title: Quantifying interhospital patient sharing as a mechanism for infectious disease spread.
abstract: BACKGROUND
  Assessments of infectious disease spread in hospitals seldom account for interfacility patient sharing. This is particularly important for pathogens with prolonged incubation periods or carrier states.
  
  
  METHODS
  We quantified patient sharing among all 32 hospitals in Orange County (OC), California, using hospital discharge data. Same-day transfers between hospitals were considered "direct" transfers, and events in which patients were shared between hospitals after an intervening stay at home or elsewhere were considered "indirect" patient-sharing events. We assessed the frequency of readmissions to another OC hospital within various time points from discharge and examined interhospital sharing of patients with Clostridium difficile infection.
  
  
  RESULTS
  In 2005, OC hospitals had 319,918 admissions. Twenty-nine percent of patients were admitted at least twice, with a median interval between discharge and readmission of 53 days. Of the patients with 2 or more admissions, 75% were admitted to more than 1 hospital. Ninety-four percent of interhospital patient sharing occurred indirectly. When we used 10 shared patients as a measure of potential interhospital exposure, 6 (19%) of 32 hospitals "exposed" more than 50% of all OC hospitals within 6 months, and 17 (53%) exposed more than 50% within 12 months. Hospitals shared 1 or more patient with a median of 28 other hospitals. When we evaluated patients with C. difficile infection, 25% were readmitted within 12 weeks; 41% were readmitted to different hospitals, and less than 30% of these readmissions were direct transfers.
  
  
  CONCLUSIONS
  In a large metropolitan county, interhospital patient sharing was a potential avenue for transmission of infectious agents. Indirect sharing with an intervening stay at home or elsewhere composed the bulk of potential exposures and occurred unbeknownst to hospitals.
authors.affiliation: Division of Infectious Diseases and Health Policy Research Institute, University of California Irvine, Irvine, California, USA. susan.huang@uci.edu
authors.forename: Susan S
authors.initials: SS
authors.lastname: Huang
authors.forename: Taliser R
authors.initials: TR
authors.lastname: Avery
authors.forename: Yeohan
authors.initials: Y
authors.lastname: Song
authors.forename: Kristen R
authors.initials: KR
authors.lastname: Elkins
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Nguyen
authors.forename: Sandra K
authors.initials: SK
authors.lastname: Nutter
authors.forename: Alaka A
authors.initials: AA
authors.lastname: Nafday
authors.forename: Curtis J
authors.initials: CJ
authors.lastname: Condon
authors.forename: Michael T
authors.initials: MT
authors.lastname: Chang
authors.forename: David
authors.initials: D
authors.lastname: Chrest
authors.forename: John
authors.initials: J
authors.lastname: Boos
authors.forename: Georgiy
authors.initials: G
authors.lastname: Bobashev
authors.forename: William
authors.initials: W
authors.lastname: Wheaton
authors.forename: Steven A
authors.initials: SA
authors.lastname: Frank
authors.forename: Richard
authors.initials: R
authors.lastname: Platt
authors.forename: Marc
authors.initials: M
authors.lastname: Lipsitch
authors.forename: Robin M
authors.initials: RM
authors.lastname: Bush
authors.forename: Stephen
authors.initials: S
authors.lastname: Eubank
authors.forename: Donald S
authors.initials: DS
authors.lastname: Burke
authors.forename: Bruce Y
authors.initials: BY
authors.lastname: Lee
mesh_terms: D000368:Aged; D002140:California / Q000453:epidemiology; D002648:Child; D015331:Cohort Studies; D003428:Cross Infection / Q000453:epidemiology* / Q000635:transmission*; D005260:Female; D006801:Humans; D008297:Male; D008485:Medical Audit; D008875:Middle Aged; D010351:Patient Discharge; D010360:Patient Transfer; D012189:Retrospective Studies