO in a patient reportedly on regular ICS therapy should prompt a structured adherence assessment before escalating treatment. Studies show that a significant proportion of “difficult asthma” is actually uncontrolled asthma secondary to poor adherence. Offering directly-observed ICS dosing over 2–4 weeks and repeat FeNO measurement is a practical strategy: a subsequent fall in FeNO confirms adherence-related under-treatment, while a persistent rise warrants genuine treatment escalation or specialist referral.

Diagnosis in Children Who Cannot Perform Spirometry

Reliable spirometry requires sustained effort and cooperation, which is difficult to achieve in children under 5–6 years old. FeNO’s simple slow exhalation manoeuvre can be performed by most children aged 4 and above with brief coaching. In the paediatric wheezy child, a FeNO ≥35 ppb significantly increases the probability of a diagnosis of eosinophilic asthma versus viral-induced wheeze, helping clinicians make earlier, more confident treatment decisions and avoid both over- and under-treatment.

Monitoring Asthma During Pregnancy

Asthma control changes in up to two-thirds of pregnant women, and poorly controlled asthma carries significant risks for both mother and fetus including preterm birth and low birth weight. FeNO provides a non-invasive, radiation-free method of monitoring airway inflammation throughout pregnancy. Since symptom perception may be altered in pregnancy, FeNO offers an objective measure that can justify maintaining or adjusting ICS therapy, reassuring both clinician and patient about treatment safety during this sensitive period.

Differentiating Asthma from COPD in Older Adults

In elderly patients with a smoking history and airflow limitation, distinguishing asthma from COPD or asthma-COPD overlap syndrome (ACOS) is clinically challenging. Elevated FeNO in this context strongly suggests a significant eosinophilic component — a finding associated with better ICS response even within COPD — and can guide targeted prescribing. Conversely, low FeNO in a patient with fixed airflow limitation supports a primary COPD diagnosis where ICS monotherapy provides limited benefit and increases pneumonia risk.

Limitations

FeNO Is Not a Stand-Alone Diagnostic Tool

FeNO must always be interpreted within the full clinical context. Elevated FeNO is not specific to asthma — it can occur in allergic rhinitis without asthma, eosinophilic bronchitis, atopic dermatitis, and helminth infections. Relying on FeNO in isolation risks overdiagnosis. The test is most powerful when used to support — not replace — a structured clinical history, symptom assessment, and appropriate lung function testing.

Smoking Suppresses FeNO: A Diagnostic Trap

Cigarette smoking is a potent suppressor of FeNO, potentially masking significant eosinophilic inflammation in current smokers with asthma. A “normal” FeNO in an active smoker should not be used to confidently rule out eosinophilic disease. Clinicians should factor in smoking status, request blood eosinophil counts as a complementary biomarker, and consider repeat FeNO testing after a period of smoking cessation to obtain a more accurate inflammatory picture.

Intermediate Values Require Careful Interpretation

FeNO values in the intermediate range (25–40 ppb in adults; 20–35 ppb in children) represent a diagnostic grey zone where neither eosinophilic disease nor its absence can be confidently established. These values should not be dismissed as “normal” nor trigger automatic treatment escalation. Instead, clinicians should correlate with clinical symptoms, allergy testing, blood eosinophils, and bronchodilator reversibility to triangulate the most likely diagnosis. A supervised therapeutic ICS trial with objective response assessment may be warranted.

Standardise Conditions for Reliable Results

Patient preparation significantly affects FeNO accuracy. Instruct patients to avoid eating or drinking (especially nitrate-rich foods or caffeine), smoking, strenuous exercise, and alcohol for at least one hour before testing. Spirometry should ideally be performed after FeNO measurement, as forced exhalation manoeuvres can transiently alter nitric oxide readings. Document recent corticosteroid use (oral or inhaled) as this will suppress values and must be noted when interpreting results.

Establish a Personal Baseline Early in Care

Population-derived thresholds are clinically useful starting points, but individual variability is substantial. Measuring FeNO during confirmed periods of clinical stability — when symptoms are well-controlled and treatment is consistent — establishes a personal best baseline. Subsequent deviations of >20% from this individual reference are more sensitive and specific for detecting loss of control than comparing to population norms alone. This transforms FeNO from a cross-sectional snapshot into a powerful longitudinal monitoring tool.

Using FeNO to Engage and Educate Patients

FeNO results can be a powerful communication tool in shared decision-making. Showing a patient a high FeNO value alongside the explanation that their airways are actively inflamed — even when they feel “not too bad” — can improve understanding of why daily controller therapy is necessary and motivate adherence. Similarly, demonstrating a falling FeNO in response to good inhaler technique reinforces behaviour change with objective, real-time biological feedback, which is far more compelling than symptom scores alone.