Fractional exhaled nitric oxide (FeNO) in chronic obstructive respiratory diseases is associated with the bronchodilator response

Introduction

Chronic obstructive respiratory diseases (CORDs) include asthma and chronic obstructive pulmonary diseases (COPDs). These heterogenous disorders are characterized by different phenotypes related to endotypes defined by the nature of airflow inflammation (AI). In human, AI can be caused by inhaled microbes, tobacco smoke and airborne particles, such as toxins, pollutants, irritants, and allergens (1). The interaction between these environmental exposures and genetic backgrounds can lead to various pattern of AI and consequently, frequent overlap [asthma COPD overlap (ACO)] between asthma and COPD. Therefore, in the current study we evaluated a CORD population as a whole, including asthma, ACO and COPD (2).

The fractional exhaled nitric oxide (FeNO) is a marker of eosinophilic airway inflammation. Airway inflammation can be eosinophilic-related, named type 2 (T2), or non-eosinophilic-related, named non-T2. The measurement of FeNO can contribute to diagnose type 2 asthma and to evaluate its prognosis or to tailor the treatment (3).

FeNO level correlates with airways’ eosinophilia in the sputum or in biopsies or bronchoalveolar lavage (4) and with blood eosinophils count (BEC) (5) and immunoglobulin E (IgE) levels (6). There is also concordance between higher FeNO and other T2 markers (7). A high FeNO level is associated with the diagnosis of non-smoker eosinophilic asthma (3,8). While several studies reported an association between high FeNO and the decline in lung function or exacerbations (9,10) among uncontrolled asthmatics, others studies shown no association with asthma severity (11,12) or exacerbations (13). Available data on FeNO in Vietnamese asthmatics are not consistent. The level of FeNO in non-smoker treated asthmatics was negatively related with the airflow limitation (14) while among a similar population, another study reported FeNO as a marker of uncontrolled asthma (15).

In COPD, the interest of FeNO is debated, among others because the smoking habits, disease severity and ICS use, can interfere with the measurements. Actually, FeNO is usually normal in COPD and low in current smokers (8). Few data are available in non-smoker COPD, highly prevalent in Vietnam. Though, an elevated FeNO was associated with an increased risk of exacerbations and type 2 airway inflammation in COPD (16,17). In others studies the decline of the lung function (18,19) or risk of exacerbations (20) seemed unrelated with the FeNO values. Particularly, among type 2 inflammatory COPD, the FeNO was predictive of the response to ICS (21). Smoking is probably the most relevant confounding factor to interpret FeNO among COPD because there is a negative correlation between smoking and the FeNO values (22). To our knowledge, in Vietnam, only one study had measured FeNO among COPD, the last being lower compared to ACO and asthma (23).

Due to the various environmental factors and genetic background in Vietnam, the clinical values of FeNO need to be evaluated in a population of CORD as a whole. Finally, a last interfering factor is the frequent helminthic infection in tropical environments that can interfere with BEC and FeNO (24).

In the present study, we aimed to associate the measurement of FeNO, with their clinical characteristics, lung function parameters and airflow limitation reversibility, in an unselected population of Vietnamese patients with chronic respiratory symptoms and airflow limitation. We present this article in accordance with the STROBE reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1727/rc).

Methods

The inclusion criteria were patients older than 18 years with both chronic respiratory symptoms (cough, and/or sputum production, and/or wheezing, and/or dyspnea, and/or chest tightness since >3 months) and basal airflow limitation.

Approximately 3,600 out-patients with respiratory symptoms were seen from November 2020 to May 2021 at the pulmonology unit of Nguyen Tri Phuong Hospital, a general hospital in Ho Chi Minh City. Based on chest X-ray, about 800 patients were excluded due to active tuberculosis, cancer or acute infectious diseases. Lung function tests with bronchodilator (BD) responsiveness test were performed in 2,800 remained patients and only 400 patients presented airflow limitation. We invited these patients to participate to the study and after written informed consent, a total of 235 patients were included to the study.

