An official JRS statement: The principles of fractional exhaled nitric oxide (FeNO) measurement and interpretation of the results in clinical practice

Nitric oxide (NO) is produced in the body and has been shown to have diverse actions in the abundance of research that has been performed on it since the 1970s, leading to Furchgott, Murad, and Ignarro receiving the Nobel Prize in Physiology or Medicine in 1998. NO is produced by nitric oxide synthase (NOS). NOS is broadly distributed, being found in the nerves, blood vessels, airway epithelium, and inflammatory cells. In asthma, inflammatory cytokines induce NOS activity in the airway epithelium and inflammatory cells, producing large amounts of NO. Measurement of fractional exhaled nitric oxide (FeNO) is a simple, safe, and quantitative method of assessing airway inflammation. The FeNO measurement method has been standardized and, in recent years, this noninvasive test has been broadly used to support the diagnosis of asthma, monitor airway inflammation, and detect asthma overlap in chronic obstructive pulmonary disease (COPD) patients. Since the normal upper limit of FeNO for healthy Japanese adults is 37 ppb, values of 35 ppb or more are likely to be interpreted as a signature of inflammatory condition presenting features with asthma, and this value is used in clinical practice. Research is also underway for clinical application of these measurements in other respiratory diseases such as COPD and interstitial lung disease. Currently, there remains some confusion regarding the significance of these measurements and the interpretation of the results. This statement is designed to provide a simple explanation including the principles of FeNO measurements, the measurement methods, and the interpretation of the measurement results.     

Differential role of mucus plugs in asthma: Effects of smoking and association with airway inflammation

BackgroundThe physiological importance of mucus plugs in computed tomography (CT) imaging is being increasingly recognized. However, whether airway inflammation and smoking affect the association between mucus plugs and clinical–physiological outcomes in asthma remains to be elucidated. The objective of this study is to examine how airway inflammation and/or smoking affect the correlation of CT-based mucus plug scores with exacerbation frequency and airflow limitation indices in asthma.MethodsA total of 168 patients with asthma who underwent chest CT and sputum evaluation were enrolled and classified in eosinophilic asthma (EA; n = 103) and non-eosinophilic asthma (NEA; n = 65) groups based on sputum eosinophil percentage (cut-off: 3%). The mucus plug score was defined as the number of lung segments with mucus plugs seen on CT.ResultsMore mucus plugs were detected on CT scans in the EA group than in the NEA group, regardless of smoking status. Mucus plug score and exacerbation frequency during one year after enrollment were significantly associated in the EA group but not in the NEA group after adjusting for demographics, blood eosinophil count, and fractional exhaled nitric oxide. Mucus plug score was associated with percentage of predicted forced expiratory volume in 1 s in non-smoking individuals in the EA and NEA group and in smoking individuals in the EA group but not in the NEA group after adjusting for demographics.ConclusionsThe association of mucus plug score with exacerbation frequency and reduced lung function may vary due to airway inflammatory profile and smoking status in asthma.     

Exhaled nitric oxide: A biomarker for chronic obstructive pulmonary disease

The fractional concentration of exhaled nitric oxide (FeNO) is recognized as a biomarker of type 2 inflammation in asthma, which is related to airway eosinophilia. We conducted a prospective observational study in a cohort of Japanese patients with chronic obstructive pulmonary disease (COPD) to evaluate the relationship between FeNO and clinical features and patient outcomes over a 3-year period. Participants were categorized into two groups based on FeNO levels (high and low), and the clinical features and outcomes were compared between the groups. Patients with high FeNO levels showed features of asthma and eosinophilic inflammation compared to those with low levels. However, high FeNO levels were not associated with worse outcomes (exacerbations, hospital admissions, all-cause and disease-specific mortality) compared to low levels. These results provide evidence that baseline FeNO is related to eosinophilic inflammation; however, is not a predictor of future exacerbations or prognosis in patients with stable COPD.     

