Authors: Jui Andharia (1Birmingham Women's and Children's NHS Foundation Trust, Birmingham, UK; 3Department of Paediatric Pulmonology, Bai Jerbai Wadia Hospital for Children, Mumbai, India), Prasad Nagakumar (1Birmingham Women's and Children's NHS Foundation Trust, Birmingham, UK; 2Department of Inflammation and Ageing, University of Birmingham, Birmingham, UK)
Categories: Review
Source: Breathe
Authors: Jui Andharia, Prasad Nagakumar
Paediatric asthma is a heterogenous clinical syndrome where objective tests aid, but do not replace, clinical judgment. In 2026, the challenge is determining which biomarkers and tests are most informative for different patients and purposes. This review summarises current evidence supporting key diagnostic and monitoring tools for childhood asthma (age 5–16 years): exhaled nitric oxide fraction (FENO), spirometry with bronchodilator reversibility (BDR), and additional investigations such as peak expiratory flow variability, blood eosinophils, allergen sensitisation and bronchial challenge.
Recent British Thoracic Society/National Institute for Health and Care Excellence/Scottish Intercollegiate Guidelines Network and European Respiratory Society guidelines emphasise the need for objective confirmation of asthma, but differ in the degree of diagnostic certainty and test prioritisation. FENO is now the leading inflammatory biomarker in routine paediatric care, valued for its noninvasive nature, ease and high specificity at elevated thresholds, especially for type 2 asthma. Its sensitivity, however, is modest, so it should not be relied upon as a stand-alone rule-out test. Spirometry remains a core physiological assessment, although its diagnostic accuracy alone is limited, and evidence supporting BDR thresholds in children is not robust. Additional tests should be used selectively, particularly when initial investigations are negative, discordant or technically challenging, or when severe asthma requires phenotype driven therapy. Biomarkers also play an important role in monitoring and severe asthma care, including predicting response to biologics. Emerging approaches like breathomics, alarmin-based biomarkers, and multi-omic profiling are discussed. Overall, biomarkers have enhanced the precision of paediatric asthma assessment, especially in severe cases, but their greatest value is in complementing, not replacing, thorough clinical evaluation.
Asthma in childhood remains a clinical diagnosis supported, rather than replaced, by tests. What has changed in 2026 is not that biomarkers have solved diagnostic uncertainty, but that recent guidelines define more clearly which objective tests deserve prominence and how they should be interpreted in children. The 2024 British Thoracic Society (BTS)/National Institute for Health and Care Excellence (NICE)/Scottish Intercollegiate Guidelines Network (SIGN) guideline for children aged 5–16 years places exhaled nitric oxide fraction (FENO) first in the diagnostic pathway, followed by spirometry with bronchodilator reversibility (BDR), peak expiratory flow variability, and then allergy-related tests or blood eosinophils if doubt remains [1]. By contrast, the European Respiratory Society (ERS) diagnostic guideline for the same age group advises against diagnosis from history alone or from a single abnormal objective test, and generally recommends confirmation with two positive objective tests [2]. These differences reflect a genuine tension in paediatric the need to avoid both overdiagnosis and diagnostic delay [1, 2].
For clinicians, the clinically useful question is not which single biomarker is “best”, but what each test adds, what it misses, and how it performs in real children rather than idealised trial populations. This review therefore focuses on the three areas most relevant to current FENO; spirometry and BDR; and other tests used for diagnosis and monitoring, including allergy-related measures, blood eosinophils and bronchial challenge (table 1). It also considers how biomarkers are used in routine monitoring and severe asthma, and where the field is likely to move next [1, 2]. Moreover, the availability of these tests in routine clinical practice is limited by funding and staff training [3].
ENO
FENO has become the most talked about inflammatory biomarker in school-age paediatric asthma because it is noninvasive, rapid and biologically plausible. It reflects inducible nitric oxide synthase activity in the airway epithelium, driven largely by type 2 inflammation, and therefore performs best in eosinophilic, allergic disease. In the 2024 BTS/NICE/SIGN guideline, a FENO value of 35 ppb or more in a child aged 5–16 years with a suggestive history is sufficient to confirm asthma. The American Thoracic Society (ATS) FENO guideline remains useful for values below 20 ppb in children make eosinophilic inflammation less likely, values above 35 ppb make it more likely, and intermediate values require interpretation in clinical context [1, 4, 5].
