Correlation between serum bilirubin levels and the severity as well as the prognosis of idiopathic pulmonary fibrosis

Bilirubin exerts antioxidant activity that has been associated with respiratory diseases. However, the relationship between serum bilirubin levels and idiopathic pulmonary fibrosis (IPF) is not clear. Therefore, in this study, we evaluated the relationship between serum bilirubin levels and the severity as well as the prognosis of IPF. One hundred and forty-six patients with IPF and 69 healthy individuals as the control group were enrolled as a derivation cohort. Routine blood examination and pulmonary function tests were performed and serum bilirubin levels were measured. To validate the value of serum bilirubin levels to predict the survival of patients with IPF, 40 additional IPF patients were included as a validation cohort. IPF patients were followed-up. Patients with IPF had significantly lower levels of serum total bilirubin (TBIL) and direct bilirubin (DBIL) than those in the control group (P < 0.05). Patients with acute exacerbation of IPF (AE-IPF) had significantly lower levels of serum TBIL and IBIL than those in patients with stable IPF (P < 0.05). The area under the receiver operating characteristic curve (AUROC) of serum TBIL levels for the prediction of the incidence of AE-IPF was 0.72 (95% CI: 0.56–0.87, P = 0.0057). The best cutoff value of serum TBIL level to predict the survival of patients with IPF was 8.8 μmol/l (AUC = 0.75, 95% CI: 0.64–0.87, P = 0.022). The log-rank test showed a significant difference in survival between the two groups (TBIL ≤8.8 μmol/l and TBIL >8.8 μmol/l) in derivation and validation cohort. Cox multiple regression analysis indicated that serum TBIL levels were an independent prognostic factor for IPF prognosis (HR = 0.582, P = 0.026). Serum TBIL levels might be useful for reflecting the severity and predicting the survival of patients with IPF.