Lack of utility of eNO?

To the editor: We respectfully disagree with one paragraph in Dr. Kharitonov’s comprehensive review of exhaled markers of inflammatory lung diseases [1]. The measurement of exhaled nitric oxide (eNO) seems to be a very useful diagnostic tool in the assessment of possible asthma [2, 3]. However, despite its proposed indication from Dr. Kharitonov [1], years of eNO research have failed to establish its clinical utility in the management and guidance of asthma. This seems strange in a disease which clearly needs surrogate markers to better aid physicians in treating asthma patients. Several studies have shown that eNO is increased in steroid naive asthmatic patients and that it can be decreased by inhaled corticosteroids (ICS) [4]. The differences in changes in lung function, airway hyperresponsiveness and eNO, however, do not really correlate with each other [5]. Exhaled NO increases while reducing the dose of ICS [6] and this even predicts loss of asthma control once the ICS have been completely stopped [7]. However, if, in accordance with clinical practice, the dose of ICS is decreased very gradually, eNO does not seem to have a predictive value for asthma exacerbations [8]. Outcome studies such as those of Green et al. [9], who used sputum induction as a marker despite the fact that eNO is far easier to perform than sputum induction, have suggested that aggressive control of inflammation leads to better outcomes. Surely this study must have been easy to do with eNO and perhaps the lack of published outcome studies of this type reflects the lack of utility of eNO.