title: Patient volume affects outcome in critically ill patients.
abstract: BACKGROUND
  A positive relationship between patient volume and outcome has been demonstrated for a variety of clinical conditions and procedures, but the evidence is sparse for critically ill patients.
  
  
  OBJECTIVE
  To evaluate the relationship between patient volume and outcome in a large cohort of critically ill patients.
  
  
  METHODS
  Prospective multicenter cohort study, January 1998 through December 2005.
  
  
  METHODS
  40 intensive care units in Austria.
  
  
  METHODS
  A total of 83,259 consecutively admitted patients.
  
  
  METHODS
  Structural quality of participating ICUs was evaluated using a questionnaire and merged with the prospectively collected data. Volume related indices were then calculated, representing patient turnover, occupancy rate, nursing workload and diagnostic variability.
  
  
  RESULTS
  Univariate analysis revealed that several volume variables were associated with outcome: more patients treated per year per bed in the intensive care unit and more patients treated in the same diagnostic category reduced the risk of dying in the hospital (odds ratios, 0.967 and 0.991 for each additional 10 patients treated, respectively). In contrast, an increase in the patient-to-nurse ratio and an increase in the number of diagnostic categories were associated with increased mortality rates. Multivariate analysis confirmed these results. The relationship between the number of patients treated in the same diagnostic category and their outcomes showed not a linear but a U shape, with increasing mortality rates below and above a certain patient volume.
  
  
  CONCLUSIONS
  Our results provide evidence for a relationship between patient volume and outcome in critically ill patients. Besides the total number of patients, diagnostic variability plays an important role. The relationship between volume and outcome seems, however, to be complex and to be influenced by other variables, such as workload of nursing staff.
authors.affiliation: Department of Medical Statistics, Medical University of Vienna, Vienna, Austria. philipp.metnitz@meduniwien.ac.at
authors.forename: Barbara
authors.initials: B
authors.lastname: Metnitz
authors.forename: Philipp G H
authors.initials: PG
authors.lastname: Metnitz
authors.forename: Peter
authors.initials: P
authors.lastname: Bauer
authors.forename: Andreas
authors.initials: A
authors.lastname: Valentin
mesh_terms: D001317:Austria; D001509:Bed Occupancy / Q000706:statistics & numerical data; D002983:Clinical Competence / Q000706:statistics & numerical data*; D016638:Critical Illness / Q000401:mortality / Q000451:nursing; D003627:Data Interpretation, Statistical; D006276:Health Facility Size / Q000706:statistics & numerical data*; D017052:Hospital Mortality; D006801:Humans; D007362:Intensive Care Units / Q000706:statistics & numerical data*; D007902:Length of Stay / Q000706:statistics & numerical data; D009741:Nursing Staff, Hospital / Q000600:supply & distribution; D010043:Outcome and Process Assessment, Health Care / Q000706:statistics & numerical data*; D011446:Prospective Studies; D011785:Quality Assurance, Health Care / Q000706:statistics & numerical data*; D020379:Risk Adjustment / Q000706:statistics & numerical data; D016526:Workload / Q000706:statistics & numerical data