Demographic characteristics, medical history, smoking habits of studied patients were collected from a survey questionnaire. Then, skin prick tests (SPTs), FeNO and BEC were evaluated. Patients who fully responded to all items in the questionnaire and completed all para-clinical investigations were enrolled to this cross-sectional study.

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of Pham Ngoc Thach University (008/HDDD) and informed consent was taken from all the patients.

Pulmonary function tests

The forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC), in L and % of the predicted value (PV) were measured by a trained technician with a spirometer (Medisoft Ltd., Sorinnes, Belgium) after stopping of long-acting BD for >12 hours and/or of short-acting BD for >8 hours, according to the European Respiratory Society (ERS) guidelines (25). For all patients, the BD responsiveness was evaluated 10 minutes after inhalation of 400 mcg salbutamol. Airflow limitation was defined as FEV1/FVC < LLN (lower limit of normality) (26). The reversibility of airflow limitation was defined as (post-BD FEV1 − basal FEV1)/basal FEV1 and expressed in %. Positive BD responsiveness was a post-BD change in FEV1 >12% and >200 mL (27).

An asthmatic patient was defined as a patient with a history of respiratory symptoms that vary in intensity and frequency, such as wheezing, shortness of breath, chest tightness, and cough, often triggered by exercise or emotional stress, with basal airflow limitation normalizing after BD and/or with a positive BD responsiveness (27).

COPD was diagnosed in the presence of persistent respiratory symptoms and post BD persistent airflow limitation (FEV/FVC <0.7) without BD responsiveness, in patient with significant exposure to tobacco smoke or noxious particles or gases (28).

ACO was defined in patients with clinical features consistent with both asthma and COPD, usually with persistent airflow limitation and a positive BD responsiveness (27).

Exacerbation

Exacerbation was defined as any acute worsening in clinical symptoms compared to the baseline, requiring a change in medication treatment, in a patient receiving prophylactic therapy. Severe exacerbation was defined as an acute episode with worsening of baseline symptoms, leading to hospitalization or intensive care unit admission. Patients with recent exacerbation in previous month were excluded from the study (27,28).

FeNO

FeNO assessment was carried out according to the recommendations of the American Thoracic Society and the ERS. FeNO was measured with the Medisoft device (Micro 6000), at the level of 50 mL/second, by technicians trained and certified according to ERS standards. Heavy meals, physical exertion and smoking cigarettes were avoided on the day of the exam. The cut-off 25 and 50 were used for analysis (3).

Other para-clinical investigations such as SPTs, BEC and chest x-ray were evaluated. SPTs were performed with the four most frequent aeroallergens in Vietnamese patients with chronic obstructive respiratory disease [Dermatophagoides farinae (DPF), Dermatophagoides pteronyssinus (DPT), Blomia tropicalis and cockroach] (29).

Statistical analysis

IBM SPSS Statistics version 23.0 (IBM Co., Armonk, NY, USA) was used for data analysis. Quantitative variables were expressed as mean with 95% confidence intervals (CIs). Categorical data were presented as number and percentage.

The differences between categorical variables were tested with χ² and Fisher’s exact test. The continuous variables without normal distribution were compared using Mann-Whitney U test. Spearman’s correlation coefficients for continuous variables were determined and logistic regression analysis was used to identify factors related with low FeNO. Odds ratios (OR) and 95% CI were calculated and P≤0.05 was defined significant.

Results

Among the 235 CORD patients there were 49 (GINA defined) asthmatics, 93 (GOLD defined) COPD and 93 ACO.

Among the whole population, the mean level of FeNO was 33.4±3.9 ppb. As shown in Figure 1A, FeNO was higher among the former- or non-smokers compared to the current-smokers (36.5±4.7 vs. 24.3±5.9 ppb; P<0.01). The mean FeNO was significantly lower among COPD (249.0±4.7 ppb) compared to asthmatics (42.3±7.7 ppb) and ACO (37.2±7.3 ppb) (Figure 1B).

