title: Relative versus absolute change in forced vital capacity in idiopathic pulmonary fibrosis.
abstract: BACKGROUND
  Decline in forced vital capacity (FVC) over time reliably predicts mortality in patients with idiopathic pulmonary fibrosis. The use of this measure in clinical practice is recommended by current evidence-based guidelines. It is unknown if the method of calculating decline in FVC (relative vs. absolute change) impacts its frequency or its ability to predict mortality.
  
  
  METHODS
  Patients with idiopathic pulmonary fibrosis from two prospective cohorts were included if they had a baseline and 12-month follow-up FVC. A ≥10% decline in FVC from baseline was calculated in two ways: a relative decline of 10% (e.g., from 60% predicted to 54% predicted) and an absolute decline of 10% (e.g., from 60% predicted to 50% predicted). The frequency of a ≥10% decline in FVC and its ability to predict 2-year transplant-free survival were compared between these two methods. Declines in FVC of ≥5% and ≥15% were similarly compared. Analyses were performed unadjusted and adjusted for age, gender, use of oxygen, baseline FVC and baseline diffusion capacity for carbon monoxide.
  
  
  RESULTS
  The frequency of any given FVC decline was significantly greater using the relative change in FVC method. For ≥10% decline, both methods predicted 2-year transplant-free survival with similar accuracy, and remained significant predictors after adjusting for baseline characteristics. The absolute change method appeared more predictive for ≥5% decline.
  
  
  CONCLUSIONS
  Using the relative change in FVC maximises the chance of identifying a ≥10% decline in FVC without sacrificing prognostic accuracy. This may not hold true for ≥5% decline in FVC. These findings have important implications for clinical practice and the design of clinical trials.
authors.affiliation: Center for Rare Lung Diseases, University of Modena and Reggio Emilia, Policlinico Hospital, Via del Pozzo 71, 41100 Modena, Italy. luca.richeldi@unimore.it
authors.forename: Luca
authors.initials: L
authors.lastname: Richeldi
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Ryerson
authors.forename: Joyce S
authors.initials: JS
authors.lastname: Lee
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Wolters
authors.forename: Laura L
authors.initials: LL
authors.lastname: Koth
authors.forename: Brett
authors.initials: B
authors.lastname: Ley
authors.forename: Brett M
authors.initials: BM
authors.lastname: Elicker
authors.forename: Kirk D
authors.initials: KD
authors.lastname: Jones
authors.forename: Talmadge E
authors.initials: TE
authors.lastname: King
authors.forename: Jay H
authors.initials: JH
authors.lastname: Ryu
authors.forename: Harold R
authors.initials: HR
authors.lastname: Collard
mesh_terms: D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D054990:Idiopathic Pulmonary Fibrosis / Q000503:physiopathology* / Q000601:surgery; D016015:Logistic Models; D016040:Lung Transplantation / Q000706:statistics & numerical data*; D008297:Male; D016019:Survival Analysis; D014797:Vital Capacity / Q000502:physiology*