Objective: To evaluate the diagnostic value of fractional exhaled nitric oxide (FeNO) in the diagnosis of chronic cough.
Methods: A total of 106 subjects with chronic cough and normal chest radiographs were recruited from October 2009 to September 2010. Based on the management guidelines of the Chinese Respiratory Society for cough, the golden standard methods were used to make the definite diagnosis of chronic cough, including sputum cell counts, pulmonary function tests, bronchial hyperresponsiveness, 24-h esophageal pH monitoring, skin pricking test and serum immunoglobulin E. All subjects received a FeNO test by a NIOXMINO analyzer. The values of FeNO to diagnose cough variant asthma (CVA) from chronic cough and EB from non-asthma cough were respectively assessed by the receiver operating characteristic (ROC) curves.
Results: Among them, the definite diagnoses were cough variant asthma (CVA, n = 39), eosinophilic bronchitis (EB, n = 30) and other causes (n = 37). The FeNO levels in CVA [(54 ± 21) ppb)] (1 ppb = 1 × 10(9) mol/L) were significantly higher than those in EB [(34 ± 17) ppb, P < 0.01] and other causes [(21 ± 10) ppb, P < 0.01]. And the FeNO levels in EB were higher than those in other causes (P < 0.01). To diagnose CVA from chronic cough, the optimal FeNO cutoff value was 40 ppb with a sensitivity of 75%, a specificity of 86%, a positive predictive value of 77%, a negative predictive value of 86% and an accuracy of 81%. To diagnose EB from non-asthma chronic cough, the optimal FeNO cutoff value was 31 ppb with a sensitivity of 63%, a specificity of 92%, a positive predictive value of 88%, a negative predictive value of 92% and an accuracy of 72% respectively.
Conclusion: There are significant differences between the FeNO levels of different causes of chronic cough. A marked elevation of FeNO level helps to make a final diagnosis of CVA or EB. FeNO test is useful for making the diagnosis and differential diagnosis of chronic cough in clinic practices.