ObjectiveTo determine the diagnostic value of serum KL-6 level in patients with interstitial lung diseases (ILD). MethodsAll the ILD patients enrolled were hospitalized from April 2013 to April 2014. Patients with other pulmonary diseases and healthy subjects were chosen as control groups simultaneously. Serum KL-6 concentrations were measured by chemiluminescent enzyme immunoassay. The association with serum KL-6 level and pulmonary function was analyzed. ResultsThere were 149 ILD patients, 155 patients with other pulmonary diseases, and 64 healthy subjects. The average serum levels of KL-6 were (1 801.86±2 831.36) U/mL, (267.00±124.41) U/mL, (201.28±81.18) U/mL in the patients with ILD, the patients with other pulmonary diseases and the healthy controls, respectively. The sensitivity and the specificity of the serum KL-6 for the diagnosis of ILD was 83.89% and 92.24% respectively when the cut-off level was set at 500 U/mL. The Kappa value was 0.767 (P < 0.001). The best cut-off value of KL-6 was 469.5 U/mL. Serum KL-6 levels in the patients with ILD were significantly higher compared with the patients with chronic obstructive pulmonary disease, pneumonia, tuberculosis, bronchiectasis and the healthy controls, respectively (all P < 0.001). The KL-6 levels in the pulmonary alveolar proteinosis patients were significantly higher compared with the patients with cryptogenic organizing pneumonia (COP), the patients with idiopathic pulmonary fibrosis (IPF) and the patients with connective tissue disease (CTD-ILD) (all P < 0.001). While the KL-6 concentration in IPF and CTD-ILD were significantly higher than that in COP (P=0.003 and P=0.008, respectively). Significant negative correlations were found between the levels of serum KL-6 and vital capacity as a percentage of the predicted value, forced vital capacity as a percentage of the predicted value, forced expiratory volume in one second as a percentage of the predicted value and carbon monoxide diffusing capacity as a percentage of the predicted value (all P < 0.001). Follow-up study showed the levels of serum KL-6 were consistent with clinical efficacy. ConclusionSerum KL-6 level is a reliable serum marker for ILD, and is related with the severity of disease and clinical efficacy.