External validation of the GAP model in Chinese patients with idiopathic pulmonary fibrosis

IntroductionThe GAP model was widely used as a simple risk "screening" method for patients with idiopathic pulmonary fibrosis (IPF). ObjectivesWe sought to validate the GAP model in Chinese patients with IPF to evaluate whether it can accurately predict the risk for mortality. MethodsA total of 212 patients with IPF diagnosed at China-Japan Friendship Hospital from 2015 to 2019 were enrolled. The latest follow-up ended in September 2022. Cumulative mortality of each GAP stage was calculated and compared based on Fine-Gray models for survival, and lung transplantation was treated as a competing risk. The performance of the model was evaluated in terms of both discrimination and calibration. ResultsThe cumulative mortality in patients with GAP stage III was significantly higher than that in those with GAP stage I or II (Gray's test p < 0.0001). The Harrell c-index for the GAP calculator was 0.736 (95% CI: 0.667-0.864). The discrimination for the GAP staging system were similar with that for the GAP calculator. The GAP model overestimated the mortality rate at 1- and 2-year in patients classified as GAP stage I (6.90% vs. 1.77% for 1-year, 14.20% vs. 6.78% for 2-year). ConclusionsOur findings indicated that the GAP model overestimated the mortality rate in mild group.