The main strength of FENO is specificity rather than sensitivity. NICE's 2024 diagnostic evidence review concluded that, in children, high FENO thresholds are highly specific for asthma, whereas sensitivity is more modest, especially at lower thresholds [4]. This is why NICE uses FENO as a rule-in rather than a rule-out test. A raised FENO in a child with episodic wheeze, cough or breathlessness strongly supports asthma; a normal FENO does not exclude it. This distinction is crucial in paediatrics, where non-atopic asthma, intermittent disease and prior corticosteroid exposure all reduce sensitivity [1, 4].
FENO must also be interpreted against phenotype. It is strongest when the clinical question is whether the child has ongoing type 2 airway inflammation. It is weaker when the question is simply whether the child has asthma of any kind. Atopy, allergic rhinitis, eczema, allergen exposure and inhaled corticosteroid treatment all influence FENO. A raised FENO is therefore not synonymous with asthma, and a normal FENO does not exclude clinically important disease. This limitation reflects asthma heterogeneity rather than technical failure of the test [4, 5]. Although FENO is not effort-dependent and is recommended from age 5 years, reliable performance is usually achieved after the age of 8 years [6].
The monitoring literature is more nuanced. NICE's monitoring review concluded that FENO monitoring may reduce exacerbations and improve lung function in some settings, and therefore recommended considering FENO measurement at routine review or when maintenance treatment may need adjustment [7]. However, the paediatric RAACENO trial found that adding FENO-guided treatment decisions to symptom-guided care did not significantly reduce asthma attacks over 12 months in children already considered at risk of exacerbation [8]. The practical lesson is not that FENO lacks value, but that FENO-driven algorithms are not uniformly superior to high-quality usual care [1, 7, 8].
In practice, FENO is most useful in three situations. 1) It supports diagnosis when the history is suggestive and the value is clearly raised. 2) It helps interpret discordance, for example when a child reports good control but FENO remains high. 3) In difficult or severe asthma, it contributes to phenotype definition, especially when interpreted alongside eosinophils, sensitisation and adherence. Persistent elevation despite prescribed inhaled corticosteroids should prompt scrutiny of adherence, inhaler technique and exposure rather than immediate escalation to biologics [1, 5].
Spirometry remains foundational in paediatric asthma, but its role is often overstated. It is indispensable because asthma is partly a disorder of variable airflow obstruction, and spirometry can identify alternative or coexisting pathology. However, NICE's 2024 evidence review found that the diagnostic accuracy of spirometry alone in children was only moderate and concluded that it should not be used in isolation to diagnose asthma [9]. This is consistent with clinical many children with asthma have normal spirometry between attacks, particularly if symptoms are intermittent or treatment has already begun [1, 9].
Spirometry is therefore best understood as a physiological test rather than a disease-specific biomarker. It characterises airflow limitation, provides baseline lung function, identifies disproportionate or fixed obstruction, and gives context to BDR. It is especially important in severe asthma, where reduced lung function may reflect cumulative burden, poor control or an alternative diagnosis. What it cannot do reliably is settle the diagnosis on its own [1, 9].
BDR is conceptually attractive because it captures one of the classic physiological features of reversibility of airflow obstruction. The BTS/NICE/SIGN guideline confirms asthma in children when forced expiratory volume in 1 s (FEV1) increases by at least 12% from baseline, or by at least 10% of predicted normal FEV1, after bronchodilator. Yet, NICE's own 2024 review found no directly relevant paediatric diagnostic accuracy studies of BDR in children being investigated for asthma. The recommendation therefore rests on extrapolation, modelling and committee consensus more than on direct paediatric accuracy data. This should encourage more cautious interpretation in clinical practice [1, 10].
A further problem is that BDR is not a fixed biological phenomenon but an interpreted physiological response. Newer ERS/ATS approaches favour expressing change relative to predicted values rather than only as percentage change from baseline. A 2025 paediatric analysis showed that using newer interpretive methods reclassified a proportion of children as bronchodilator-responsive compared with older definitions, with the greatest discordance in younger, shorter children and in girls. This highlights an important point for paediatric a “positive” bronchodilator response partly depends on the formula used, and paediatric-specific validation remains incomplete [11].
The ERS diagnostic guideline addresses this uncertainty by recommending that diagnosis should usually rely on two positive objective tests and by advising against diagnosis from history alone or from a single abnormal result [2]. This is conceptually compelling because neither spirometry nor BDR is sufficiently strong as a stand-alone discriminator. The trade-off is feasibility. In real practice, children may have only one clearly positive test during an assessment pathway, especially if symptoms are intermittent or access is limited. The UK pathway is therefore more operational; the ERS approach is more stringent. Neither is irrational. They simply prioritise different risks [1, 2].
