504 Systematic Review of Measurement and Adjustment for Colonization Pressure in Methicillin-resistant Staphylococcus aureus (MRSA) and Vancomycin-resistant enterococci (VRE)

Saturday, March 20, 2010
Grand Hall (Hyatt Regency Atlanta)
Adebola Ajao, MPH , University of Maryland, Baltimore, MD
Anthony Harris, MD, MPH , University of Maryland School of Medicine, Baltimore, MD
Mary-Claire Roghmann, MD, MS , University of Maryland and VA Maryland Health Care System, Baltimore, MD
J. Kristie Johnson, PhD , University of Maryland, Baltimore, MD
Min Zhan, PhD , University of Maryland, Baltimore, MD
Jessina C. McGregor, PhD , Oregon State University College of Pharmacy, Portland, OR
Jon Furuno, PhD , University of Maryland, Baltimore, MD
Background: Colonization pressure (CP), the proportion of patients colonized or infected in an intensive care unit or ward, has been identified as an important risk factor for acquisition of MRSA and VRE among hospitalized patients. Most MRSA and VRE acquisition studies do not measure CP and studies that have measured CP have used different methods. 

Objective: This study aimed to determine the proportion of MRSA and VRE acquisition studies that measured CP and describe the heterogeneity that exists in the measurement of CP in these studies

Methods: A systematic literature search of MRSA and VRE acquisition studies was performed in PubMed using the keywords “MRSA acquisition”, “VRE acquisition” and “colonization pressure.” The search was restricted to articles published before July 1, 2009. Studies were excluded if they were not conducted in humans, were not written in English language, were review articles, were not MRSA or VRE acquisition studies, or did not assess risk factors for MRSA or VRE acquisition.

Results: The initial search yielded 378 articles for MRSA and 72 articles for VRE. Following initial review and application of the inclusion criteria, 31 MRSA and 17 VRE articles were included.  Eight (26%) of the MRSA studies and 7 (41%) of the VRE studies measured CP. Thirteen (85%) of the 15 studies with CP measured were prospective cohort studies, while 1 was a retrospective cohort study and 1 was a case control study. The measurement of CP varied between studies. Five studies measured CP as the average daily proportion of MRSA/VRE positive patients on the unit while 6 studies used the proportion of MRSA/VRE patient days in a study period (week or month). Four studies used only the number or proportion of MRSA/VRE positive patients on admission or patient days from admission. The outcomes used to calculate CP varied between studies also, 5 studies used colonization only while 10 studies used both colonization and infection. Few studies described the assumptions they made about inclusion of patients with prior history of colonization or infection. Among 11 studies that performed multivariate analysis, 7 (64%) identified CP as independently associated with MRSA/VRE acquisition.

Conclusions: There is need for a simple method to quantify CP in MRSA and VRE acquisition studies. Consideration and explicit decisions must be made regarding inclusion of patients with prior history of colonization or infection and patients positive on admission. Assumptions regarding the length of positivity of patients positive on admission and timing of acquisition for nosocomial cases should be clearly stated. While heterogeneity in the methods for measurement of CP in existing studies have made it difficult to quantify it’s true effect, CP is likely an important factor associated with MRSA and VRE acquisition.