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BackgroundAlthough hospitalization in idiopathic pulmonary fibrosis (IPF) is associated with poor outcomes, it occurs during the disease course and may lead to overestimation of its prognostic impact when analyzed as a fixed exposure.ObjectivesWe aimed to evaluate the association between hospitalization and transplantation-free survival in patients with IPF by modeling hospitalization as a time-dependent exposure and applying a standardized definition of respiratory-related hospitalization.DesignThis was a retrospective cohort study conducted at Asan Medical Center, a tertiary referral center in Seoul, South Korea.MethodsA total of 223 patients with IPF were included. Hospitalization was treated as a time-dependent exposure in Cox regression models. Changes in pulmonary function and exercise capacity before and after hospitalization were assessed using linear mixed-effects models.ResultsSeventy patients (31%) experienced hospitalization during follow-up, whereas 153 (69%) did not. Hospitalized patients had significantly worse lung function and exercise capacity at baseline compared with non-hospitalized patients. When modeled as a time-dependent exposure, hospitalization was strongly associated with increased risk of transplantation or death (adjusted hazard ratio: 8.87, 95% confidence interval [CI]: 5.57-14.11; P<0.001). After excluding patients who died within 10 days of hospitalization, long hospital stay (≥10 days) was associated with poorer transplantation-free survival than short stay (<10 days) (P=0.022). Pulmonary function and exercise capacity declined significantly during hospitalization. Forced vital capacity decreased both before (-7.6%, 95% CI: -11.2 to -4.1; P<0.001) and after hospitalization (-4.8%, 95% CI: -9.4 to -0.2; P=0.04). Minimum oxygen saturation during the six-minute walk test decreased significantly both before (-2.5%, 95% CI: -3.8 to -1.3; P<0.001) and after hospitalization (-2.3%, 95% CI: -4.4 to -0.3; P=0.029).ConclusionHospitalization in IPF was associated with poorer transplantation-free survival and subsequent physiological decline. It may serve as an important time-dependent marker of clinical deterioration and disease progression.