MUC1 and respiratory system disorder: Similarly, a low serum KL-6 level (≤ 436 U/mL) with a high CRP level was also associated with non-ILD respiratory disease or non-severe TR-ILD rather than severe TR-ILD (OR: 0.41, 95% CI: 0.18–0.98; p = 0.045) (Fig. 4b).