Colonization pressure is an important infection control metric that quantifies the burden of antibiotic-resistant bacteria in a hospital unit over a period of time. Colonization pressure has been shown to be an important risk factor for nosocomial acquisition of MRSA,13,17,21
and C. difficile
Previous studies have also assessed risk factors for acquisition of other antibiotic-resistant bacteria, such as extended-spectrum β
and Escherichia coli
, and have concluded that patient-to-patient transmission is likely an important contributor.22,23
However, these studies were not included in this review because of the limited data on colonization pressure and in an attempt to focus the manuscript. The aim of this study was to describe the definition and measurement of and adjustment for colonization pressure in nosocomial-acquisition risk factor studies of MRSA, VRE, and C. difficile
We systematically reviewed the colonization pressure literature for MRSA, VRE, and C. difficile
acquisition studies, and we found significant heterogeneity in the definition of and adjustment for colonization pressure. To summarize, colonization pressure was broadly defined as the proportion of antibiotic-resistant-bacteria-positive patients, the proportion of antibiotic-resistant bacteria–positive patient-days, or the total number of antibiotic-resistant bacteria–positive patients or patient-days in the unit or the mean number of antibiotic-resistant bacteria–positive patients in the unit daily, weekly, monthly, or for the duration of the study period. Positivity was determined using surveillance cultures, clinical cultures, or both. This review did not provide sufficient data to determine the most accurate definition of colonization pressure, but it is clear that there is a need for a simple and consistent but optimal definition of colonization pressure for use in both research and routine clinical care.24
The majority of the studies included in this review were performed in ICUs, where patients are screened more often to identify asymptomatic carriers of antibiotic-resistant bacteria. However, there are other healthcare settings, such as non-ICU hospital wards and long-term care facilities, where MRSA, VRE, and C difficile are endemic but surveillance for these bacteria is not routinely performed. In these settings, clinical cultures from symptomatic patients may be the only data available to quantify colonization pressure, and a definition of colonization pressure that quantifies only asymptomatic carriers could be problematic. The use of a definition that incorporates clinical-culture positivity or possibly prior history of colonization or infection may be more applicable and may still provide some useful data in these settings.
However, it is important to note that using clinical cultures from symptomatic patients to quantify colonization pressure may be prone to ascertainment bias. Clinical cultures are requested by the treating physician as clinically indicated; therefore, sicker patients and patient populations are more likely to have clinical cultures collected. This may lead to a subgroup of the patient population who are less sick and thus are less likely to have clinical cultures collected. This would likely result in an underestimation of colonization pressure.
For a definition of colonization pressure to be useful, it should be applicable in both research and routine clinical care. For example, a useful definition of colonization pressure should be applicable in a healthcare unit to routinely monitor colonization pressure, and when colonization pressure is found to be especially high (ie, above an indicated level), for example, during an outbreak, more intensive infection prevention efforts can be implemented.
In summary, the optimal definition of colonization pressure would quantify asymptomatic carriers present in the unit daily. Colonization pressure would thus be defined as the average daily proportion of patients colonized with the bacterial species under study during the period prior to acquisition or discharge from the unit. However, because of limited resources in routine clinical care, daily surveillance cultures are not often feasible, and this definition of colonization pressure may not be applicable in every healthcare setting. In the absence of daily surveillance culture data, the best available data, such as weekly surveillance data, clinical cultures, or possibly prior history of colonization or infection, may prove useful for quantifying colonization pressure. Computer simulation models may also be useful in estimating colonization pressure. Readers interested in the use of computer simulation models to study transmission of antibiotic-resistant bacteria may benefit from several available resources.25–27
In conclusion, further study is needed to determine a simple and consistent method to quantify colonization pressure in research and routine clinical care to accurately assess the effect of colonization pressure on cross-transmission of antibiotic-resistant bacteria.