title: Measuring readmissions: focus on the time factor.
abstract: OBJECTIVE
  To assess the effects of choosing different time-intervals of observation when using unplanned readmissions as an outcome indicator.
  
  
  METHODS
  A conceptual model was developed based on the risk curve. The model assigned readmissions above a background level as 'related' to the earlier episode of illness. The characteristics of the hazard curve were used to estimate how the rates of related and unrelated readmissions varied with time.
  
  
  METHODS
  Patients living in a region of Middle Norway served by eight acute-care hospitals and discharged in the year 1996.
  
  
  METHODS
  The conditional risk (hazard rate) of having an unplanned readmission. The information gathered allowed inclusion of readmissions to all hospitals in the area, and to make risk corrections for deaths.
  
  
  RESULTS
  The identified proportion of readmissions judged as related to the earlier episode of illness was found to be very sensitive to changes in the time interval. With the commonly used interval of 30 days, 0.5 of all related readmissions were identified, while 0.7 of the readmissions included at this time were estimated as related ones ('true positives'). The hazard curve was different for medical and surgical patients, but the corresponding proportions of related and unrelated readmissions were relatively similar. Adjusting for deaths in the observation period did not result in significantly different risk curves.
  
  
  CONCLUSIONS
  When unplanned readmissions are used as an outcome indicator, the measure is susceptible to the choice of time interval. The operative characteristics must be interpreted in the context of where it is intended that the indicator should be used.
authors.affiliation: Sintef Unimed Health Services Research, Trondheim, Norway. torhild.heggestad@sintef.no
authors.forename: Torhild
authors.initials: T
authors.lastname: Heggestad
authors.forename: Solfrid E
authors.initials: SE
authors.lastname: Lilleeng
mesh_terms: D005260:Female; D006761:Hospitals / Q000592:standards / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D008875:Middle Aged; D017063:Outcome Assessment, Health Care; D010359:Patient Readmission / Q000706:statistics & numerical data*; D016016:Proportional Hazards Models; D019984:Quality Indicators, Health Care; D018570:Risk Assessment / Q000379:methods*; D013997:Time Factors