Is Exhaled Nitric Oxide a Useful Adjunctive Test for Assessing Asthma?

Rationale. To determine the general utility of clinical (Asthma Control Test) and physiologic (forced expiratory volume in the first second of exhalation [FEV1] and fractionated exhaled nitric oxide level [FeNO]) parameters for characterizing asthma patients. Methods. Two cross-sectional independent studies simultaneously enrolled 100 patients in the US and 109 patients in Spain >= 18 years of age with a physician-diagnosis of asthma and confirmed by a >= 12% improvement in FEV1 after bronchodilators or the presence of airway hyperresponsiveness, a central feature of asthma, as measured by methacholine challenge (PC20 <10 mg/mL). There was no restriction on asthma severity or treatment. Patients were excluded if they had a diagnosis of chronic obstructive pulmonary disease and/or were current cigarette smokers. Statistical analyses were performed to compare ACT, FeNO, and spirometry within and between sites. Results. Population characteristics revealed significant differences in distributions of age, percent-predicted FEV1 (%FEV1), FeNO, inhaled corticosteroid usage, and atopy between the two populations. The Spain site enrolled younger patients with milder asthma, based on higher %FEV1 values and less frequent treatment with inhaled corticosteroids. At each site, mean FeNO levels decreased as asthma control categories increased, and means were lower in the US. There was a negative correlation between ACT and FeNO that was statistically significant for Spain patients not treated with inhaled corticosteroids. Conclusions. The results of this study support the use of FeNO as an adjunctive tool for assessing asthma primarily in mild inhaled corticosteroid (ICS)-naive asthma patients. The lack of correlation of ACT with FeNO in this and other studies across the entire population appears to reflect the heterogeneity of asthma patients who have an admixture of asthma severity and treatment regimens making it very difficult to appreciate the nuances of sensitive tests like FeNO.