In
idiopathic pulmonary fibrosis, the quantitation of disease severity using pulmonary function tests is often confounded by
emphysema. We have identified the composite physiologic index (
CPI) most closely reflecting the morphologic extent of
pulmonary fibrosis. Consecutive patients with a clinical/computed tomography (CT) diagnosis of
idiopathic pulmonary fibrosis (n = 212) were divided into group I (n = 106) and group II (n = 106). The
CPI was derived in group I (by fitting pulmonary function tests against disease extent on CT) and was tested in Group II. The formula for the
CPI was as follows: extent of disease on CT = 91.0 - (0.65 x percent predicted diffusing capacity for
carbon monoxide [DLCO]) - (0.53 x percent predicted FVC) + (0.34 x percent predicted FEV1). In group II, the
CPI correlated more strongly with disease extent on CT (r2 = 0.51) than the individual pulmonary function test (DLCO the highest value, r2 = 0.38). A subanalysis demonstrated that the better fit of the
CPI was ascribable to a correction of the confounding effects of
emphysema. Mortality was predicted more accurately by the
CPI than by a pulmonary function test in all clinical subgroups, including a separate cohort of 36 patients with histologically proven
usual interstitial pneumonia (
CPI, p < 0.0005; FVC, p = 0.002; PO2, p = 0.002). In conclusion, a new
CPI, derived against CT and validated using split sample testing, is a more accurate prognostic determinant in
usual interstitial pneumonia than an individual pulmonary function test.