Figure 1 The mean FeNO values and phenotypes (A-C) and its relationship with age (D), BEC (E), FEV1 reversibility (F) when BEC ≥300 cells/µL (F1) or <300 cells/µL (F2) in former or non-smokers. **, P<0.01; ***, P<0.001. ACO, asthma COPD overlap; BEC, blood eosinophils count; CI, confidence interval; COPD, chronic obstructive pulmonary disease; FeNO, fraction of exhaled nitric oxide; FEV1, forced expiratory volume in one second; ns, not significant; ppb, parts per billion.

Patient characteristics in CORD patients (n=235) according to FeNO value (Table 1)

A FeNO value ≥25 ppb was more frequent (P<0.001) in asthmatics (69%) compared to COPD (34%), in ACO (56%) compared to COPD (34%) (P<0.01) but not in ACO (56%) compared to asthmatics (69%).

A low FeNO (i.e., <25 ppb) was associated with male, COPD diagnosis, history of tuberculosis, more frequent (severe or not) exacerbations, heavy smokers, FEV1 <60% PV or with emphysema. Age <60 years was associated with a high FeNO but only among the non-current smokers (P<0.01).

Similar observations were found using the cut-off of 50 ppb FeNO.

Patient characteristics in asthmatics, ACO and COPD according to FeNO values (Table 2)

In Table 2, we evaluated FeNO among asthmatics (n=49) compared to ACO (n=93) and COPD patients (n=93). The exacerbations and radiological emphysema were more frequent in patients with <25 ppb FeNO.

Among asthmatics, the T2 inflammation (positive SPTs, BEC ≥150 cells/µL and eosinophils means) was significantly associated with the FeNO ≥25 ppb, while a low FeNO (<25 ppb) was more frequent in patients with exacerbations.

In the COPD group, FeNO ≥25 ppb was associated with rhinitis and history of allergy, but unrelated with T2 AI (i.e., BEC ≥150 cells/µL, eosinophils mean and positive SPTs). A low FeNO was associated with the risk of severe exacerbation and Rx emphysema.

All the 93 COPD patients, defined by post-BD FEV1/FVC <0.7, were ex/current smokers. Only 3 asthmatics were smokers. Conversely, among the ACO, the never smokers and ex/current smokers were 22 (24%) and 71 (76%), respectively. In Table 3, we compared the ACO phenotypes in regard with the smoking habits. The non-smoking ACO (exposed to biomass fumes, to environmental pollutants and infectious agents) were more frequently females, with allergies (history and positive SPTs) and had less airflow limitation.

Correlation between FeNO values and characteristics of former-and non-smoker CORD

We showed that current smoking was associated with a lower FeNO (Figure 1A), among ACO and asthmatics but not COPD (Figure 1C). The FeNO was higher in asthma and ACO compared to COPD but comparable in asthma and ACO (Figure 1B). For the following analysis, the current smokers were excluded (Figure 1D-1F). By doing so, we shown that the FeNO negatively related with age (Figure 1D) while the eosinophilia and the airflow limitation reversibility were related positively with the FeNO (Figure 1E,1F). The association between the airflow limitation reversibility and the FeNO was more significant among the patients with BEC ≥300 cells/µL (Figure 1F1), compared to BEC <300 cells/µL (Figure 1F2).

The post-BD FEV1 and FEV1/FVC, correlated positively with the FeNO levels (Figure 2). This relationship was significant among the former and non- smokers but not among the current smokers (Figure 2). The basal FEV1 and FEV1/FVC correlated also positively with the FeNO levels (data not shown).

Figure 2 Relationship between FeNO and basal airflow limitation, in former- or non-smokers and current smokers. FeNO, fraction of exhaled nitric oxide; FEV1, forced expiratory volume in one second; FVC, forced vital capacity; NS, not significant; Post-BD FEV1, post bronchodilator forced expiratory volume in one second; ppb, parts per billion; PV, predicted value.

Factors related to low FeNO on multivariate analysis

We developed a multivariate model adjusted for age, sex and pack-year number which was applied to each parameter with P<0.2 on univariate analysis from Table 2. The FeNO <25 ppb remained significantly higher among patients with severe exacerbation and radiological emphysema.

