Idiopathic Pulmonary Fibrosis: Gender-Age-Physiology Index Stage for Predicting Future Lung Function Decline.
Idiopathic pulmonary fibrosis is a progressive lung disease with variable course in individuals. The Gender-Age-Physiology (GAP) Index stage uses clinical variables to stage mortality risk. It is unknown whether clinical staging predicts future pulmonary function decline. We assess whether the GAP stage predicts future pulmonary function decline, and whether interval pulmonary function change predicts mortality after accounting for stage.
Patients with Idiopathic Pulmonary Fibrosis (n=657) were identified retrospectively at three tertiary referral centers and baseline GAP stage assessed. Mixed models describe average trajectories of forced vital capacity (FVC) and diffusion capacity (DLCO). Multivariable Cox proportional hazards models assess whether pulmonary function declines of 10% or more in 6 months predict mortality after accounting for stage.
Over 2 years, GAP stage was not associated with differences in yearly lung function decline. After accounting for stage, a 10% decrease in FVC or DLCO over 6 months independently predicted death or transplant (FVC HR=1.37, DLCO HR 1.30, both p≤0.03). GAP stage 2 patients with declining pulmonary function experienced a survival profile similar to GAP 3 patients with 1-year event-free survival of 59.3% (CI95% 49.4-67.8) versus 56.9% (CI95% 42.2-69.1).
Baseline GAP stage predicts death or lung transplantation, but not the rate of future pulmonary function decline. After accounting for GAP stage, a decline of 10% or more over 6 months independently predicts death or lung transplant.