Pathophysiological Basis

Pediatric asthma encompasses diverse phenotypes, prominently Type 2 (T2) High Asthma, which is driven by an eosinophilic inflammatory cascade.
Allergen exposure triggers the release of epithelial alarmins (TSLP, IL-33, IL-25), activating T-helper 2 (Th2) cells and Type 2 Innate Lymphoid Cells (ILC2s) to secrete IL-4, IL-5, and IL-13.
IL-13 profoundly upregulates the expression of inducible Nitric Oxide Synthase (iNOS) within bronchial epithelial cells.
iNOS converts L-arginine into L-citrulline and Nitric Oxide (NO).
FeNO serves as a direct, highly sensitive, quantitative, and non-invasive biomarker of IL-13-driven T2 eosinophilic airway inflammation.

Standardization of Measurement

American Thoracic Society (ATS) and European Respiratory Society (ERS) protocols mandate inhalation to total lung capacity, followed by exhalation at a strictly controlled flow rate of 50 mL/second for 6 to 10 seconds.
FeNO must be measured prior to spirometry or bronchodilator administration, as forced maneuvers transiently lower NO levels.

FeNO Level	Clinical Interpretation	Therapeutic Implication
Low (<20 ppb)	T2 inflammation unlikely.	Symptoms likely stem from non-T2 phenotypes (neutrophilic, obesity-related) or alternate diagnoses (vocal cord dysfunction, primary ciliary dyskinesia); unlikely to respond robustly to Inhaled Corticosteroids (ICS).
Intermediate (20–35 ppb)	Gray area.	Indicates evolving inflammation or partial suppression by ongoing ICS therapy.
High (>35 ppb)	Active T2 inflammation.	Strongly indicates active, symptomatic eosinophilic inflammation; predicts a highly robust and rapid response to ICS therapy.

Note: For adolescents ≥12 years, the cut-offs shift to <25 ppb (Low), 25–50 ppb (Intermediate), and >50 ppb (High).

Diagnostic Adjunct: An elevated FeNO (>35 ppb in children, >50 ppb in adolescents) strongly supports the diagnosis of atopic, eosinophilic asthma, particularly when spirometry is normal or unattainable.
Assessing ICS Adherence: Non-adherence is the leading cause of poor pediatric asthma control. A FeNO >60 ppb in a child prescribed high-dose ICS indicates profound non-adherence or improper inhaler technique, rather than true steroid-resistant biology.
Predicting Exacerbations: A progressively rising FeNO in an asymptomatic child on maintenance therapy acts as an early warning system for an impending exacerbation, preceding lung function (FEV1) decline.
Guiding Step-Down Therapy: Stepping down ICS therapy is safe when FeNO is securely <20 ppb, whereas stepping down with a FeNO remaining at 45 ppb strongly predicts relapse.

Global Initiative for Asthma (GINA) guidelines mandate FeNO assessment during the phenotypic evaluation of severe asthma.
A baseline FeNO ≥20 ppb indicates eligibility for targeted biologic therapies.
Dupilumab (Anti-IL-4Rα): Blocks the IL-13/iNOS pathway, making FeNO the most accurate biomarker for predicting clinical response and monitoring drug efficacy as levels rapidly plummet.
Tezepelumab (Anti-TSLP): Profoundly suppresses FeNO by blocking the inflammatory cascade at the epithelial alarmin level.
Omalizumab and Anti-IL-5 agents (Mepolizumab/Benralizumab): Exert minimal, variable effects on FeNO as they do not directly block the IL-13/iNOS pathway.

Factors Falsely Elevating FeNO	Factors Falsely Depressing FeNO
Allergic rhinitis	Active or secondhand cigarette smoke exposure
Consumption of dietary nitrates (spinach, beetroot)	Bronchoconstriction
Viral upper respiratory tract infections (Rhinovirus)	Forced spirometry maneuvers performed prior to test

The Indian Academy of Pediatrics (IAP) advocates the integration of FeNO in tertiary pediatric pulmonary clinics primarily for managing difficult-to-treat asthma, monitoring adherence, and rationalizing the use of expensive biologic therapies.
Recent 2025 health economic modeling proves routine FeNO monitoring is highly cost-effective by averting emergency department visits, preventing unnecessary hospitalizations, and identifying non-adherence.