Review of Respiratory Medicine - Volumen 25, Número 2 - June 25

Review Article

Fractional Exhaled Nitric Oxide (FeNO): An Approach to its Use in Asthma

Fracción exhalada de óxido nítrico (FeNO): Un acercamiento para su uso en asma

Recibido: 09/20/2024

Aceptado: 06/26/2025

PHYSIOLOGY

Nitric oxide (NO) is a gas that is present in all human organ systems and has many functions: it acts as a vasodilator, bronchodilator, neurotrans­mitter, and inflammatory mediator. Its properties against free radicals make it highly bactericidal and cytotoxic. ⁽¹⁾

In the respiratory system, NO acts in both a pro- and anti-inflammatory manner. This duality is very important to understand its role in type-2 airway inflammation in asthma. ⁽¹⁾

In the lungs, it is generated by epithelial, vas­cular endothelial, and neuronal cells. ⁽²⁾

It is produced enzymatically and non-enzymat­ically. When produced through enzymatic process­es, it is formed by the nitric oxide synthase (NOS) in the airways from L-arginine, which is absorbed by epithelial cells. There are three isoforms of NOS in the lungs: constitutive neuronal NOS (nNOS), constitutive endothelial NOS (eNOS), and induc­ible NOS (iNOS). ⁽²⁾

The constitutive forms of NOS are known as cNOS and generate low quantities of NO in bursts. The iNOS expression depends on the transcription and produces large quantities of NO for prolonged periods. ⁽²⁾

In asthma, the production of NO derived from cNOS is reduced, whereas NO levels generated by iNOS increase, and this causes bronchoconstric­tion and airway inflammation. ⁽²⁾

Also, the presence of pro-inflammatory cy­tokines, such as TNF (tumor necrosis factor), interferon-gamma, and interleukin-13 triggered by allergen exposure leads to an overexpression of iNOS.

This increase in NO significantly enhances its physiological functions, thus producing hyperemia, hypotension, and edema. NO derived from iNOS prolongs T2 inflammation, causing pulmonary epi­thelial damage, mucus hypersecretion, increased vascular permeability, bronchial hyperreactivity, and eosinophilia. ⁽²⁾

ASTHMA AND FENO

The use of chemiluminescence analyzers allowed the detection of NO in exhaled air in the early 1990s.

It was found that asthma patients had high fractional exhaled nitric oxide (FeNO) in their exhaled air, and reduced response to corticosteroid treatment. This quickly encouraged the evalua­tion of FeNO as a possible non-invasive method to diagnose and monitor the response to the anti-inflammatory therapy in the disease.

There is solid evidence that FeNO levels corre­late with the characteristics of T2 inflammation, especially eosinophil levels in peripheral blood and induced sputum, providing a non-invasive way to assess T2 airway inflammation in asthma ⁽³⁾.

FeNO levels do not correlate well with the basal degree of airway obstruction but do correlate with the severity of symptoms and the characteristics of “indirect” or “endogenous” airway hyperreactivity, such as exercise-induced bronchoconstriction ⁽³⁾.

TECHNOLOGIES FOR FENO ANALYSIS

Chemiluminescence techniques are considered the “gold standard” for measuring FeNO. Calcula­tions can be very precise, with thresholds in parts per billion (ppb), and very fast response times of less than one second. But these technologies use large and expensive devices that require constant calibration. ⁽⁴⁾

On the other hand, electrochemical FeNO detec­tion systems convert the NO concentration into an electrical signal that is read later.

Some of the advantages include a smaller size and lower weight, which make them potentially portable, and they do not require external calibra­tion. And two of the disadvantages would be lower sensitivity and longer response time. ⁽⁴⁾

USE AS DIAGNOSIS IN ASTHMA

The FeNO test is classified as an inflammometry test.

It can be positive, associated with type 2 (T2) inflammation, or negative, in the neutrophilic asthma phenotype. Additionally, it may show moderate to strong correlations with eosinophil levels in blood and sputum.

However, as a standalone test outside the clini­cal context, FeNO cannot be considered as useful to definitively rule out or confirm asthma, because FeNO levels can be influenced by external factors that may cause false positives or false negatives. ⁽⁵⁾ (see Table 1).

In asthma diagnosis, international guidelines recommend using bronchoprovocation tests when lung function is normal to further evaluate symp­toms suggestive of asthma. However, these tests can be cumbersome, costly, and carry the risk of acute bronchospasm. ⁽²⁾

FeNO is an established biomarker that reflects underlying airway inflammation and provides an easy, non-invasive, and reproducible means of detecting airway inflammation. ⁽⁶⁾

FENO MEASUREMENT METHOD

The patient must be seated comfortably, with the mouthpiece placed at the correct height and position. No nasal clip should be used, since it may allow the nasal accumulation of nitric oxide (NO) and cause NO to leak through the posterior nasopharynx. The patient inserts a mouthpiece, inhales for 2-3 seconds through the mouth until reaching total lung capacity (TLC), and then ex­hales immediately, since holding their breath can affect the value obtained. The resulting pressure at the mouthpiece should be at least 5 cm H₂O to ensure closure of the soft palate and prevent nasal NO from contaminating the exhaled air. An exhala­tion flow rate of 50 ml/second is recommended. ⁽⁷⁾

Repeated and reproducible exhalations should be performed to obtain at least two NO plateau values with a difference of less than 10%. So, the FeNO is calculated as the average of these two values. For certain purposes, it may be better to obtain three reproducible FeNO values. There should be at least 30 seconds of normal breathing between each maneuver. ⁽⁷⁾

SPECIAL CONSIDERATIONS

It is important to take into account the daily variation of FeNO according to the time of day the measurement is taken. The range of diurnal variation was between 24% and 29%, with higher values observed in the morning. ⁽⁸⁾

