ObjectiveTo compare the expressive differences of plasma Kerbs von den lungen-6 (KL-6) in patients with idiopathic interstitial pneumonia (IIP) and connective tissue disease associated secondary interstitial pneumonia (CTD-SIP), and analyze the clinical significances.MethodsThe clinical data and peripheral blood of 399 inpatients with interstitial pneumonia and 50 healthy controls were collected from January 2011 to December 2014 in Nanjing Drum Tower Hospital. The level of plasma KL-6 was measured by chemiluminescence immunoassay method. The subjects were divided into IIP (n=233) group and CTD-SIP (n=166) group, usual interstitial pneumonia (UIP) pattern and non-UIP pattern, and stable (S) UIP group and acute exacerbation (AE) UIP group. Statistical analyses were performed by using IBM SPSS 19.0 (SPSS, Inc., Chicago IL, USA) to compare the differences of plasma KL-6 in groups.ResultsThere were more male subjects (61.8%) in the IIP group, and the average age of (62.3±12.5) years was significantly older (both P<0.01). Plasma KL-6 levels in the IIP ［(1 822.7±1 505.2) U/ml) and the CTD-SIP group ［(1 846.7±1 625.3) U/ml］ were significantly higher than the healthy control group ［(190.2±88.7) U/ml］ (both P<0.001). However, there was no any difference of KL-6, white blood cell count (WBC), lactate dehydrogenase (LDH), C-reactive protein (CRP) and erythrocyte sedimentation rate between the IIP and the CTD-SIP group. The level of plasma KL-6 was positively correlated to WBC, LDH and CRP in the IIP group (r=0.159, P=0.016; r=0.380, P<0.001; r=0.158, P=0.015, respectively); and it was positively correlated to LDH and CRP in the IIP group (r=0.187, P=0.016 and r=0.068, P=0.032) in the CTD-SIP group. There was no significant difference of plasma KL-6 between the UIP and non-UIP subgroups (P>0.05). The difference of plasma KL-6 between the S-UIP and AE-UIP subgroup was significant (P<0.001 and P=0.023). There was no any significant difference of plasma KL-6 among the subgroups with CTD patients (primary Sjögren’s syndrome, n=90; rheumatoid arthritis, n=20; polymyositis/dermatomyositis, n=26; undifferentiated connective tissue disease, n=10; anti-neutrophil cytoplasmic antibody associated vasculitis, n=15 and systemic sclerosis, n=5) (P=0.785 2).ConclusionsPlasma KL-6 may be a useful biomarker for interstitial pneumonia. It can show the disease activities, but is not able to distinguish IIP from SIP.