• Am. J. Respir. Crit. Care Med. · Dec 2011

    Forced vital capacity in patients with idiopathic pulmonary fibrosis: test properties and minimal clinically important difference.

    • Roland M du Bois, Derek Weycker, Carlo Albera, Williamson Z Bradford, Ulrich Costabel, Alex Kartashov, Talmadge E King, Lisa Lancaster, Paul W Noble, Steven A Sahn, Michiel Thomeer, Dominique Valeyre, and Athol U Wells.
    • Imperial College, London, UK. ron@du-bois.co.uk
    • Am. J. Respir. Crit. Care Med. 2011 Dec 15; 184 (12): 138213891382-9.

    RationaleForced 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.ObjectivesTo assess the reliability, validity, and responsiveness of FVC and estimate the MCID in patients with IPF.MethodsThe 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.Measurements And Main ResultsCorrelation 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%.ConclusionsFVC 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.

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