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

Introduction: The GAP model was widely used as a simple risk "screening" method for patients with idiopathic pulmonary fibrosis (IPF).

Objectives: We sought to validate the GAP model in Chinese patients with IPF to evaluate whether it can accurately predict the risk for mortality.

Methods: A 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.

Results: The 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).

Conclusions: Our findings indicated that the GAP model overestimated the mortality rate in mild group.