MRSA Nasal Burden and Risk of MRSA Infection Open Access

Stenehjem, Edward Alan (2012)

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Introduction: The burden of nasal Staphylococcus aureus colonization has been linked to an increased risk of staphylococcal infection. The cycle threshold (Ct) of molecular diagnostic tests used to identify methicillin-resistant S. aureus (MRSA) nasal colonization is a quantitative value of burden and may have clinical implications.

Methods: A retrospective cohort study was used to assess the effect MRSA nasal colonization burden has on subsequent MRSA infection. Veterans were classified as non-carriers, low burden MRSA (Ct > 24) carriers or high burden MRSA carriers (Ct ≤ 24) based on admission nasal surveillance swabs. MRSA infections were identified prospectively. Chart reviews were performed to obtain important clinical information such as demographics, comorbidities, and presence of wounds or devices. A multivariate logistic regression model was used to assess the association of MRSA nasal colonization burden (negative, low, and high) and risk of subsequent MRSA infection.

Results: From October 1st, 2007, to February 1st, 2008, 205 patients were admitted to the Atlanta VA Medical Center with positive MRSA nasal colonization, 68.8% had a Ct > 24 and 31.2% had a Ct ≤ 24. A comparison group from the same time period, consisting of 141 patients with negative MRSA nasal colonization, was randomly selected. 6 MRSA infections occurred in patients with negative colonization (4.3%), 26 with low burden (18.5%), and 11 with high burden (17.2%). In multivariate analysis, MRSA nasal colonization was a risk factor for subsequent MRSA infection (p = 0.0081). Low burden (RR 3.62, 95% CI 1.47 - 8.93) and high burden (RR 2.71, 95% CI 0.95 - 7.72) were both associated with subsequent MRSA infection when compared to patients without nasal MRSA colonization. High burden of nasal MRSA was not a significant risk factor (RR 0.75, 95% CI 0.36 - 1.55) when compared to low nasal burden of MRSA.

Conclusions: Among Atlanta veterans, MRSA nasal colonization was a risk factor for subsequent MRSA infection. However, patients with a higher nasal burden of MRSA, as defined by the Ct from the Xpert MRSA assay, were not at an increased risk of MRSA infection when compared to patients with low nasal burden of MRSA.

Table of Contents

Introduction...1 - 2
Background...3 - 9

The Evolution of MRSA...3
Nasal Colonization and Risk of Infection...4
Active Surveillance and Molecular Methods of Detection and Quantification...7

Methods...10 - 16

Study Design...10
Study Setting...10
Inclusion / Exclusion Criteria...10
Colonization Data...11
MRSA Infection Data...12
Identification of Covariates...13
Sample Size Calculation...14
Statistical Analysis...14
IRB Approval and Funding Source...16

Results...17 - 20

Description of Cohort...17
Characterization of Colonization Strata...18
Multivariate Analysis...18

Discussion...21 - 26

Future Directions...25

References...27 - 30
Tables and Figures...31 - 35

Figure 1. Distribution of the cycle threshold (CT) of positive admission nasal MRSA screens (Xpert MRSA assay) among Atlanta veterans (n = 205)...31
Table 1. Baseline patient characteristics among MRSA colonized and non-colonized (N = 346)...32
Table 2. A comparison of subsequent MRSA infection types, death during follow-up, and readmission within 4 years stratified by colonization status among a cohort of Atlanta veterans...33
Table 3. Baseline patient characteristics among patients without MRSA colonization, low MRSA colonization burden, and high MRSA colonization burden (N = 346)...34
Table 4. Univariate analysis of potential risk factors for subsequent MRSA infection in veterans (n=346)...35
Table 5. Multivariate analysis of predictors of subsequent MRSA infection among veterans...36


Table 1. Testing for statistical significance of interaction terms between colonization status and selected covariates

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