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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 × percent predicted diffusing capacity for carbon monoxide [DL_CO]) − (0.53 × percent predicted FVC) + (0.34 × 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 (DL_CO the highest value …