Accuracy of objective tests for diagnosing adult asthma in symptomatic patients: A systematic literature review and hierarchical Bayesian latent-class meta-analysis

BackgroundWe obtain summary estimates of the accuracy of additional objective tests for the diagnosis of adult asthma using systematic review and meta-analysis of diagnostic test accuracy studies.MethodsMedline, Embase, and other relevant electronic databases were searched for papers published between January 1989 and December 2016. Studies were included if they evaluated the diagnostic accuracy of objective tests, including airway reversibility (AR), airway hyperresponsiveness (AHR), and fractionated exhaled nitric oxide (FeNO) for the diagnosis of adult asthma in patients with symptoms suggestive of asthma. If papers were assessed appropriate using the adapted QUADAS-2 tool, meta-analysis was conducted using the hierarchical bivariate model. This hierarchical model accounts for both within and between study variability.ResultsSixteen studies reported the performance of the evaluated objective tests at presentation. For diagnosis of adult asthma, overall sensitivity and specificity for AR were 0.39 (95% confidence interval [CI] 0.18 to 0.66) and 0.95 (95% CI 0.86 to 1.00); for AHR, 0.86 (95% CI 0.61 to 1.00) and 0.95 (95% CI 0.77 to 1.00); for FeNO, 0.65 (95% CI 0.53 to 0.77) and 0.83 (95% CI 0.75 to 0.90). Comprehensive comparison of three diagnostic tools for adult asthma using the back-calculated likelihood rate (LR) showed that AR and AHR corresponded to a higher LR+, and AHR gave a lower LR-.ConclusionsIn the current situation of no gold standard for diagnosis of adult asthma, AR and AHR are appropriate for ruling-in the true diagnosis, and AHR is superior for ruling-out a diagnosis. Since each objective test had a specific characteristic, it should be chosen depending on the situation, such as the capacity of the institution and the conditions of patients.     

Pathophysiological relevance of sputum MUC5AC and MUC5B levels in patients with mild asthma

BackgroundAirway mucus hypersecretion is an important pathophysiological feature of asthma. MUC5AC and MUC5B are the major secreted polymeric mucins in airways, and their compositions affect mucus properties. Despite the increasing appreciation of MUC5AC and MUC5B compositions in asthmatic airways, their pathophysiological relevance remains to be fully understood in humans.MethodsIn this cross-sectional study, we prospectively enrolled newly referred steroid-untreated patients with mild asthma and healthy controls. We compared induced sputum MUC5AC and MUC5B levels between patients and controls. Subsequently, we assessed the correlation between MUC5AC and MUC5B levels and clinical indices in patients. Sputum MUC5AC and MUC5B levels were measured using enzyme-linked immunosorbent assays.ResultsSputum MUC5AC and MUC5B levels were significantly higher in patients (n = 87) than in controls (n = 22) (p = 0.0002 and p = 0.006, respectively). The ratio of sputum MUC5AC to MUC5B tended to be higher in patients than in controls (p = 0.07). Sputum MUC5AC levels significantly and positively correlated with fractional exhaled nitric oxide at expiratory flow of 50 mL/s (Spearman's rho = 0.29, p = 0.006), sputum eosinophil proportion (rho = 0.34, p = 0.0013), and airway sensitivity (rho = 0.39, p = 0.0005). By contrast, sputum MUC5B levels significantly and positively correlated with airway sensitivity (rho = 0.35, p = 0.002) and negatively correlated with airway reactivity (rho = ?0.33, p = 0.004).ConclusionsSputum MUC5AC is increased by protein levels and involved in airway type 2/eosinophilic inflammation and airway hyperresponsiveness in steroid-untreated patients with mild asthma.     

