title: Studying outcomes of intensive care unit survivors: measuring exposures and outcomes.
abstract: BACKGROUND
  Measurement of long-term outcomes and the patient and intensive care unit (ICU) factors predicting them present investigators with unique challenges. There is little systematic guidance for measuring these outcomes and exposures within the ICU setting. As a result measurement methods are often variable and non-comparable across studies.
  
  
  METHODS
  We use examples from the critical care literature to describe measurement as it relates to three key elements of clinical studies: subjects, outcomes and exposures, and time. Using this framework we review the principles and challenges of measurement and make recommendations for long-term outcomes research in the field of critical care medicine.
  
  
  CONCLUSIONS
  Relevant challenges discussed include: (a) selection bias and heterogeneity of ICU research subjects, (b) appropriate selection and measurement of outcome and exposure variables, and (c) accounting for the effect of time in the exposure-outcome relationship, including measurement of baseline data and time-varying variables.
  
  
  CONCLUSIONS
  Addressing these methodological challenges will advance research aimed at improving the long-term outcomes of ICU survivors.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Johns Hopkins University, Baltimore, MD 21205, USA. dale.needham@utoronto.ca
authors.forename: Dale M
authors.initials: DM
authors.lastname: Needham
authors.forename: David W
authors.initials: DW
authors.lastname: Dowdy
authors.forename: Pedro A
authors.initials: PA
authors.lastname: Mendez-Tellez
authors.forename: Margaret S
authors.initials: MS
authors.lastname: Herridge
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Pronovost
mesh_terms: D003422:Critical Care; D017052:Hospital Mortality; D006801:Humans; D007902:Length of Stay; D017063:Outcome Assessment, Health Care / Q000379:methods*; D011788:Quality of Life; D016019:Survival Analysis