Forced Vital Capacity in Patients with Idiopathic Pulmonary Fibrosis

TLDR

Forced vital capacity (FVC) is an established pulmonary function measure in idiopathic pulmonary fibrosis, yet its measurement properties and minimal clinically important difference remain poorly characterized. The study aims to evaluate FVC’s reliability, validity, responsiveness, and to estimate its minimal clinically important difference in IPF patients. FVC and other functional status measures were assessed at baseline and every 24 weeks, with reliability examined through proximal measurements, validity via correlations with other status indicators, responsiveness via 24‑week change relationships, and MCID estimated using distribution‑ and anchor‑based methods. FVC proved highly reliable (r = 0.93), showed modest correlations with other functional measures (max r = 0.38), and a 5–10 % decline over 24 weeks doubled 1‑year mortality risk; the estimated minimal clinically important difference was 2–6 %, confirming FVC as a reliable, valid, and responsive clinical status marker in IPF.

Abstract

Forced vital capacity (FVC) is an established measure of pulmonary function in idiopathic pulmonary fibrosis (IPF). Evidence regarding its measurement properties and minimal clinically important difference (MCID) in this population is limited.To assess the reliability, validity, and responsiveness of FVC and estimate the MCID in patients with IPF.The study population included all 1,156 randomized patients in two clinical trials of IFN-γ1b. FVC and other measures of functional status were measured at screening or baseline and 24-week intervals thereafter. Reliability was assessed based on two proximal measures of FVC, validity was assessed based on correlations between FVC and other measures of functional status, and responsiveness was assessed based on the relationship between 24-week changes in FVC and other measures of functional status. Distribution-based and anchor-based methods were used to estimate the MCID.Correlation of percent-predicted FVC between measurements (mean interval, 18 d) was high (r = 0.93; P < 0.001). Correlations between FVC and other parameters were generally weak, with the strongest observed correlation between FVC and carbon monoxide diffusing capacity (r = 0.38; P < 0.001). Correlations between change in FVC and changes in other parameters were slightly stronger (range, r = 0.16-0.37; P < 0.001). Importantly, 1-year risk of death was more than twofold higher (P < 0.001) in patients with a 24-week decline in FVC between 5% and 10%. The estimated MCID was 2-6%.FVC is a reliable, valid, and responsive measure of clinical status in patients with IPF, and a decline of 2-6%, although small, represents a clinically important difference.