Lung function tests are the traditional method for assessing severity of asthma. Spirometry (meaning the measuring of breath) is the most common test measuring lung function, specifically the measurement of the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. This is compared to typical readings for people without airways obstruction. Peak flow readings are also taken. These show how hard and quickly a patient can blow air out of their lungs.
However, it is increasingly recognized that these tests do not necessarily reflect the level of underlying inflammation. Patients can suffer a serious reduction in their ability to exhale for a number of reasons that may not be linked to asthma. Conversely, lung function may appear to be normal, even when the underlying inflammation is quite serious.
Another marker that is often used to assess severity is reported symptoms. However, again it seems to be the case that the patient’s assessment of their own status does not necessarily correspond to the underlying severity of the disease.
Exhaled NO levels on the other hand accurately reflect the level of underlying inflammation. Most studies into the relative utility of standard tests confirm that there is little or no correlation between their results and exhaled NO levels (and therefore inflammation) [Sippel 2000, Stirling 1998, Langley 2003, Piacentini 1999].
While levels of exhaled NO respond rapidly to changes in allergen exposure, pulmonary function – and therefore the tests that measure it – respond more slowly [Piacnetini 1999, Baraldi 1999].
As a result, a treatment strategy based on symptom management may not effectively manage the underlying inflammation. This is particularly important if the patient has co-existing allergic rhinitis, as this may be largely asymptomatic, providing no clues to the severity of the underlying condition.
- Improve the diagnosis of asthma and inflammatory airways disease
- predict and assess the patient’s response to anti-inflammatory treatment (usually with steroids)
- optimise the dosage of anti-inflammatory treatment
- predict loss of control and possible relapse
- monitor compliance with treatment
More on the clinical application of exhaled NO measurements can be found in the Health care professionals section.