A more difficult problem to address in the NO field has been the establishment of normal healthy population values for Feno. In a recent large population study,16 the mean Feno (17.9 ppb) was higher than that found for the smaller sample of healthy control subjects included in the study by Kostikas et al.,12 This discrepancy may be a result of the relatively smaller sample size in the study by Kostikas et al,12as well as their inclusion of smokers in the healthy control population. While several studies13,16–18 have tried to address this issue of normative values, they were done in different populations, addressed different potential confounders, and reported their results in different ways. Furthermore, “reference values” derived from a “normal” population may not be applicable in patients with asthma. This raises the question whether normal values are at all useful when it comes to the use of Feno in asthma. It is very clear from reviewing the literature that the Feno value by itself is not sufficient; rather, it needs to be taken within the clinical context. Beyond the confounding variables such as atopy and smoking, several issues need to be considered. Was the measurement obtained in someone who has symptoms, or in an asymptomatic individual? Was it performed as a screening, or to aid in the diagnosis? Is the individual known to have asthma? And if so, is he/she receiving therapy? Do they have previous levels, and how does this level compare?