This hard data confirms some suspicions and helps those working in the infection control field to better hone our screening recommendations.
Dr. Lance Peterson, Northwestern Healthcare

Findings from a new study provide doctors with a more complete epidemiologic picture of high-risk groups most likely to carry deadly organisms. Presumed carriers of superbugs, (multidrug-resistant bacteria) responsible for causing a growing number of U.S. hospital-acquired infections (HAIs), were identified through a universal surveillance program encompassing more than 90 percent of admitted patients over 12 months.

Conducted by Evanston Northwestern Healthcare in Evanston, Ill., the study, Prevalence of Colonization with Methicillin-Resistant Staphylococcus aureus (MRSA) Among 24,045 Patients Screened at an Acute Care Hospital, sampled patients at least once from Aug. 1, 2005 until July 31, 2006. Results show how healthcare organizations can better target patients colonized with MRSA. Significant predictors of MRSA colonization include: being male; older age; living in a nursing home or assisted living facility, and hospital admission within the prior two years.

Rapid results of new molecular diagnostics enabled the universal surveillance of over 20,000 patients screened for MRSA upon admittance to the hospital, says Dr. Lance Peterson, director of microbiology and infectious disease research at Evanston Northwestern Healthcare and one of the authors of the study. This hard data confirms some suspicions and helps those working in the infection control field to better hone our screening recommendations and definition of those patients at highest-risk of carrying MRSA.

According to the Centers for Disease Control and Prevention (CDC), HAIs cause 90,000 deaths per year in the United States and can cause an estimated 8 million excess hospital days and more than $5 billion in excess healthcare costs, with MRSA being the leading microbe of all HAIs. Lurking on the body of one in every 20 patients entering the hospital is one of the most troublesome bacteria, MRSA.1 Past research has shown that of those patients who have it on their bodies at admission, up to 1 in 5 develops an infection.2 For patients who acquire MRSA while in the hospital, up to 25 percent develop a potentially lethal infection3 and in the United States alone MRSA accounts for nearly 60 percent of the Staphylococcus aureus infections in intensive care units.4

In the Evanston Northwestern Healthcare study, 1,309 patients (5.4 percent) were found to carry MRSA. Most importantly, there was a trend of demographic variables identified that might help healthcare institutions to develop a profile of those at highest risk of MRSA colonization.

The data showed that colonization was most prevalent at extremes of age, reaching 19 percent among those aged 90 and older (1,443 patients) and that at least 21 percent of all MRSA-positive patients listed a multi-resident facility as their home address. Also of statistical significance was the fact that colonization was more prevalent in patients with an admission in the prior two years. Evanston Northwestern Healthcare used the BD GeneOhm MRSA assay to conduct all of this research and for its ongoing universal screening program.

Universal surveillance is the most thorough way to identify patients who are carriers of MRSA upon admission, says Dr. Ari Robicsek, the associate epidemiologist at Evanston Northwestern Healthcare who presented this work. But when it isnt feasible, this study suggests that risk factors can be identified by an organization to direct targeted active surveillance.

Additional data from Evanston Northwestern Healthcare indicate that universal surveillance upon admission is the ideal method of identifying and isolating all carriers of MRSA. However targeted surveillance of high-risk admissions, though less robust, can also be a cost-effective alternative to identify carriers. A second new study released by Evanston Northwestern Healthcare finds that the majority of patients admitted harboring MRSA are not typically identified via passive or targeted active surveillance. Passive surveillance detects patients with clinical cultures only and is the method most commonly used for identifying MRSA in hospital patients in the U.S. Targeted active surveillance is used when particular areas of the hospital are labeled as high risk and all patients in those wards are screened.

This study characterized the decay pattern of MRSA colonization in a real-life population of retested patients in a universal surveillance and decolonization program. Results showed that recolonization can occur, demonstrating the need for re-screening to detect re-colonization of patients upon each admission to the hospital.

Both of these studies show that active MRSA surveillance at patient admittance is critical to reduce the ongoing spread of MRSA within U.S. hospitals, says Peterson. Universal surveillance may not be immediately feasible for many organizations, but in the interim targeted surveillance is an acceptable alternative that can identify MRSA carriers that could spread the bacteria, particularly in hospitals with a high percentage of total bed capacity being ICU beds.

Evanston Northwestern Healthcare has employed universal surveillance since August 2005, and in 24,045 admissions tested, universal surveillance correctly detected MRSA on 1,309 incoming patients, or in 5.4 percent of admissions, says Robicsek. ICU-based targeted active surveillance correctly identified only 478 MRSA carriers, or 37 percent of those detected by universal surveillance. With passive surveillance, currently what the majority of U.S. hospitals practice, only 247 patients would have been identified as MRSA carriers, missing fully 80 percent of the MRSA reservoir for spread.

Results of the second study of their intervention data reinforce the importance of repeated MRSA testing, even when patients have been MRSA-negative in the past. In a cohort of 613 MRSA patients who were tested multiple times, approximately 50 percent were negative on subsequent tests. Mupirocin therapy for decolonization was significantly associated with a loss of MRSA on subsequent tests. 


1. Hidron, Alice l et al, CID 2005; 41: 167-9.

2. Davis, K.A. et al, CID 2004; 39: 776-82.

3. Davis, K.A. et al, CID 2004; 39: 776-82.

4. The Lancet Infectious Diseases, October 2005, page 653; National Nosocomial Infections Surveillance (NNIS) System Report, data summary from January 1992 through June 2004, issued October 2004. Am J Infect Control2004; 32: 470-85 (quoted in The Lancet Infectious Diseases, October 2005, page 653.