Minimum clinically important difference in Quantitative Lung Fibrosis score associated with all-cause mortality in idiopathic pulmonary fibrosis: subanalysis from two phase II trials of pamrevlumab

OBJECTIVES: Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease. Chest high-resolution CT (HRCT) is instrumental in IPF management, and the Quantitative Lung Fibrosis (QLF) score is a computer-assisted metric for quantifying lung disease using HRCT. This study aimed to assess the change in QLF score associated with a minimum clinically important difference (MCID) of IPF symptoms and physiological lung function, and also determine the MCID of QLF change associated with all-cause mortality to serve as an imaging biomarker to confirm disease progression and response to therapy. DESIGN AND STUDY SETTING: We conducted post hoc analyses of prospective data from two IPF phase II studies of pamrevlumab, a fully human monoclonal antibody that binds to and inhibits connective tissue growth factor activity. PARTICIPANTS: Overall, 152 patients with follow-up visits after week 24. METHODS: We used the anchor-based Jaeschke's method to estimate the MCID of the QLF score that corresponded with the already established MCID of St. George's Respiratory Questionnaire (SGRQ) and percent-predicted forced vital capacity (ppFVC). We also conducted a Cox regression analysis to establish a sensitive and robust MCID of the QLF score in predicting all-cause mortality. RESULTS: QLF changes of 4.4% and 3.6% corresponded to the established MCID of a 5-point increase in SGRQ and a 3.4% reduction in ppFVC, respectively. QLF changes of 1% (HR=4.98, p=0.05), 2% (HR=4.04, p=0.041), 20 mL (HR=6.37, p=0.024) and 22 mL (HR=6.38, p=0.024) predicted mortality. CONCLUSION: A conservative metric of 2% can be used as the MCID of QLF for predicting all-cause mortality. This may be considered in IPF trials in which the degree of structural fibrosis assessed via HRCT is an endpoint. The MCID of SGRQ and FVC corresponds with a greater amount of QLF and may reflect that a greater amount of change in fibrosis is required before there is functional change. TRIAL REGISTRATION NUMBER: NCT01262001, NCT01890265.