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Colonization pressure may affect nosocomial MRSA transmission

Monitoring colonization pressure can decrease MRSA transmission by emphasizing infection control measures.

Issue: August 2007

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Monitoring colonization pressure had a significant effect on decreasing transmission of methicillin-resistant Staphylococcus aureus in general medicine units, according to recent study data.

Victoria R. Williams, an infection prevention and control coordinator at Sunnybrook Health Sciences Centre in Toronto, presented the study results at the 34th Annual Education Conference & International Meeting of the Association for Professionals in Infection Control, held recently in San Jose, Calif.

Sunnybrook Health Sciences Centre is a 1,200-bed acute and long-term care facility. The study began after an MRSA outbreak in June 2005. Researchers sought to determine the role of colonization pressure in nosocomial MRSA transmission and to find thresholds of colonization pressure for implementing additional infection control practice.

Colonization pressure

All patients admitted to a 36-bed general medicine unit were screened on admission for MRSA from the nares, perianal area and exit site of indwelling devices, as well as from any open wounds or incisions. The study endpoint was monthly colonization pressure and the presence or absence of MRSA transmission during the next month.

Nosocomial MRSA transmission was considered for colonized patients who had been in the unit for more than 48 hours or who had been admitted to the unit within the past year without contact with another health care facility.

Monthly MRSA nosocomial incidence rates were calculated by finding the number of new cases per 1,000 patient days, and colonization pressure was calculated by multiplying the number of MRSA–positive patient days per total patient days by 100. The rates were calculated for January 2005 to December 2006. Patients were considered to be MRSA carriers from the date of positive sampling until discharge or death.

Nosocomial rates decrease

During the study period, 21 cases of nosocomially acquired MRSA were detected. MRSA outbreaks occurred on two occasions. In both outbreaks, colonization pressure for the prior month was greater than the 6.7% median.

Transmission occurred in eight separate months. Median colonization pressure was 6.7% for the two-year period. The relative risk for MRSA acquisition increased as colonization pressure increased above the median (RR=7.64; 95% CI, 1.11-52.55). During months in which colonization pressure was less than or equal to the median in preceding months, overall incidence of nosocomial MRSA was 0.078 per 1,000 patient days vs. 1.82 per 1,000 patient days for months when preceding colonization pressure was greater than the median. – by Kirsten H. Ellis

For more information:

• Williams VR, Callery S, Vearncombe M, Simor AE. The role of colonization pressure in nosocomial transmission of methicillin-resistant Staphylococcus aureus. Presented at: The 34th Annual Education Conference & International Meeting of the Association for Professionals in Infection Control; June 24-27, 2007; San Jose, Calif.