For monitoring, the evidence base is thinner than routine practice implies. NICE found little direct evidence that spirometry-driven monitoring improves outcomes beyond standard review; yet, spirometry still has clinical value at follow-up because it identifies persistent obstruction, tracks lung function trajectory and highlights discordance between symptoms and physiology. In severe paediatric asthma, serial spirometry remains important for severity assessment and for consideration of alternative diagnoses, but it should be viewed as one dimension of assessment rather than a stand-alone marker of control [1, 7].
In a quality improvement study of 111 children in the West Midlands, UK, referred from primary care for asthma diagnosis, spirometry was normal in 96.3% of those with cough and wheeze, and FENO was normal in 64.2% of those tested, suggesting limited diagnostic yield when tests are performed opportunistically rather than during symptomatic periods [12]. Moreover, definitive cut-off values for these tests are yet to be determined. In the Swiss Paediatric Airway Cohort, the NICE algorithm had a sensitivity of 69% and specificity of 67%, whereas the Global Initiative for Asthma algorithm had lower sensitivity (42%) but higher specificity (90%) [13]. In a population-based birth cohort, NICE paediatric cut-offs for FEV1/forced vital capacity, FEV1 and BDR showed sensitivities below 10%, while FENO had a sensitivity of 44% and specificity of 84% [14].
Tests beyond FENO and spirometry are clinically important, but mainly because they resolve uncertainty or refine phenotype rather than because they function well as universal first-line tools. Peak expiratory flow variability remains in the UK diagnostic pathway as a pragmatic option when spirometry is unavailable or delayed; a variability of 20% or more over a 2-week period supports asthma in a child with a compatible history. However, NICE's monitoring review found that routine peak flow monitoring was not helpful for most people and could even be associated with increased attacks, leading to a recommendation against routine use except in selected individuals, such as those who under-perceive deterioration [1, 7, 15].
Allergy-related tests occupy a different niche. In the BTS/NICE/SIGN pathway, if FENO and BDR are negative or not possible but suspicion remains, house dust mite skin prick testing or total IgE with blood eosinophils may add weight to the diagnosis. These tests do not diagnose asthma directly; instead, they increase the probability that compatible symptoms represent type 2 or allergic airway disease. NICE's paediatric evidence review found useful specificity for some allergy-related measures, but the evidence base was limited and did not justify their use as first-line tests. Blood eosinophils behave a raised count is more helpful when present than when absent, especially in difficult asthma, but normal values do not exclude asthma [1, 16].
Bronchial challenge remains the best-established specialist investigation when first-line tests are unrevealing. NICE recommends specialist referral for methacholine or histamine challenge when diagnostic uncertainty persists. The paediatric evidence review suggested that methacholine challenge can perform reasonably well diagnostically, whereas exercise challenge has less support as a general diagnostic tool. This distinction fits clinical exercise testing is most useful when exertional symptoms predominate, whereas direct bronchial provocation is a more general marker of airway hyperresponsiveness [1, 17].
For monitoring, validated symptom questionnaires remain useful as structured tools, but they are not biomarkers. Their value lies in standardising review and ensuring that control is assessed consistently. In severe asthma, however, combining FENO, eosinophils, total IgE, sensitisation profile and spirometry is far more informative than any single test. This is where biomarkers matter not in replacing clinical judgement, but in determining whether uncontrolled disease is allergic, eosinophilic or mixed, and in distinguishing persistent inflammation from poor adherence or comorbidity [1, 18].
Biomarkers are more useful for augmenting asthma monitoring than for replacing clinical review. In routine paediatric practice, FENO is the most informative monitoring biomarker because it reflects current type 2 airway inflammation and can identify discordance between symptoms and airway activity. The ATS guideline emphasises that serial change is more informative than a single value, and NICE recommends considering FENO at review when treatment adjustment is being contemplated. Blood eosinophils may add prognostic value, especially in children with recurrent exacerbations, but they are less responsive than FENO to short-term change and are influenced by corticosteroid exposure [1, 5, 7].
