The fraction of exhaled nitric oxide (FeNO) is higher in patients with asthma than in the normal population. However, other conditions other than asthma can cause elevated readings.
Method of measurement: portable nitric oxide analyzer calibrated with a certified reference gas
Reported units: parts per billion (ppb)
Role of exhaled nitric oxide measurements:
(1) making the diagnosis of asthma, when used in conjunction with pulmonary function and other tests
(2) excluding the diagnosis of asthma
(3) identifying a patient likely to respond to inhaled steroid therapy
(4) monitoring the response to therapy (effective therapy should reduce; disease that is resistant to therapy should show little or no changes)
(5) detecting an impending exacerbation (which is associated with rising levels)
Factors affecting the ability to interpret the readings:
(1) other sources of airway inflammation, including chronic bronchitis in smokers
(2) allergic rhinitis
Schneider et al used an FeNO <= 16 ppb to exclude asthma.
Kostikas et al used an FeNO > 25 ppb as a good marker for asthma in smokers and nonsmokers (> 19 ppb was suggestive of the diagnosis in nonsmokers).
Smith et al used a FeNO > 47 ppb to identify asthma patients likely to respond to inhaled corticosteroids.
