GDF15 and idiopathic pulmonary fibrosis: In multivariate Cox regression models, a serum GDF-15 level above 1,075.76 pg/ml and PaO2/FiO2 were the independent predicting factors for total survival in IPF patients (HR = 0.991, 95% CI: 0.986-0.996, p < 0.001 and HR = 0.428, 95% CI: 0.232-0.790, p = 0.007, respectively) (Table 5).