Abstract: Objective To investigate the impact of a history of atopy on the value of fractional exhaled nitric oxide (FENO) for predicting sputum eosinophils in patients with chronic cough. Methods A total of 868 patients with persistent cough lasting more than 3 weeks without pulmonary infection were enrolled, including 119 patients with subacute cough (defined as cough lasting 3-8 weeks) and 749 with chronic cough (longer than 8 weeks). The predictive value of FENO level for sputum eosinophilia was analyzed using receiver-operating characteristic (ROC) curve analysis, and the area under the curve (AUC) was calculated. The atopy status of the patients was determined by screening for history of allergy, hay fever, or animal or food allergies. Results Of the 868 patients enrolled, 173 patients (19.9%) had eosinophilic airway inflammation (EAI). In the overall patients, the median (Q1, Q3) FENO level was 18 (12, 35) ppb, ranging from 5 to 300 ppb. The patients with chronic cough and a positive history of atopy had a higher median FENO level than those without atopy (24 [13, 50] vs 18 [11, 34]; Z=2.25, P=0.029), and FENO level was significantly correlated with EAI (r=0.281, P<0.001). The AUCs of FENO for diagnosis of airway eosinophilia in patients with atopy and those without atopy were 0.677 (95% CI: 0.548-0.806) and 0.708 (95% CI: 0.660-0.756), respectively. The optimal cut-off value of FENO for diagnosing EAI was higher in patients with atopy than in those without atopy (72 vs 28.5 ppb). Conclusion A history of atopy reduces the predictive value of FENO level for EAI in patients with chronic cough, suggesting the importance of examining the atopic status when interpreting test results of FENO.

Key words: atopy; exhaled nitric oxide measurements; eosinophilic airway inflammation; chronic cough