Discussion

This study showed that FeNO was associated with asthma and ACO compared to COPD and was related positively with T2 inflammation, BD responsiveness and negatively with active/cumulative smoking, the risk of exacerbations and, among non-current smokers, the age and the basal airflow limitation.

The concentrations of FeNO were 24.9 ppb among COPD and 42.3 ppb among asthmatics. Consistently in Vietnam, Sy Duong-Quy et al. reported a level of 15±8 ppb FeNO in COPD (including 75% current smokers) of, significantly lower compared to 46±28 ppb in asthmatics (23).

FeNO ≥25 ppb was associated with asthma and T2 inflammation markers

The asthmatics were frequently young women with atopic status. A higher FeNO has been reported in allergic status (30), high BEC (7) and asthma (3,8). Consistently, in the present study, the rates of allergic status, BEC ≥150 cells/µL and asthma diagnosis were higher in FeNO ≥25 ppb group, confirming an association between higher FeNO level and allergic asthma.

While a lower prevalence of eosinophilic asthma was observed in the LMICs compared to the HICs (31,32), the asthma eosinophilic AI was frequent (>70%) in the present study. The asthmatics (but not the ACO or COPD) with FeNO ≥25 ppb were associated with BEC >150 cells/µL and with a higher mean BEC. Moreover, among the CORD patients, not currently smoking, the FeNO concentration was positively related to the BEC.

FeNO <25 ppb was associated with COPD characteristics

The COPD are frequently associated with cigarette smoking, older age and pulmonary emphysema. A history of pulmonary tuberculosis has also been reported as a risk factor for COPD in LMICs (2). In COPD patients, the radiological emphysema, history of tuberculosis and risk of severe exacerbations were associated with FeNO <25 ppb, while type 2 inflammation (rhinitis and history of allergy) was associated with FeNO ≥25 ppb. This suggested a non-type-2 inflammation, among COPD with low FeNO, often characterized by neutrophils (33).

In COPD, the role of FeNO to predict lung function or exacerbation is still debated. Elevated exhaled nitric oxide fraction was associated with increased risk of exacerbation and allergic airflow inflammation in COPD (16,17) whereas decline lung function or risk of exacerbations seemed to have no association with FeNO values (18,19). In Chinese COPD, acute exacerbations in the past year, ICS users, leukocytes and eosinophils counts were associated with high FeNO level (20). Our data were not in line with these observations. However, while the mean concentration of FeNO were comparable between studies, our COPD patients were less frequently current smokers (41%) and less frequently treated with ICS (43%). Since current smoking and ICS interact with blood eosinophils and FeNO, it can explain the discrepancy observations. Among non-current smokers only, we showed a significant relationship between FeNO and blood eosinophilia, as reported among the Chinese COPD.

Since blood eosinophilia is related to FeNO and might be a predictor of the response to inhaled corticosteroids (ICS), the question is to consider FeNO as a predictor on the ICS response among COPD. Though, a systematic review on COPD and ACO, including 38 articles, concluded that there was no cut-off of FeNO to guide ICS therapy in COPD patients (34).

Low level of FeNO was associated with poorer lung function and severe exacerbation in both asthma and COPD patients

In moderate to severe asthma, the FeNO level was associated with the lung function decline and future exacerbations (9,10) and could be a biomarker in tailoring asthma treatment (11). Though in mild asthma, there was no association between the FeNO level and exacerbations (12). In Chinese asthmatics, future exacerbations were associated with poor adherence to the treatment, but not with the FeNO level (13). On the contrary with our data, a weak inverse correlation between FeNO with FEV1 values was reported in non-current smoking Vietnamese asthmatics (14). However, this relation was only present among asthmatics treated with LABA and moreover our patients were more severe (mean FEV1 =64.5% compared to 85.8% PV).