The evidence base is mixed. NICE judged that FENO monitoring may reduce exacerbations in some settings, but the RAACENO trial showed that a FENO-guided strategy did not significantly outperform optimised symptom-guided care in children prone to attacks [8]. The practical implication is that biomarkers should not be used in isolation to escalate or de-escalate treatment. Their greatest value lies in targeted suspected nonadherence, persistent type 2 inflammation despite apparently good control, or uncertainty about whether symptoms reflect asthma, dysfunctional breathing or comorbidity. The most defensible paediatric approach remains multidimensional symptoms, exacerbation history, lung function and selected biomarkers interpreted together [1, 7, 8].
Biomarkers are most clinically consequential in severe paediatric asthma, where they move from being descriptive to treatment-enabling. The ERS/ATS severe asthma guideline explicitly considered whether biomarkers should guide initiation of anti-interleukin (IL)-5/IL-5Rα and anti-IgE therapy, reflecting the central role of blood eosinophils, FENO, total IgE and allergen sensitisation in specialist practice. In the current UK pathway, difficult and severe asthma management signposts phenotype-directed biologics including omalizumab, mepolizumab, dupilumab and tezepelumab [18, 19].
The strongest paediatric predictive evidence currently exists for dupilumab. In a 2024 paediatric analysis, baseline blood eosinophils and FENO were both prognostic and predictive higher levels identified children at greater exacerbation risk and with larger treatment benefit, with the greatest responses seen when both biomarkers were raised. This aligns with the NICE guidance for dupilumab, which positions it for severe asthma with type 2 inflammation and reflects the practical reliance on eosinophils and FENO in specialist selection [20, 21].
For mepolizumab and omalizumab, biomarker use is more phenotype based than driven by refined paediatric response-prediction models. Omalizumab remains linked to severe allergic asthma with relevant sensitisation and total IgE in range, while mepolizumab is linked to severe eosinophilic asthma in specialist pathways. Tezepelumab broadens the landscape NICE recommends it for severe asthma from age 12 years irrespective of classical biomarker status, although efficacy is generally greater in biomarker-high subgroups. In practice, biomarkers in severe asthma should therefore be used to refine phenotype, estimate biologic responsiveness and monitor residual inflammation, but only after adherence, inhaler technique and comorbidity have been addressed systematically [18, 22–24].
The most promising future biomarkers in paediatric asthma are unlikely to be single molecules. They are more likely to be integrated signatures combining inflammatory, metabolic and physiological information. Breathomics, particularly analysis of exhaled volatile organic compounds, is one of the most attractive approaches because it is entirely noninvasive. Recent reviews suggest that breathomic profiling has potential applications in diagnosis, phenotyping, assessment of control and prediction of exacerbations in childhood asthma, but methodological heterogeneity and the lack of standardised signatures remain major barriers [25].
Recent paediatric data are encouraging but still preliminary. A 2024 multicentre study reported that exhaled volatile organic compound profiles could classify controlled versus uncontrolled asthma in children with moderate-to-severe disease, supporting the idea that breath analysis may eventually provide a practical point-of-care biomarker platform. However, this remains a research tool rather than a clinically validated assay [26].
Beyond breathomics, alarmin-related biomarkers may become important for prognosis and remission. A 2025 paediatric study suggested that serum thymic stromal lymphopoietin was independently associated with clinical remission in type 2-high childhood asthma, although its performance alone was modest and likely greatest as part of a composite panel rather than as a stand-alone test. More broadly, multi-omic approaches incorporating transcriptomic, proteomic, metabolomic and microbiome data are likely to matter most in severe asthma, where current biomarkers incompletely capture non-type 2 disease. Oscillometry may also expand the physiological armamentarium, particularly in children who struggle with spirometry. The main challenge for the next phase is not biomarker discovery alone, but validation in real-world paediatric cohorts and proof that biomarker-guided decisions improve exacerbations, steroid burden, lung growth and quality of life [11, 25, 27].
In 2026, biomarkers in paediatric asthma are clinically useful, but unevenly so. FENO has the strongest claim to routine value because it provides inflammatory information not available from symptoms or spirometry and, at higher thresholds, is highly specific for asthma in school-age children. Spirometry remains indispensable, but mainly as a physiological framework; as a stand-alone diagnostic test it is insufficient, and BDR thresholds remain under-validated in paediatric populations. Other tests, including peak flow variability, allergy-related measures, blood eosinophils and bronchial challenge, are best used selectively, especially when first-line tests are negative or discordant, or when severe asthma requires phenotype-directed therapy. The central educational message is not that paediatric asthma has become a biomarker-defined disease. It has not. Rather, we now have better evidence about which tests deserve prominence, which deserve caution, and why guideline differences often reflect honest uncertainty rather than error [1, 2, 9, 15].