Estimation of methicillin-resistant Staphylococcus aureus transmission by considering colonization pressure at the time of hospital admission

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Summary

Our objective was to evaluate the accuracy of a methicillin-resistant Staphylococcus aureus (MRSA) rate using the imported MRSA reservoir identified at the time of hospital admission. Two indicators were used: the number of imported MRSA patient-days/total number of patient-days [representing colonization pressure (CP) at the time of admission] and the incidence of hospital-acquired MRSA isolated from clinical samples expressed as density/100 patient-days for carriers identified at the time of admission [representing the incidence taking CP into account (ICP)]. The variations of these indicators were analysed and compared with two more common indicators: percentage of MRSA acquired in our hospital and the incidence of hospital-acquired MRSA isolated from clinical samples expressed as density/1000 patient-days within three four-month periods during 2002. Common indicators varied similarly, with marked decline during the third period; first-period CP was twice that of other periods (P<10−6) and the highest (>two-fold) ICP was seen in the summer (second) period (P<0.001) when the personnel/patient ratio was the lowest. Thus, comparison of different indicators within four-month periods underlines important differences between common and novel indicators. Despite several limitations, ICP should be helpful in the interpretation of MRSA surveillance data, particularly for estimating the extent of MRSA transmission.

Introduction

Hospital-acquired methicillin-resistant Staphylococcus aureus (MRSA) has been an increasing problem worldwide since the initial reports 35 years ago.1 Colonized and infected patients are the major institutional reservoir, and transient carriage on the hands of hospital personnel is the most common mechanism of patient-to-patient transmission.2

MRSA surveillance in hospitals is universally recommended.3, 4 To assess colonization and infection rates over time or to compare them with those of other hospitals, infection control personnel must use rates adjusted for the major risk factors.5, 6, 7 For MRSA, infection and/or colonization rates more accurately reflect the risk of acquisition when they are adjusted for the length of stay. Therefore, the most common indicator is the number of MRSA cases/1000 patient-days. To estimate MRSA transmission in a hospital or a ward, only cases of MRSA acquired in the setting are considered. However, this value does not take into account the duration of exposure to the imported MRSA reservoir (length of stay of patients infected or colonized at the time of admission). This has been identified as an independent risk factor of MRSA acquisition in a recent study conducted in a French intensive care unit (ICU).8

Our objective was to evaluate the usefulness of an indicator taking the imported MRSA reservoir into account.

Section snippets

Methods

Data were collected from January to December 2002, in a 600-bed public teaching hospital that provides a comprehensive range of departments including two ICUs, two surgical wards, a maternity unit, a psychiatric ward, a paediatric department, two internal medicine wards and a 120-bed long-term care facility for the elderly. Any patient admitted to an acute ward was included in the study (the long-term care facility was excluded). An MRSA carrier was defined as a patient from whom MRSA was

Results

During the study period, 14 202 patients were admitted to acute wards in our hospital, and 1128 (7.9%) of them were screened for MRSA. In 2002, no screening samples were taken from maternity, paediatric and psychiatry patients because none presented known risk factors for MRSA carriage at the time of admission. Samples were taken at admission from 17.0% (1128/6652) of the other patients. When assessed in three successive four-month periods, the percentage was lowest during the third period (

Discussion

CP is an indicator for the risk of MRSA transmission. Each additional MRSA carrier substantially increases the MRSA acquisition risk for other patients. In their study conducted in an ICU, Merrer et al. defined CP as the number of imported+hospital-acquired MRSA/patient-days in the week/total number of patient-days.8 We chose to consider only imported MRSA pateint-days because this parameter reflects the true unavoidable MRSA pressure.

In our study, the incidence of acquired MRSA/100

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