Exhaled nitric oxide and asthma

The landmark study by Smith et al.(May 26 issue) shows that measurement of exhaled nitric oxide (FeNO) can reduce the dose of inhaled corticosteroids in patients with asthma without impairing control of asthma and, in particular, exacerbations of asthma. Because of the high variability of FeNO among both healthy persons and patients with asthma, it may make sense to consider a personalized “best” cutoff FeNO level, as seen after the dose-optimization phase or after administration of a dose of oral prednisone. In addition, the relatively high exhalation flow rate used (250 ml per second), as compared with the recommended flow rate of 50 ml per second, may be a factor. The difference in terms of parts per billion between a patient when in stable condition and the same patient in unstable condition will be much smaller, and perhaps less discriminatory, at 250 ml per second than at 50 ml per second. Finally, the numerical trend seen in the reduction of exacerbations suggests that the study was underpowered to assess this outcome.