MUC1 and interstitial lung disease: The cut-off serum KL-6 value for predicting severe TR-ILD (grade 2 or higher) in categories 1 and 2 was determined by receiver operating characteristic (ROC) curve analysis, and the optimal value was 436 U/mL, with a sensitivity of 75.0% and a specificity of 61.9% (area under the curve [AUC]: 0.696, 95% CI: 0.594–0.785; p < 0.001) (Fig. 3d).