Development of a surveillance system for methicillin-resistant Staphylococcus aureus in German hospitals.

OBJECTIVE: To determine the appropriate method to calculate the rate of methicillin-resistant Staphylococcus aureus (MRSA) infection and colonization (hereafter, MRSA rates) for interhospital comparisons, such that the large number of patients who are already MRSA positive on admission is taken into account. DESIGN: A prospective, multicenter, hospital-based surveillance of MRSA-positive case patients from January through December 2004. SETTING: Data from 31 hospitals participating in the German national nosocomial infections surveillance system (KISS) were recorded during routine surveillance by the infection control team at each hospital. RESULTS: Data for 4,215 MRSA-positive case patients were evaluated. From this data, the following values were calculated. The median incidence density was 0.71 MRSA-positive case patients per 1,000 patient-days, and the median nosocomial incidence density was 0.27 patients with nosocomial MRSA infection or colonization per 1,000 patient-days (95% CI, 0.18-0.34). The median average daily MRSA burden was 1.13 MRSA patient-days per 100 patient-days (95% CI, 0.86-1.51), with the average daily MRSA burden defined as the total number of MRSA patient-days divided by the total number of patient-days times 100. The median MRSA-days-associated nosocomial MRSA infection and colonization rate, which describes the MRSA infection risk for other patients in hospitals housing large numbers of MRSA-positive patients and/or many patients who were MRSA positive on admission, was 23.1 cases of nosocomial MRSA infection and colonization per 1,000 MRSA patient-days (95% CI, 17.4-28.6). The values were also calculated for various MRSA screening levels. CONCLUSIONS: The MRSA-days-associated nosocomial MRSA rate allows investigators to assess the extent of MRSA colonization and infection at each hospital, taking into account cases that have been imported from other hospitals, as well as from the community. This information provides an appropriate incentive for hospitals to introduce further infection control measures.