A serological biomarker in connective tissue disease-associated interstitial lung disease: Krebs von den lungen-6

Objective To evaluate the serum Krebs von den Lungen (KL)-6 for the diagnosis of interstitial lung disease (ILD) associated with connective tissue diseases (CTD) and its lung-CT subtypes. Methods Seventy-five CTD patients were employed for this study, 44 CTD with ILD and 31 ILD without ILD. The 44 CTD patients with ILD were further divided into different subgroups based on lung-CT imaging and clinical indexes. The enzyme-linked immune sorbent assay (ELISA) was used to measure the serum KL-6 level. For those data that was abnormally distributed, the differences between groups was compared with independent samples nonparametric tests. Results The level of serum KL-6 in the CTD with ILD was significantly higher than that without ILD [(1 118±877) U/ml vs (253±144) U/ml] (Z=-6.047, P<0.01). By using a criterion of 500 U/ml, our data suggested that the serum KL-6 level was useful for the ILD-CTD diagnosis; the sensitivity, specificity, positive and negative predictive values were 72.7%, 87.1%, 88.9% and 69.2%, respectively. The serum KL-6 level, however, showed no statistical differences between ILD subtypes, i.e., usual interstitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP) and indeterminate [(1 104±843) U/ml, (1 242±1 039) U/ml, (815±400) U/ml, respectively] (χ2=0.35, P=0.84). Our data further showed that the KL-6 level was significantly higher in CTD-ILD patients with intensive lung lesions than those with limited lung lesions [(1 910±918) U/ml vs (459±268) U/ml] (Z=-4.364, P<0.01). In addition, the KL-6 level was significantly higher in active ILDs than in inactive ILDs[(1 478±917) U/ml vs (598±475) U/ml] (Z=-3.915, P<0.01). Conclusion The serum KL-6 is a valuable biomarker for CTD-ILD diagnosis and even for the assessment of the extent and activity of lung damage. Key words: Lung diseases, interstitial; Connective tissue diseases; Biological markers; Krebs von den lungen-6