On one side, in severe asthma, BD-responsiveness was associated to a risk of exacerbations (35) and to rapid decline of FEV1. On the other side, the association between elevated FeNO levels and larger BD response was found in non-smoking non-asthmatic and asthmatic subjects in a population of 4,257 participants (36). A relationship between FeNO level and a greater decline in FEV1 over 20 years has also been reported in non-smoking asthmatic subjects. The same authors speculated that achieving control of type-2 inflammation might result in slower lung function decline. Also, in a small population of 29 asthmatics, FeNO may be a predictor of BD-response (37). Consistently, our data showed a significant relationship between FeNO level and the BD-responsiveness, in non-current smokers, whatever the diagnosis of asthma or COPD.

This observation is important because FeNO could be a predictor of ICS response, among the non-smoking CORD and not requiring a specific diagnosis of asthma or COPD. A prospective longitudinal study should be done among COPD and ACO patients (in terms of exacerbations and quality of life) randomized with or without ICS in regard with the FeNO threshold. Sustained this, a Japanese study including a small population of 43 symptomatic COPD, suggested that FeNO predict the response to ICS treatment (21). In adult asthma a systematic review has concluded that FeNO can guide the ICS treatment and reduce the number of exacerbations (38).

If we observed an association between FeNO and BD-responsiveness (and consequently the need for ICS), though the present study shown that the retrospective risk of exacerbations was associated with a low FeNO. One explanation could be that the patients with a low FeNO were more severe, heavy smokers with frequent emphysema and therefore with a higher risk of exacerbations.

Limitations of the study

This cross-sectional design couldn’t determine the causality between FeNO level and the prognosis of asthma and COPD such as lung function decline and risk for future exacerbations. FeNO testing is often unavailable in clinical care in Vietnam leading to lack of local literature resources for more comparison. In this study, FeNO values by themselves do not justify a diagnosis or a change in treatment and should be interpreted according to the clinical context and future longitudinal studies. Additionally, we cannot exclude whether lung development disorders such as airway dysanapsis had interfered with the reported data.

Conclusions

In Vietnam, among chronic respiratory obstructive diseases, FeNO is a biomarker of inflammation, clinical severity and airways’ reactivity, independently of the specific diagnosis (i.e., asthma or COPD). Being related with the BD responsiveness, it could be a predictor of ICS response, whatever the diagnosis of asthma, ACO or COPD. Future research based on longitudinal studies is needed to evaluate the role of FeNO in treatment strategy of CORD, in Vietnam.

Acknowledgments

We thank the participants and field workers for their time and cooperation.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1727/rc

Data Sharing Statement: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1727/dss

