title: Forced vital capacity in patients with idiopathic pulmonary fibrosis: test properties and minimal clinically important difference.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  To assess the reliability, validity, and responsiveness of FVC and estimate the MCID in patients with IPF.
  
  
  METHODS
  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.
  
  
  RESULTS
  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%.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Imperial College, London, UK. ron@du-bois.co.uk
authors.forename: Roland M
authors.initials: RM
authors.lastname: du Bois
authors.forename: Derek
authors.initials: D
authors.lastname: Weycker
authors.forename: Carlo
authors.initials: C
authors.lastname: Albera
authors.forename: Williamson Z
authors.initials: WZ
authors.lastname: Bradford
authors.forename: Ulrich
authors.initials: U
authors.lastname: Costabel
authors.forename: Alex
authors.initials: A
authors.lastname: Kartashov
authors.forename: Talmadge E
authors.initials: TE
authors.lastname: King
authors.forename: Lisa
authors.initials: L
authors.lastname: Lancaster
authors.forename: Paul W
authors.initials: PW
authors.lastname: Noble
authors.forename: Steven A
authors.initials: SA
authors.lastname: Sahn
authors.forename: Michiel
authors.initials: M
authors.lastname: Thomeer
authors.forename: Dominique
authors.initials: D
authors.lastname: Valeyre
authors.forename: Athol U
authors.initials: AU
authors.lastname: Wells
mesh_terms: D000368:Aged; D005260:Female; D006801:Humans; D054990:Idiopathic Pulmonary Fibrosis / Q000175:diagnosis*; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011379:Prognosis; D015203:Reproducibility of Results; D014797:Vital Capacity