Identification of risk times for dissemination of methicillin-resistant Staphylococcus aureus in an intensive care unit

BACKGROUND AND OBJECTIVE: The dissemination of methicillin-resistant Staphylococcus aureus (MRSA) in patients admitted to an Intensive Care Unit (ICU) depends, among other reasons, on the time interval between obtention of the first positive sample and the establishment of measures for contact isolation. The objective of this study was to identify the risk intervals for the spread of MRSA in ICU patients and to assess the relationship between these periods and the development of new cases of MRSA acquired in the ICU. MATERIAL AND METHOD: Observational and prospective study, which was carried out in a 18-bed polyvalent ICU during a 49-month period (October 1998-October 2002). The «exposure risk period» was defined as the time elapsed between obtention of the first positive sample and contact isolation of the index case, and the «window period» as the time elapsed between recovery of the last negative sample to the first positive sample. Infection sources of MRSA were classified into community-acquired and hospital-acquired (nosocomial extra-ICU and nosocomial intra-ICU infections). RESULTS: MRSA was isolated in 69 (2.73%) of 2,531 patients admitted to the ICU during the study period and in all patients measures of contact isolation were indicated. Community-acquired MRSA was diagnosed in 9 (13%) cases, nosocomial intra-ICU in 29 (42%), and nosocomial extra-ICU in 31 (44.9%). The mean duration of the exposure risk period was 3.1 (SD 2.2) (median 3, range 0-9) days and the window period 2.9 (SD 4.6) (median 1, range 0-28) days. In 18 of the 29 cases of intra-ICU-acquired MRSA (62.1%; 95% CI, 42.3-79.3), the infection was acquired within the exposure risk and/or window periods of other patients with MRSA. CONCLUSIONS: The exposure risk periods and the window periods showed a strong relationship between detection of new cases of intra-ICU colonization and/or infection by MRSA.