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1727/prf

Funding: This work was supported by a Research Project for Development (PRD-Projet de Recherche pour le Développement) granted from the Academy of Research and Higher Education (ARES) of Belgium.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1727/coif). All authors report that the study was partially granted by ARES of Belgium. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of Pham Ngoc Thach University (008/HDDD) and informed consent was taken from all the patients.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Moldoveanu B, Otmishi P, Jani P, et al. Inflammatory mechanisms in the lung. J Inflamm Res 2009;2:1-11.
2. Nguyen TC, Tran HVT, Tran MHT, et al. Phenotyping chronic respiratory diseases with airways obstruction. Int J Tuberc Lung Dis 2024;28:287-94. [Crossref] [PubMed]
3. Dweik RA, Boggs PB, Erzurum SC, et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FENO) for clinical applications. Am J Respir Crit Care Med 2011;184:602-15. [Crossref] [PubMed]
4. Riise GC, Torén K, Olin AC. Subjects in a Population Study with High Levels of FENO Have Associated Eosinophil Airway Inflammation. ISRN Allergy 2011;2011:792613. [Crossref] [PubMed]
5. Rabe KF, Rennard S, Martinez FJ, et al. Targeting Type 2 Inflammation and Epithelial Alarmins in Chronic Obstructive Pulmonary Disease: A Biologics Outlook. Am J Respir Crit Care Med 2023;208:395-405. [Crossref] [PubMed]
6. Chipps BE, Busse WW, Luskin AT, et al. Baseline IgE Levels as a Marker of Type 2 Asthma Among Patients Enrolled in the Prospective Observational Study to Evaluate Predictors of Clinical Effectiveness in Response to Omalizumab (prospero). Am J Respir Crit Care Med 2016;193:A6473.
7. Al Ghobain MO, Alsubaie AS, Aljumah WA, et al. The Correlation Between Fractional Exhaled Nitric Oxide (FeNO), Blood Eosinophil Count, Immunoglobulin E Levels, and Spirometric Values in Patients With Asthma. Cureus 2023;15:e35289. [Crossref] [PubMed]
8. Schneider A, Brunn B, Hapfelmeier A, et al. Diagnostic accuracy of FeNO in asthma and predictive value for inhaled corticosteroid responsiveness: A prospective, multicentre study. EClinicalMedicine 2022;50:101533. [Crossref] [PubMed]
9. Murugesan N, Saxena D, Dileep A, et al. Update on the Role of FeNO in Asthma Management. Diagnostics (Basel) 2023;13:1428. [Crossref] [PubMed]
10. Busse WW, Wenzel SE, Casale TB, et al. Baseline FeNO as a prognostic biomarker for subsequent severe asthma exacerbations in patients with uncontrolled, moderate-to-severe asthma receiving placebo in the LIBERTY ASTHMA QUEST study: a post-hoc analysis. Lancet Respir Med 2021;9:1165-73. [Crossref] [PubMed]
11. Ulrik CS, Lange P, Hilberg O. Fractional exhaled nitric oxide as a determinant for the clinical course of asthma: a systematic review. Eur Clin Respir J 2021;8:1891725. [Crossref] [PubMed]
12. Semprini R, Williams M, Semprini A, et al. Type 2 Biomarkers and Prediction of Future Exacerbations and Lung Function Decline in Adult Asthma. J Allergy Clin Immunol Pract 2018;6:1982-1988.e1. [Crossref] [PubMed]
13. Yuan Y, Li B, Huang M, et al. Fractional exhaled nitric oxide was not associated with the future risk of exacerbations in Chinese asthmatics: a non-interventional 1-year real-world study. J Thorac Dis 2019;11:2438-47. [Crossref] [PubMed]
14. Nguyen VN, Chavannes NH. Correlation between fractional exhaled nitric oxide and Asthma Control Test score and spirometry parameters in on-treatment-asthmatics in Ho Chi Minh City. J Thorac Dis 2020;12:2197-209. [Crossref] [PubMed]
15. Nguyen Nhu V, Le An P, Chavannes NH. Combination of Fractional Exhaled Nitric Oxide (FeNO) Level and Asthma Control Test (ATC) in Detecting GINA-Defined Asthma Control in Treated Asthmatic Patients in Vietnam. Can Respir J 2020;2020:5735128. [Crossref] [PubMed]
16. Liu X, Zhang H, Wang Y, et al. Fractional Exhaled Nitric Oxide is Associated with the Severity of Stable COPD. COPD 2020;17:121-7. [Crossref] [PubMed]
17. Alcázar-Navarrete B, Ruiz Rodríguez O, Conde Baena P, Romero Palacios PJ, Agusti A. Persistently elevated exhaled nitric oxide fraction is associated with increased risk of exacerbation in COPD. Eur Respir J 2018;51:1701457. [Crossref] [PubMed]
