Duration of colonization with methicillin-resistant Staphylococcus aureus in an acute care facility: a study to assess epidemiologic features.

Background: Patients with a history of methicillin-resistant Staphylococcus aureus (MRSA) colonization or infection are often presumed to remain colonized when they are readmitted to the hospital. This assumption underlies the hospital practice that flags MRSA-positive patients so that these patients can be placed in contact isolation at hospital admission and, when necessary, be given the appropriate empirical therapy and/or antibiotic prophylaxis.

Methods: To determine the duration of and factors associated with MRSA colonization among patients following discharge, we designed a cohort study of patients hospitalized between October 1, 2007, and July 31, 2009, at the Atlanta Veterans Affairs Medical Center, a 128-bed acute care facility. We defined 3 cohorts: cohort A; patients with both a MRSA infection during hospitalization and nasal colonization at discharge; cohort B; patients with a MRSA infection but no nasal colonization at discharge; and cohort C; patients only nasally colonized at discharge. We collected information on demographic characteristics, underlying conditions, infections, and antibiotic use. We cultured nasal swabs obtained from patients at home. We calculated hazard ratios (HR), comparing cohorts A, B, and C after controlling for other factors.

Results: We obtained 231 swabs (23 in cohort A, 34 in cohort B, and 174 in cohort C). We documented MRSA colonization in 92 (39.9%) of the 231 patients who returned swabs. The median duration of colonization was 33.3 months. Factors significantly associated with persistent MRSA colonization were (1) total duration of hospital stay from previous admissions prior to study entry and (2) a member of cohort A who had a longer duration of colonization compared with cohorts B and C (P < .001).

Conclusion: Our data suggest that higher initial inocula of bacteria may be an important determinant of persistent colonization with MRSA.