The usefulness of a disease severity staging classification system for IPF in Japan: 20 years of experience from empirical evidence to randomized control trial enrollment

Since 1991, the severity of idiopathic pulmonary fibrosis (IPF) has been classified into 4 stages—stage I (characterized by a resting PaO2≥80 Torr), stage II (70–79 Torr), stage III (60–69 Torr), or stage IV (<60 Torr)—to aid decisions on medical care subsidization in Japan. Among patients with stage II/III IPF, the severity should be increased by one stage if the lowest oxygen saturation on pulse oximetry (SpO2) is <90% during a 6-min walk...