title: Systematic review of measurement and adjustment for colonization pressure in studies of methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and clostridium difficile acquisition.
abstract: OBJECTIVE
  Colonization pressure is an important infection control metric. The aim of this study was to describe the definition and measurement of and adjustment for colonization pressure in nosocomial-acquisition risk factor studies of methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), and Clostridium difficile.
  
  
  METHODS
  We performed a computerized search of studies of nosocomial MRSA, VRE, and C. difficile acquisition published before July 1, 2009, through MEDLINE. Studies were included if a study outcome was MRSA, VRE, or C. difficile acquisition; the authors identified risk factors associated with MRSA, VRE, or C. difficile acquisition; and the study measured colonization pressure.
  
  
  RESULTS
  The initial MEDLINE search yielded 505 articles. Sixty-six of these were identified as studies of nosocomial MRSA, VRE, or C. difficile acquisition; of these, 18 (27%) measured colonization pressure and were included in the final review. The definition of colonization pressure varied considerably between studies: the proportion of MRSA- or VRE-positive patients (5 studies), the proportion of MRSA- or VRE-positive patient-days (6 studies), or the total or mean number of MRSA-, VRE-, or C. difficile-positive patients or patient-days (7 studies) in the unit over periods of varying length. In 10 of 13 studies, colonization pressure was independently associated with MRSA, VRE, or C. difficile acquisition.
  
  
  CONCLUSIONS
  There is a need for a simple and consistent method to quantify colonization pressure in both research and routine clinical care to accurately assess the effect of colonization pressure on cross-transmission of antibiotic-resistant bacteria.
authors.affiliation: Department of Epidemiology and Public Health, University of Maryland School of Medicine, 685 West Baltimore Street, MSTF Room 360 Baltimore, Maryland 21201, USA. aajao@epi.umaryland.edu
authors.forename: Adebola O
authors.initials: AO
authors.lastname: Ajao
authors.forename: Anthony D
authors.initials: AD
authors.lastname: Harris
authors.forename: Mary-Claire
authors.initials: MC
authors.lastname: Roghmann
authors.forename: J Kristie
authors.initials: JK
authors.lastname: Johnson
authors.forename: Min
authors.initials: M
authors.lastname: Zhan
authors.forename: Jessina C
authors.initials: JC
authors.lastname: McGregor
authors.forename: Jon P
authors.initials: JP
authors.lastname: Furuno
mesh_terms: D016360:Clostridioides difficile; D003015:Clostridium Infections / Q000635:transmission; D015169:Colony Count, Microbial; D003428:Cross Infection / Q000453:epidemiology*; D016983:Enterococcus; D016908:Gram-Positive Bacterial Infections / Q000635:transmission*; D006801:Humans; D017053:Infection Control / Q000379:methods; D055624:Methicillin-Resistant Staphylococcus aureus; D018570:Risk Assessment; D012307:Risk Factors; D013203:Staphylococcal Infections / Q000635:transmission; D020713:Vancomycin Resistance