Abstract: Objective To investigate the value of exhaled nitric oxide (eNO) in the diagnosis and differential diagnosis of interstitial lung disease (ILD). Methods This study was conducted among 45 patients with interstitial lung disease, including 18 with connective tissue disease- related ILD (CTD- ILD) and 27 with non-CTD-ILD, with 68 healthy subjects as the control group. According to European Respiratory Association Guidelines, alveolar nitric oxide (CaNO) concentration and fractional exhaled nitric oxide (FeNO) level were measured at the flow rates of 50 and 200 mL/s. The predictive level of CaNO was analyzed using receiver-operating characteristic curve (ROC), and the correlations between CaNO and pulmonary function indicators were examined in the patients with ILD. Results CaNO, FeNO50, and FeNO200 levels were significantly higher in patients with ILD than in the healthy controls. Logistic regression analysis showed that lowered levels of CaNO and FeNO200 were risk factors for ILD. ROC curve analysis showed that the area under the curve (AUC) of CaNO combined with FeNO200 was 0.829 (95% CI: 0.752-0.906) for the diagnosis of ILD. In patients with ILD, CaNO levels were negatively correlated with DLCO%pred (r=-0.471, P<0.05). Subgroup comparison showed a significantly higher CaNO level in CTD- ILD group than in non-CTD- ILD group. The AUC for CaNO to discriminate CTD- ILD from non-CTD-ILD was 0.725 (95% CI: 0.576 to 0.875). Conclusion CaNO has a potential value in the diagnosis of ILD and differential diagnosis of CTD-ILD.

Key words: interstitial lung disease; connective tissue disease related interstitial lung disease; fraction of exhaled nitric oxide; concentration of alveolar nitric oxide