Clinical usefulness of fractional exhaled nitric oxide for diagnosing prolonged cough.

Background: Prolonged cough is one of the troublesome symptoms commonly seen in daily practice. Especially, detection of allergic cough such as bronchial asthma (BA), cough variant asthma (CVA) and eosinophilic bronchitis without asthma (EB) is important because the prevalence of these disorders are high. We previously reported fractional exhaled nitric oxide (FeNO) can be a non-invasive marker of allergic airway inflammation. We examined whether FeNO could be applicable for the proper diagnosis of prolonged cough.

Method: About 71 consecutive subjects complaining prolonged cough who gave informed consent for the study were enrolled. FeNO, pulmonary function tests, bronchial hyperresponsiveness (BHR), IgE, and eosinophils in induced sputum and peripheral blood were measured. Final diagnosis of the subjects was 30 with BA, 18 with CVA, 8 with EB, and 15 with other respiratory disorders (Others).

Result: FeNO had significant correlations with non-specific IgE, mite-specific IgE, FEV/FVC, BHR, and eosinophils. The level of cedar-specific IgE was significantly higher in subjects with EB than CVA. FeNO levels in BA and CVA were significantly higher than those in EB and Others. The optimal cutoff level of FeNO was 38.8 ppb with sensitivity of 79.2% and specificity of 91.3% for distinguishing BA and CVA from EB and Others.

Conclusion: FeNO could be used as a diagnostic marker of prolonged cough, especially for the differential diagnosis BA and CVA from EB and others.