MUC1 and idiopathic interstitial pneumonia: In the multivariable analysis including age, KL-6 level (≥ 800 U/mL), FVC, lowest oxygen saturation during 6MWT, and diagnosis of IPF, only KL-6 levels (≥ 800 U/mL) were the independent predictive factor for disease progression (HR 2.689, 95% CI 1.445–5.004, P = 0.002).