MUC1 and interstitial lung disease: The area under the ROC curve (AUC) of CYFRA21-1 in RA-ILD was 0.73 (<i>p</i> < 0.001; sensitivity = 61.86%; specificity = 78.05%), while that in RA-advanced ILD was 0.88 (<i>p</i> < 0.001; sensitivity = 84.09%; specificity = 78.05%).<h4>Conclusion</h4>CYFRA21-1 exhibited graded expression and a positive correlation with KL-6, serving as a valuable serum biomarker for risk stratification and severity assessment in RA-ILD.