title: An empirical derivation of the optimal time interval for defining ICU readmissions.
abstract: BACKGROUND
  Intensive care unit (ICU) readmission rates are commonly viewed as indicators of ICU quality. However, definitions of ICU readmissions vary, and it is unknown which, if any, readmissions are associated with ICU quality.
  
  
  OBJECTIVE
  Empirically derive the optimal interval between ICU discharge and readmission for purposes of considering ICU readmission as an ICU quality indicator.
  
  
  METHODS
  Retrospective cohort study.
  
  
  METHODS
  A total of 214,692 patients discharged from 157 US ICUs participating in the Project IMPACT database, 2001-2008.
  
  
  METHODS
  We graphically examined how patient characteristics and ICU discharge circumstances (eg, ICU census) were related to the odds of ICU readmissions as the allowable interval between ICU discharge and readmission was lengthened. We defined the optimal interval by identifying inflection points where these relationships changed significantly and permanently.
  
  
  RESULTS
  A total of 2242 patients (1.0%) were readmitted to the ICU within 24 hours; 9062 (4.2%) within 7 days. Patient characteristics exhibited stronger associations with readmissions after intervals >48-60 hours. By contrast, ICU discharge circumstances and ICU interventions (eg, mechanical ventilation) exhibited weaker relationships as intervals lengthened, with inflection points at 30-48 hours. Because of the predominance of afternoon readmissions regardless of time of discharge, using intervals defined by full calendar days rather than fixed numbers of hours produced more valid results.
  
  
  CONCLUSIONS
  It remains uncertain whether ICU readmission is a valid quality indicator. However, having established 2 full calendar days (not 48 h) after ICU discharge as the optimal interval for measuring ICU readmissions, this study will facilitate future research designed to determine its validity.
authors.affiliation: Center for Clinical Epidemiology and Biostatistics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104-6021, USA. sydney2@mail.med.upenn.edu
authors.forename: Sydney E S
authors.initials: SE
authors.lastname: Brown
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Ratcliffe
authors.forename: Scott D
authors.initials: SD
authors.lastname: Halpern
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D015897:Comorbidity; D005260:Female; D006304:Health Status; D006801:Humans; D007362:Intensive Care Units / Q000458:organization & administration* / Q000592:standards; D008297:Male; D008875:Middle Aged; D017063:Outcome Assessment, Health Care; D010359:Patient Readmission / Q000592:standards / Q000706:statistics & numerical data*; D019984:Quality Indicators, Health Care / Q000458:organization & administration* / Q000592:standards; D011787:Quality of Health Care / Q000458:organization & administration; D012189:Retrospective Studies; D012737:Sex Factors; D013997:Time Factors; D014481:United States