IC4: a new combined predictive index of mortality in idiopathic pulmonary fibrosis

While a number of individual patient characteristics are associated with survival in idiopathic pulmonary fibrosis (IPF), their incorporation into combined indexes, such as the GAP index, has been shown to increase the predictive capacity. It is unknown whether the predictive capacity of GAP-derived indexes that also include anthropometric and exercise parameters is superior to the original instrument.

We tested the four-year survival predictive capacity of a modified, adimensional and multiplicative GAP index (IC4) that included percent forced vital capacity (FVC%), diffusing capacity of the lung for carbon monoxide (DLCO%), Body Mass Index (BMI), and six-minute walk distance (6MWD) in 90 IPF patients recruited from two centers in France and Italy.

In ROC comparisons, the AUC of the IC4 (0.859, 95% CI 0.770-0.924 P<0.0001) was significantly higher than the AUCs of the individual components, their two-three component combinations, and the original GAP index, with 77% sensitivity and 89% specificity. Mean survival was 14.0±11.7, 23.2±12.7, 34.9±14.8, and 40.8±12.9 months, and survival rate was 0%, 14%, 39% and 73%, in IC4 quartile 1, 2, 3, and 4, respectively.

The IC4, a combined non-dimensional index incorporating FVC%, DLCO%, BMI and 6MWD, provides superior capacity to predict mortality, when compared to its individual components, their other combinations, and the GAP index, in patients with IPF.