Type‐2 Biomarkers, Asthma Diagnosis and Lung Function Impairment in the General Population

The diagnosis of asthma is typically centred around the presence of four key hallmark features: variable respiratory symptoms, chronic airway inflammation, airway hyperresponsiveness and variable expiratory airflow limitation [1-3]. However, bronchodilator reversibility (BDR) is present in < 20% of (mild-to-moderate) asthma cases in primary care, limiting its diagnostic value [4]. Recent guidelines have shifted towards type-2 (T2) biomarkers. The 2022 ERS guideline supports FeNO > 50 ppb to confirm asthma when spirometry is normal or BDR absent, while recommending blood eosinophil counts (BEC) only for phenotyping [1]. The 2020 NAEPP guideline similarly endorses FeNO as an adjunct test, but not BEC [5]. In contrast, the 2024 BTS/SIGN/NICE guideline proposes FeNO (> 50 ppb) or elevated BEC as first-line diagnostic tests, removing the requirement for spirometry [2]. The Global Initiative for Asthma (GINA) recommends the use of FeNO and BEC mainly for asthma phenotyping [3]. We evaluated this biomarker-first approach as a potential screening tool for asthma in the general population using U.S. NHANES 2007–2012 data from 18,319 participants (aged 6–80) with FeNO and BEC measurements and asthma status (self-reported physician-diagnosed current asthma). FeNO was measured with NIOX MINO (NIOX MINO, Aerocrine, Stockholm, Sweden). FeNO was defined as normal (< 20 ppb in children and adolescents aged 6–18/< 25 ppb in adults), intermediate (20–35/25–50 ppb) and high (≥ 35/≥ 50 ppb). Associations with asthma status were assessed using multivariable regression adjusted for demographic and clinical covariates. Associations between asthma status, high biomarkers and airflow obstruction were restricted to adults ≥ 18 years without emphysema (N = 12,575). Airflow obstruction was defined as prebronchodilator FEV1/FVC below the lower limit of normal using the GLI-2012 reference values. The average age in the study group was 36 years. This study group included 1584 participants with current asthma (8.6%). Compared to controls, participants with current asthma were generally younger (32.7 years), had higher BMI (28.1 ± 8.7 kg/m2) and 56.2% were female. Reported use of oral and inhaled corticosteroids in asthmatics amounted to 3.6% and 21.8%, respectively (of which 12.8% reported use of ICS/LABA inhaler). Overall asthma prevalence increased from 5.9% in participants with normal FeNO and BEC < 150/μL to 31.7% in those with high FeNO and BEC > 500/μL (Figure 1A). In each of the three FeNO categories, asthma prevalence increased gradually according to increasing BEC levels (5.9% to 13% in the case of low FeNO, 8.8% to 23.6% in intermediate FeNO and 4.4% to 31.7% in participants with high FeNO). Compared to individuals with low BEC and FeNO, moderate increases in BEC (150–300/μL) were associated with 1.3- to 3.2-fold higher odds of having current asthma depending on FeNO levels. In those with the highest BEC and FeNO, 3.7- and 7.1-fold higher odds of asthma were found (online supplement). Using cut-offs of FeNO > 50 ppb and/or BEC > 300 cells/μL, we found heterogeneity between biomarker-positive individuals, those with an asthma diagnosis and those with airflow obstruction (Figure 1B). A distinct subgroup exists: individuals without a formal asthma diagnosis who nonetheless have elevated biomarkers, and in some cases, co-existing airflow obstruction. Significantly lower FEV1% predicted was also seen in non-asthmatics with BEC 300–500/μL and > 500/μL but normal FeNO compared to those with normal FeNO and BEC < 300/μL (Supporting Information). In this large population-based study, we demonstrate that higher levels of FeNO and BEC consistently relate to increased asthma prevalence in the general population. Perhaps most strikingly, over two thirds of people with high FeNO and BEC above 500/μL did not have a diagnosis of asthma, potentially indicating underdiagnosis of (T2-) asthma. In support of this hypothesis, we identified a subgroup of participants without an asthma diagnosis but with increased T2-biomarkers and airflow obstruction, and found that participants without asthma and high T2 biomarkers had significantly lower FEV1 compared to those without asthma but low T2 biomarkers. These findings support the use of both T2 biomarkers FeNO and BEC in screening for (T2-high) asthma in adults in the general population. While T2 biomarkers may be easier to perform in primary care compared to spirometry with BDR, the isolated use of FeNO and/or BEC may lead to overdiagnosis of asthma. Diagnostic accuracy studies in high pre-test likelihood populations have previously indicated moderate diagnostic accuracy of FeNO and BEC [6]. Therefore, clinicians should be advised to interpret elevated FeNO and BEC in conjunction with objective lung function testing (e.g., spirometry with BDR or bronchial challenge), where available, and to remain vigilant for alternative diagnoses or asthma phenotypes that may not present with elevated T2 biomarkers [7, 8]. The strengths of the NHANES study include its large-scale representative setup with systematically collected questionnaires, spirometry, FeNO measurements and BEC. The NHANES data we have used were collected more than a decade ago when diagnostic asthma algorithms did not yet recommend FeNO or BEC measurements, providing us the opportunity to investigate the diagnostic utility of these T2 biomarkers without diagnostic bias. Limitations of our study include the self-reported nature of physician-diagnosed asthma, which might give rise to recall bi as, misclassification and over- or underdiagnosis [9]; the lack of allergy testing and the fact that clinically relevant outcomes (asthma attacks, ER visits for asthma) were only collected in those with an asthma diagnosis. In summary, our analysis highlights the potential of T2 biomarkers FeNO and BEC as a first screening tool for T2 asthma in the general population to detect undiagnosed asthma patients more rapidly. However, the isolated upfront use of these biomarkers in the diagnostic pathway could lead to overdiagnosis of asthma and is unable to identify all cases of asthma (e.g., T2-low asthma). Study concept and design: G.G.B., A.M.; Data curation: S.R., A.M.; Data analysis: S.R.; Interpretation of data: S.R., A.M., G.G.B.; Manuscript draft: S.R.; Manuscript review and editing: A.M., G.G.B.; Visualisation: S.R. We would like to thank all participants and study staff of the NHANES study. IRB Statement: NHANES was approved by the NCHS research ethics review board (https://wwwn.cdc.gov/Nchs/Nhanes/), NCHS ERB Protocol Number #2005–06 for NHANES 2007–2008 and 2009–2010, and Protocol Number #2011–17 for NHANES 2011–2012. Written informed consent was obtained from all participants. S.R. reports conference support from AstraZeneca, GlaxoSmithKline and Sanofi outside the submitted work; A.M. reports advisory board fees from Chiesi, honoraria from Boehringer-Ingelheim and support from NIOX and Thermo Fisher Scientific in investigator-initiated studies; G.G.B. reports honoraria from AstraZeneca, Boehringer-Ingelheim, Chiesi, GlaxoSmithKline, Merck Sharp & Dohme, Novartis and Sanofi Regeneron. NHANES data is publicly available at https://wwwn.cdc.gov/Nchs/Nhanes/, with the exception of sensitive data types. An additional online supplement is available on Zenodo in the following repository: https://doi.org/10.5281/zenodo.17276627.