The utility of measurements of exhaled
nitric oxide (FeNO) will likely depend on context, being most helpful in moderate and severe
asthma, rather than mild asthmatics and community based studies. Atopy on its own is a cause of elevation in FeNO. Adult and paediatric studies have clearly established that measurement of some aspect of airway
inflammation is part of state of the art management of
asthma, but it is as yet unclear which of several techniques is most useful. The relationship between FeNO and sputum eosinophils is relatively loose, but this does not preclude it being a useful test in clinical practice. In fact, there are only poor correlations between sputum, proximal mucosal, and distal eosinophils, and the importance of these different compartments is unclear. A low FeNO in the setting of supposedly poorly controlled
asthma should cast doubt on the diagnosis. We certainly cannot treat an isolated elevation in FeNO, which may be due to a simple viral cold, or constitutional. If FeNO is elevated, particularly if
asthma is uncontrolled, it suggests an imbalance between anti-inflammatory
therapy and pro-inflammatory environmental influences. Inadequate anti-inflammatory
therapy may be due to the prescribed dose being too low; the
drug delivery device not being used correctly; or the medication not being taken. Adverse pro-inflammatory environmental influences driving up FeNO include
IgE and non-
IgE mediated
allergen sensitivity in the home, and even in the child's school. Novel technology allows home monitoring of FeNO, but the role of these devices is less clear. Although more data is needed properly to define the role of FeNO measurements in clinical practice, there is sufficient data already published to conclude that 'inflammometry' is an important part of
asthma management at the more severe end of the spectrum, and that FeNO measurements are probably the most useful at the moment.