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Exhaled NO and nitrite as potential biomarkers in asthma
The measurement of exhaled biomarkers has gained increasing interest in recent years, mainly driven by the unmet clinical need to monitor airway inflammation and the response to anti-inflammatory treatment. The current issue of Thorax contains two important publications in this rapidly growing field. The study by Pijnenburg et al shows how exhaled nitric oxide (NO) measurement can serve clinical practice,1 while the investigation by Marteus et al draws attention to the potential pitfalls of measuring nitrite in exhaled breath condensate (EBC).2
It was hardly more than a decade between the discovery by Gustaffson et al in 1991 that the exhaled breath contains NO and the approval of such a measurement for clinical practice to monitor the effect of anti-inflammatory treatment in asthma.3,4 The road has been paved by approximately 2000 publications on the measurement of the fractional concentration of exhaled nitric oxide (FENO) in health and disease, including three guidelines which provide methodological recommendations by internationally known experts in the field and endorsed by the European Respiratory Society (ERS) and/or the American Thoracic Society (ATS).5–7 By using these recommendations, exhaled NO can be measured reproducibly and data from different laboratories can be compared.
Exhaled NO has been extensively studied as a marker of airway inflammation in asthma and it serves as a prototype for the application of biomarkers to the management of the inflammatory component of asthma. Can monitoring FENO in addition to symptoms and spirometry contribute to asthma control? The paper by Pijnenburg et al in this issue of Thorax provides a positive answer to this question.1 In a longitudinal study the authors determined whether FENO predicted asthma relapse in 40 children with asymptomatic asthma followed for 24 weeks after discontinuation of treatment with inhaled corticosteroids (ICS). The children were enrolled in the study at the moment when discontinuation of ICS was considered because of lack of symptoms for more than 6 months at a stable dose of ICS. This ensured that the study was undertaken in a real clinical context (and the withdrawal of treatment did not occur solely for the purpose of the study). The main finding was that an increase in FENO predicted loss of asthma control in patients with no symptoms or changes in spirometric parameters. The authors found that increased FENO predicted asthma relapse with a sensitivity of 71% and a specificity of 93% using a cut-off FENO value of 49 ppb. This finding has important clinical implications because an increase in FENO warns the clinician of worsening airway inflammation, indicating the need to start treatment before symptoms appear. In another longitudinal study Jones et al8 studied FENO as a predictor of loss of asthma control in relation to withdrawal of steroids in adults. Exhaled NO levels were measured weekly for 11 weeks in 78 subjects with asthma who abruptly stopped treatment with ICS. The authors found that, in subjects who eventually experienced loss of control, exhaled NO levels increased more rapidly and to significantly higher levels than in those remaining clinically stable. Similar to the results of Pijnenburg et al in children, they found that exhaled NO levels measured at the visit before loss of control occurred predicted the upcoming exacerbation (positive predictive value of 80–90%) at a time when symptoms were stable.
Although as yet we do not know whether using exhaled NO measurements to guide anti-inflammatory treatment in asthma in addition to traditional means of monitoring would improve asthma control, both studies indicate that exhaled NO can serve as a marker of loss of asthma control and may be useful in clinical decision making.
While a decade was enough for exhaled NO measurement to enter clinical practice, the same decade was only good enough to give a boom to research for measurement of biomarkers in EBC. It is easy to collect, requiring only the non-invasive collection of exhaled breath for 10–20 minutes in a cold trap. The fluid obtained is a complex diluted solution of diverse biomarkers with various chemical stabilities including a number of constituents.9,10 Because of the complexity of EBC and the fact that it is a much diluted sample, there are still a number of uncertainties surrounding it. Although there is an expectation that this sampling method will be clinically useful, there are still several unresolved issues, many of which are highlighted in the report by the ERS/ATS Task Force entitled “Exhaled Breath Condensate” which is awaiting ATS approval for publication.
In this issue of Thorax an important study by Marteus et al2 addresses the relation between exhaled NO and EBC nitrite/nitrate concentration. The authors performed a carefully designed study which assessed the source of nitrite in orally collected EBC, compared nitrite levels between oral and tracheal EBC samples, and investigated the influence of nitrate intake and antibacterial mouthwash on the nitrite concentration in oral EBC samples, nasal air condensates, and on FENO. Their findings can be summarised as follows: (1) nitrate levels in EBC are influenced by dietary intake; (2) nitrate is reduced to nitrite primarily by bacterial activity which takes place mainly in the oropharyngeal tract in healthy subjects; and (3) there is a substantial contribution of nitrite from the oropharyngeal tract during oral EBC collection. The findings of this study draw attention to the acknowledged pitfall of oral EBC sampling—namely, the potential naso-oropharyngeal influence on mediator levels. They also question, to some extent, the reliability of oral EBC nitrite in reflecting lower airway NO production and its ability to serve as a marker of airway inflammation.11–15 The study also highlights the importance of potential external contamination (the authors covered the condensing surface with a specific plasma layer to minimise nitrite contamination) and emphasises the need for great care when measuring mediators which occur in such a low concentration in the EBC.
Exhaled NO and nitrite as potential biomarkers in asthma
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