18. Lu Z, Huang W, Wang L, et al. Exhaled nitric oxide in patients with chronic obstructive pulmonary disease: a systematic review and meta-analysis. Int J Chron Obstruct Pulmon Dis 2018;13:2695-705. [Crossref] [PubMed]
19. Dilka E, Tashi E, Nushi E, et al. The use of FENO in COPD: the relationship to pulmonary function tests and its importance in differential diagnosis. Eur Respir J 2017;50:PA1099.
20. Xu X, Zhou L, Tong Z. The Relationship of Fractional Exhaled Nitric Oxide in Patients with AECOPD. Int J Chron Obstruct Pulmon Dis 2023;18:3037-46. [Crossref] [PubMed]
21. Yamaji Y, Oishi K, Hamada K, et al. Detection of type2 biomarkers for response in COPD. J Breath Res 2020;14:026007. [Crossref] [PubMed]
22. Herath P, Wimalasekera S, Amarasekara T, et al. Effect of cigarette smoking on smoking biomarkers, blood pressure and blood lipid levels among Sri Lankan male smokers. Postgrad Med J 2022;98:848-54. [Crossref] [PubMed]
23. Duong-Quy S, Tran Van H, Vo Thi Kim A, et al. Clinical and Functional Characteristics of Subjects with Asthma, COPD, and Asthma-COPD Overlap: A Multicentre Study in Vietnam. Can Respir J 2018;2018:1732946. [Crossref] [PubMed]
24. Brown M. Manson's tropical diseases. Lancet Infect Dis 2009;9:407-8.
25. Agnew M. Spirometry in clinical use: practical issues. Breathe 2010;6:196-203.
26. Quanjer PH, Stanojevic S, Cole TJ, et al. Multi-ethnic reference values for spirometry for the 3-95-yr age range: the global lung function 2012 equations. Eur Respir J 2012;40:1324-43. [Crossref] [PubMed]
27. Global Initiative for Asthma. Global strategy for asthma management and prevention 2021. Available online: https://ginasthma.org/wp-content/uploads/2023/04/GINA-Main-Report-2021-V2-WMSA.pdf (accessed 11th October 2023).
28. Global Initiative for Chronic Obstructive Lung Disease. Global Strategy for Prevention, Diagnosis and Management of COPD: 2021 Report. https://goldcopd.org/wp-content/uploads/2020/11/GOLD-2021-POCKET-GUIDE-v1.0-16Nov20_WMV.pdf (accessed 11th October 2023).
29. Chu HT, Godin I, Phuong NT, et al. Allergen sensitisation among chronic respiratory diseases in urban and rural areas of the south of Viet Nam. Int J Tuberc Lung Dis 2018;22:221-9. [Crossref] [PubMed]
30. Maspero J, Adir Y, Al-Ahmad M, et al. Type 2 inflammation in asthma and other airway diseases. ERJ Open Res 2022;8:00576-2021. [Crossref] [PubMed]
31. Shi B, Li W, Dong H, et al. Distribution of inflammatory phenotypes among patients with asthma in Jilin Province, China: a cross-sectional study. BMC Pulm Med 2021;21:364. [Crossref] [PubMed]
32. Pembrey L, Brooks C, Mpairwe H, et al. Asthma inflammatory phenotypes on four continents: most asthma is non-eosinophilic. Int J Epidemiol 2023;52:611-23. [Crossref] [PubMed]
33. Quint JK, Wedzicha JA. The neutrophil in chronic obstructive pulmonary disease. J Allergy Clin Immunol 2007;119:1065-71. [Crossref] [PubMed]
34. Mostafavi-Pour-Manshadi SM, Naderi N, Barrecheguren M, et al. Investigating Fractional Exhaled Nitric Oxide in Chronic Obstructive Pulmonary Disease (COPD) and Asthma-COPD Overlap (ACO): A Scoping Review. COPD 2018;15:377-91. [Crossref] [PubMed]
35. Denlinger LC, Phillips BR, Ramratnam S, et al. Inflammatory and Comorbid Features of Patients with Severe Asthma and Frequent Exacerbations. Am J Respir Crit Care Med 2017;195:302-13. Erratum in: Am J Respir Crit Care Med 2018;197:971. [Crossref] [PubMed]
36. Nerpin E, Ferreira DS, Weyler J, et al. Bronchodilator response and lung function decline: Associations with exhaled nitric oxide with regard to sex and smoking status. World Allergy Organ J 2021;14:100544. [Crossref] [PubMed]
37. Provenzano BC, Bartholo TP, Kirk KM, et al. Use of fractional exhaled nitric oxide as a potential predictor of bronchodilator response. Medicine (Baltimore) 2023;102:e34073. [Crossref] [PubMed]
38. Petsky HL, Kew KM, Turner C, et al. Exhaled nitric oxide levels to guide treatment for adults with asthma. Cochrane Database Syst Rev 2016;9:CD011440. [Crossref] [PubMed]