The type of analyzer used can influence the re­sults. For example, the average values in children using NObreath (Intermedical, Aylesford, UK) and NIOX Vero were 20 ppb and 27 ppb, respectively. Thus, caution should be exercised when using a low value with the NObreath device to rule out asthma. ⁽⁸⁾

It is recommended to discontinue inhaled cor­ticosteroids (ICS) for at least two weeks before conducting a FeNO test when trying to establish an asthma diagnosis in uncertain cases. If fluticasone furoate (FF) is used, up to 21 days of discontinua­tion may be necessary to reach baseline levels. ⁽⁸⁾

A FeNO suppression test with ICS can help iden­tify patients with severe refractory asthma who aren’t adhering to ICS treatment. This step can be crucial before prescribing expensive biological drugs to these patients. This test involves measur­ing FeNO levels before and after administering supervised, high-dose corticosteroids over a short period (usually 7-14 days). A significant drop in FeNO levels following treatment indicates that the inflammation is responsive to corticosteroids and suggests good adherence, whereas a lack of suppression might indicate biological resistance to corticosteroids or poor treatment adherence. ^{(9) (10)}

Since the ACQ (Asthma Control Questionnaire) and blood eosinophil counts are strong predictors of future exacerbations, FeNO alone should not be used to stratify risk. A FeNO concentration greater than or equal to 50 ppb, together with a blood eosinophil count ≥ 300/mL, was associated with a 73% increase in the exacerbation risk, whereas a FeNO concentration ≥ 25 ppb with blood eo­sinophils ≥ 300/mL resulted in a 70% increase. In turn, this clearly emphasizes the importance of not relying on FeNO alone as a marker of type 2 inflammation, especially since eosinophils are primarily driven by IL-5. ⁽⁸⁾

RECOMMENDATIONS FOR DIAGNOSIS

– Low FeNO measurement (< 25 ppb in adults and < 20 ppb in children).

The likelihood of eosinophilic inflammation and corticosteroid response is low.

– Intermediate FeNO measurement (between 25 and 50 ppb in adults and between 20 and 35 ppb in children).

Evaluate clinical context and monitor changes in FeNO levels over time.

– High FeNO measurement (> 50 ppb in adults and > 35 ppb in children).

Presence of eosinophilic airway inflammation and a likely positive response to corticosteroids. ⁽¹⁾

FOLLOW-UP OF PATIENTS DIAGNOSED WITH ASTHMA

In 2021, the ATS (American Thoracic Society) is­sued an official clinical practice guideline in which a panel of experts addressed a single question: Should asthma patients who are being considered for treatment undergo a FeNO test?

The panel concluded that the evidence sup­ports performing a FeNO test during the evalu­ation of a patient with asthma, in addition to standard care. Although the desirable effects of the intervention were relatively modest in magnitude, the panel prioritized outcome mea­surements that showed a desirable effect before the analysis and revealed a reduction in the frequency of exacerbations and the use of oral corticosteroids, two critical outcomes in asthma management. ⁽³⁾

In a post-hoc analysis of a double-blind study of dupilumab vs. placebo, it was observed that in patients with uncontrolled moderate to severe asthma (treated with inhaled corticosteroids + two controller medications, and with one or more exacerbations in the last year), higher baseline FeNO levels (> 50 ppb) were associated with an increased risk of severe asthma exacerbations—es­pecially when combined with an elevated eosino­phil count (>150 cells/μL) and a history of previous exacerbations. This supports the added value of FeNO as a prognostic biomarker. The situation is less clear in patients with mild asthma not treated with ICS. To date, only one study has assessed the prognostic and predictive value of type 2 inflam­matory biomarkers in this group of patients. It found that the blood eosinophil count identified patients at risk of severe exacerbations and those likely to respond well to maintenance treatment with ICS. However, FeNO didn’t show this ability and, consequently, did not provide any additional prognostic information beyond that delivered by the blood eosinophil count. ⁽¹¹⁾

FENO VALUES IN THE FOLLOW-UP OF PATIENTS DIAGNOSED WITH ASTHMA

– Low FeNO measurement (< 25 ppb in adults and < 20 ppb in children).

A. Symptomatic patients: possible alternative di­agnosis/unlikely to benefit from increasing ICS dose.

B. Asymptomatic patients: adequate ICS dose/good adherence/consider dose reduction.

– Intermediate FeNO measurement (between 25 and 50 ppb in adults and between 20 and 35 ppb in children).

A. Symptomatic patients: persistent allergen ex­posure/inadequate ICS dose/poor adherence/ steroid resistance.

B. Asymptomatic patients: adequate ICS dose/good adherence/monitor FeNO levels.

– High FeNO measurement (> 50 ppb in adults and > 35 ppb in children) or > 40% increase from stable levels.

A. Symptomatic patients: persistent allergen expo­sure/inadequate ICS dose/poor adherence/steroid resistance/poor inhalation technique.

B. Asymptomatic patients: withdrawing or reduc­ing ICS dose may lead to relapse. ⁽¹⁾

CONCLUSIONS

Establishing or ruling out an asthma diagnosis in a significant group of patients is often complicated due to many factors. The variability of symptoms over time and in intensity, normal results in pul­monary function tests, especially spirometry, and the frequent use of self-medication or isolated treatment during acute or subacute episodes make it difficult to reach a conclusion in these patients.

The availability of a new tool that can evaluate another dimension, which we might call “inflam­mometry”, does not completely eliminate the previously mentioned uncertainties, but it can provide more information before making a clini­cal decision.

The objective of this article is to explore this tool (FeNO), which is currently rare in our setting, but may become more widely used soon thanks to the development of new, more affordable, and portable devices.

Conflict of interest

The author has no conflict of interest to declare.

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