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.