Researchers found that the incorporation of exercise capacity into the existing gender-age physiology (GAP) index may improve the prediction of the prognosis in patients with idiopathic pulmonary fibrosis (IPF), as published in Thorax.

A commonly used risk prediction model for IPF is the GAP index. “This model provides valuable insight through simple objective predictors of disease progression (gender, age, forced vital capacity (FVC) and diffusing capacity of the lung for carbon monoxide (DLCO)),” the authors wrote.

One fundamental difference in the GAP index is that it does not take into consideration exercise capacity. Hence, it may be limited in its ability to accurately capture decline over time. The research team sought to build on the GAP index by incorporating exercise capacity as a factor that can be applied during initial clinical evaluations. 

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The researchers reviewed patients with IPF at the Inova Advanced Lung Disease Program and at the European Hospital Georges Pompidou in Paris, France. Predictor variables were prespecified based on the evidence of their association with a poor prognosis; these indicators were exertional hypoxia and the 6-Minute Walk Distance (6MWD) scores. “The addition of these variables to the GAP index significantly improved model discrimination,” the researchers wrote.

The original GAP index was found to not be well-calibrated and it overestimated mortality, particularly in mild and moderate disease stages. The researchers then introduced exertional hypoxia and the 6MWD into the GAP index and termed it the “DO-GAP” index. This new index was able to discriminate outcomes when applied to patients with a recent diagnosis of IPF, those on antifibrotic medication, and those from a geographically distinct external cohort.

“We found that the original index was not well calibrated to predict outcomes observed in our cohort,” the authors concluded. “6MWD and exertional hypoxia were significant prognostic factors strongly associated with overall survival.”


Chandel A, Pastre J, Valery S, King CS, Nathan SD. Derivation and validation of a simple multidimensional index incorporating exercise capacity parameters for survival prediction in idiopathic pulmonary fibrosisThorax. Published online March 24, 2022. doi:10.1136/thoraxjnl-2021-218440