Comparison of CPI and GAP models in patients with idiopathic pulmonary fibrosis: a nationwide cohort study

The clinical course of idiopathic pulmonary fibrosis (IPF) is difficult to predict, partly owing to its heterogeneity. Composite physiologic index (CPI) and gender-age-physiology (GAP) models are easy-to-use predictors of IPF progression. This study aimed to compare the predictive values of these two models. From 2003 to 2007, the Korean Interstitial Lung Disease (ILD) Study Group surveyed ILD patients using the 2002 ATS/ERS criteria. A total of 832 patients with IPF were enrolled in this study. CPI was calculated as follows: 91.0 − (0.65 × %DL(CO)) − [0.53 × %FVC + [0.34 × %FEV(1). GAP stage was calculated based on gender (0–1 points), age (0–2 points), and two physiologic lung function parameters (0–5 points). The two models had similar significant predictive values for patients with IPF (p < 0.001). The area under the curve (AUC) was higher for CPI than GAP for prediction of 1-, 2-, and 3-year mortality in this study. The AUC was higher for surgically diagnosed IPF patients than for clinically diagnosed patients. However, neither CPI nor GAP yielded good predictions of outcomes; the AUC was approximately 0.61~0.65. Although both CPI and GAP stage are significantly useful predictors for IPF, they have limited capability to accurately predict outcomes.