title: The utility of 6-minute walk distance in predicting waitlist mortality for lung transplant candidates.
abstract: BACKGROUND
  The lung allocation score (LAS) has led to improved organ allocation for transplant candidates. At present, the 6-minute walk distance (6MWD) is treated as a binary categorical variable of whether or not a candidate can walk more than 150 feet in 6 minutes. In this study, we tested the hypothesis that 6MWD is presently under-utilized with respect to discriminatory power, and that, as a continuous variable, could better prognosticate risk of waitlist mortality.
  
  
  METHODS
  A retrospective cohort analysis was performed using the Organ Procurement and Transplantation Network/United Network for Organ Sharing (OPTN/UNOS) transplant database. Candidates listed for isolated lung transplant between May 2005 and December 2011 were included. The population was stratified by 6MWD quartiles and unadjusted survival rates were estimated. Multivariable Cox proportional hazards modeling was used to assess the effect of 6MWD on risk of death. The Scientific Registry of Transplant Recipients (SRTR) Waitlist Risk Model was used to adjust for confounders. The optimal 6MWD for discriminative accuracy in predicting waitlist mortality was assessed by receiver-operating characteristic (ROC) curves.
  
  
  RESULTS
  Analysis was performed on 12,298 recipients. Recipients were segregated into quartiles by distance walked. Waitlist mortality decreased as 6MWD increased. In the multivariable model, significant variables included 6MWD, male gender, non-white ethnicity and restrictive lung diseases. ROC curves discriminated 6-month mortality was best at 655 feet.
  
  
  CONCLUSIONS
  The 6MWD is a significant predictor of waitlist mortality. A cut-off of 150 feet sub-optimally identifies candidates with increased risk of mortality. A cut-off between 550 and 655 feet is more optimal if 6MWD is to be treated as a dichotomous variable. Utilization of the LAS as a continuous variable could further enhance predictive capabilities.
authors.affiliation: Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Anthony
authors.initials: A
authors.lastname: Castleberry
authors.affiliation: Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA. Electronic address: mike.mulvihill@dm.duke.edu.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Mulvihill
authors.affiliation: Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Babatunde A
authors.initials: BA
authors.lastname: Yerokun
authors.affiliation: Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Brian C
authors.initials: BC
authors.lastname: Gulack
authors.affiliation: Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Brian
authors.initials: B
authors.lastname: Englum
authors.affiliation: Division of Advanced Oncologic and GI Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Laurie
authors.initials: L
authors.lastname: Snyder
authors.affiliation: Division of Advanced Oncologic and GI Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA; Department of Visceral Surgery and Medicine, Inselspital, Berne University Hospital, Berne, Switzerland.
authors.forename: Mathias
authors.initials: M
authors.lastname: Worni
authors.affiliation: Department of General Surgery, Massachusetts General Hospital, Boston, Massachusetts, USA.
authors.forename: Asishana
authors.initials: A
authors.lastname: Osho
authors.affiliation: Division of Pulmonary, Allergy, and Critical Care Medicine, Department of Medicine, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Scott
authors.initials: S
authors.lastname: Palmer
authors.affiliation: Cardiovascular Institute, Florida Hospital, Orlando, Florida, USA.
authors.forename: R Duane
authors.initials: RD
authors.lastname: Davis
authors.affiliation: Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Hartwig
mesh_terms: D000328:Adult; D005080:Exercise Test; D017079:Exercise Tolerance; D005260:Female; D006801:Humans; D008171:Lung Diseases / Q000401:mortality* / Q000503:physiopathology* / Q000601:surgery; D016040:Lung Transplantation; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D015996:Survival Rate; D014850:Waiting Lists; D000070857:Walk Test; D016138:Walking
keywords: 6-minute walk distance; candidate assessment; lung transplantation; organ allocation; waitlist mortality