MUC1 and interstitial lung disease: Furthermore, in categories 1 and 2, a low serum KL-6 level (≤ 436 U/mL) with a high procalcitonin (PCT) level (> 0.5 ng/mL) was associated with non-ILD respiratory disease or non-severe TR-ILD rather than severe TR-ILD (OR: 0.14, 95% CI: 0.21–0.93; p = 0.042) (Fig. 4b).