Derivation and validation of a simple multidimensional index incorporating exercise capacity parameters for survival...

The gender-age-physiology (GAP) index is an easy-to-use baseline mortality prediction model in idiopathic pulmonary fibrosis (IPF). The GAP index does not incorporate exercise capacity parameters such as 6 min walk distance (6MWD) or exertional hypoxia. We evaluated if the addition of 6MWD and exertional hypoxia to the GAP index improves survival prediction in IPF.

Patients with IPF were identified at a tertiary care referral centre. Discrimination and calibration of the original GAP index were assessed. The cohort was then randomly divided into a derivation and validation set and performance of the GAP index with the addition of 6MWD and exertional hypoxia was evaluated. A final model was selected based on improvement in discrimination. Application of this model was then evaluated in a geographically distinct external cohort.

There were 562 patients with IPF identified in the internal cohort. Discrimination of the original GAP index was measured by a C-statistic of 0.676 (95% CI 0.635 to 0.717) and overestimated observed risk. 6MWD and exertional hypoxia were strongly predictive of mortality. The addition of these variables to the GAP index significantly improved model discrimination. A revised index incorporating exercise capacity parameters was constructed and performed well in the internal validation set (C-statistic: 0.752; 95% CI 0.701 to 0.802, difference in C-statistic compared with the refit GAP index: 0.050; 95% CI 0.004 to 0.097) and external validation set (N=108 (C-statistic: 0.780; 95% CI 0.682 to 0.877)).

A simple point-based baseline-risk prediction model incorporating exercise capacity predictors into the original GAP index may improve prognostication in patients with IPF.