Evaluation of olfactory dysfunction to estimate the presence of eosinophilic chronic rhinosinusitis in patients with asthma

BackgroundEosinophilic chronic rhinosinusitis (ECRS) is often complicated by asthma and can be difficult to diagnose. This study aimed to clarify the usefulness of the self-administered odor questionnaire (SAOQ) and visual analog scale (VAS) to identify olfactory disorders in patients with asthma.MethodsThis retrospective study was conducted on patients with asthma who were referred to the Otolaryngology clinic between May and September 2018. The treatment step of asthma, asthma control test (ACT), pulmonary function test, peripheral blood eosinophils, and fractional exhaled nitric oxide (FeNO) were analyzed. ECRS was diagnosed based on the Japanese Epidemiological Survey of Refractory Eosinophilic Chronic Rhinosinusitis Study score. Olfactory dysfunction was evaluated using the SAOQ and VAS for olfactory disorders.ResultsThe study included 56 patients (18 males and 38 females), who were divided into two groups; those with ECRS (n = 18) and those without ECRS (n = 38). Age, sex, treatment step, ACT score, and pulmonary function were not significantly different between the groups. The ECRS group had a significantly higher FeNO value (89.1 ppb vs. 39.1 ppb) and a significantly lower SAOQ score (40.1% vs. 96.1%). The area under the receiver operating characteristic curve for the efficacy of ECRS diagnosis was 0.88, 0.889, 0.799, and 0.757 for SAOQ, VAS, blood eosinophil count, and FeNO, respectively.ConclusionThe SAOQ and VAS scores were useful tools that presented similar results to the blood eosinophil count and FeNO, and may help to improve the diagnosis of ECRS in patients with asthma.     

Abundant TNF-LIGHT expression in the airways of patients with asthma with persistent airflow limitation: Association with nitrative and inflammatory profiles

BackgroundThe role of the inflammatory secretory protein TNF-LIGHT (LIGHT) in the molecular mechanisms underlying persistent airflow limitation (PAL) in asthma remains unclear. We hypothesized that high airway LIGHT expression may be a feature of asthma with PAL associated with specific expression patterns of inflammatory molecules.MethodsThis hypothesis was tested in 16 patients with asthma on inhaled corticosteroid treatment. Induced sputum was collected, the expression of LIGHT and 3-nitrotyrosine (NT), which reflects the footprint of reactive nitrogen species content, was measured using immunohistochemical staining, and the inflammatory molecules in the sputum supernatant were analyzed using a magnetic bead array.ResultsLIGHT staining in the cells had a significantly higher intensity in participants with PAL than in participants without PAL (47.9 × 10⁴/ml vs. 5.4 × 10⁴/ml; p < 0.05). The array analysis indicated that IL-8, IL-19, matrix metalloproteinase 2, and osteopontin, were associated with high LIGHT immunoreactivity. The fractionation of 3-NT-positive cells was positively correlated with that of LIGHT-positive cells (r = 0.57, p < 0.05) and the TGF-β1 level (r = 0.61, p < 0.05). LIGHT- and 3-NT-positive cells showed significant positive correlation with the differential cell counts of neutrophils, macrophages, and eosinophils in the induced sputum. Intense immunoreactivities of LIGHT (r = −0.54, p < 0.05) and 3-NT (r = −0.42, p = 0.1) were negatively associated with decreased forced expiratory volume in 1/forced vital capacity ratio.ConclusionsThe findings suggest that LIGHT is a key component in the association between airway inflammation and airflow limitation in patients with asthma, and its expression may be persistently correlated with the abundance of inflammatory cells and inflammatory and profibrogenic radical/molecules.     

Determination of the cutoff values of Th2 markers for the prediction of future exacerbation in severe asthma: An analysis from the Hokkaido Severe Asthma Cohort Study

BackgroundWe recently reported that severe asthma patients with frequent exacerbations showed high blood eosinophil counts (Eo) and fractions of exhaled nitric oxide (FeNO) compared with non-exacerbators. However, we did not determine exact cutoff values related to exacerbation. The aim of this study was to determine the cutoff values of Eo and FeNO that could be related to the exacerbation of severe asthma. We also aimed to confirm the clinical utility of Th2 markers related to exacerbation.MethodsThis study included 105 severe asthma patients who completed a three-year follow-up of a severe asthma cohort study, including smokers. Three Th2 markers were selected, viz., Eo, FeNO, and positive atopic status. Regarding Eo and FeNO, we determined the cutoff values for the definition of “positive” Th2 features using receiver operating characteristic curve analyses, based on the comparisons between frequent exacerbators and other patients.ResultsThe cutoff values for positive Th2 features were Eo ≥ 250 cells/μL and FeNO ≥31 ppb. Sixteen patients (15.2%) had no Th2 features, 40 (38.1%) had one, 25 (23.8%) had two, and 24 (22.9%) had three. A high number of positive Th2 features was significantly associated with exacerbation frequencies over three years (p < 0.05). Similarly, compared to patients with one or no Th2 features, those with three Th2 features had a significantly higher probability of having more than one exacerbation (p < 0.05).ConclusionsThe cutoff values determined in the current analysis were good predictors of future exacerbations in severe asthma patients.     

Reflux-related symptoms reflect poor asthma control and the presence of airway neuronal dysfunction

BackgroundGastroesophageal reflux may be associated with the worsening of asthma by increasing cough reflex sensitivity. Hull Airway Reflux Questionnaire (HARQ) consists of 14 prevalent reflux-related symptoms. It may be useful in predicting the presence of cough reflex hypersensitivity in asthma.MethodsFrom August 2018 to July 2020, 266 asthmatic patients completed the HARQ. They underwent blood analysis, spirometry, fraction of exhaled nitric oxide (FeNO) measurement, and the capsaicin cough challenge test. Patients were considered to have reflux-related symptoms if their HARQ scores were 13 points or higher. We evaluated the association between reflux-related symptoms and clinical asthma outcomes. Finally, we performed a multivariate analysis to determine the clinical significance of the HARQ for asthma. This study was registered in the University Hospital Medical Information Network (UMIN000040732).ResultsThe mean HARQ scores were 13.1 (standard deviation 12.0). Patients in the high HARQ scores group (HARQ ≥13, n = 105) showed a lower prevalence of atopic predisposition, lower levels of FeNO, heightened capsaicin cough reflex sensitivity, poorer asthma control, and more frequent admissions due to asthma than those in the low HARQ groups (all p values < 0.05). The HARQ was useful in selecting patients with poor controlled asthma and those with severe cough when the cut-off value was set at 13. Multivariate analysis revealed that heightened capsaicin cough reflex sensitivity affected reflux-related symptoms, as well as lower levels of FeNO and younger age.ConclusionsHigher HARQ scores (≥13) may be useful in predicting not only poor asthma condition but also the presence of airway neuronal dysfunction in patients with asthma to some extent.     

Clinical significance of fractional exhaled nitric oxide and periostin as potential markers to assess therapeutic efficacy in patients with cough variant asthma

BackgroundIn Japan, cough variant asthma (CVA) is the most common etiology of chronic cough. Contrary to substantial progress in understanding the roles of various factors in classic asthma, little is known regarding the pathogenesis and development of CVA. Furthermore, few studies have explored valuable biomarkers for evaluating the therapeutic efficacy of patients with CVA.MethodsWe conducted a single-center, prospective study to investigate the clinical significance of various clinical factors as potential “therapeutic” markers for CVA.ResultsFrom December 2019 to September 2020, we enrolled 20 patients with CVA and 10 age-matched healthy control subjects. Fractional exhaled nitric oxide (FeNO) values were significantly higher in patients with CVA than those in healthy controls. All patients with CVA commenced treatment at the initial visit, which markedly alleviated symptoms 12 weeks after treatment. FeNO values and serum periostin levels were significantly decreased following treatment, and altered FeNO values correlated with improved visual analogue scale scores of symptoms. Moreover, changes in both FeNO values and serum periostin levels were significantly correlated with increased values of some pulmonary function tests while also correlating with each other.ConclusionsOur observations indicate the usefulness of FeNO and periostin as potential “therapeutic” markers for CVA. To the best of our knowledge, this is the first report demonstrating the clinical significance of these factors as potential biomarkers to assess therapeutic efficacy in patients with CVA.