MRSA Risk Lingers from ICU Room's Prior Occupant

BOSTON, Oct. 10 -- National standards for ICU decontamination do not completely prevent transmission of antibiotic-resistant infections, according to a federally sponsored study.

Despite above-average cleaning protocols, the antibiotic-resistant infection transmission risk was 40% higher for patients in an ICU room previously occupied by someone with methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant enterococci (VRE), the investigators found.

Acquisition of an antibiotic-resistant infection was significantly higher among patients whose prior room occupant had a MRSA infection (3.9% versus 2.9%, adjusted odds ratio 1.4, P=0.04) or VRE positive infection (4.5% versus 2.8%, adjusted OR 1.4, P=0.02), reported. Susan S. Huang, M.D., M.P.H., of Brigham and Women's Hospital and Harvard Medical School, and colleagues, in the Oct. 9 Archives of Internal Medicine.

However, this increased risk accounted for only a small fraction of acquired antibiotic-resistant infections, which "suggests that levels of contamination do not pose a high risk for transmission or that current cleaning methods generally reduce contamination below levels required for transmission," the researchers wrote.

The population-attributable risk among exposed patients for either organism was 2% and accounted for 5.1% of all incident MRSA cases and 6.8% of all incident VRE cases. "Based on our findings, the prevention of one case of acquisition due to room contamination could require more intensive cleaning of 94 rooms vacated by MRSA carriers and of 59 rooms vacated by VRE carriers," they wrote.

Objects in a hospital room can be persistently contaminated, such as the beds, doorknobs, and blood pressure cuffs, even after end-of-stay cleaning. The study, though, found that the prior occupant may be a more direct transmission risk factor.

Exposure to a room where the prior occupant harbored either resistant infection significantly predicted transmission. This was so even after multivariate analysis controlling for clustering by ICU as well as variables such as age, sex, comorbidities, length of hospital stay before admission to the intensive care unit, prior occupant's hospital length of stay, and duration of room vacancy before occupancy.

The study included eight different adult ICUs at a tertiary care hospital and a total of 8,203 patients with 11,528 ICU room stays. Patients were swabbed for MRSA and VRE upon admission and weekly during their stay per protocol.

A little less than 10% of patients (809 patients, 1,377 room stays) were MRSA carriers on ICU admission while 658 patients (1,179 room stays) were VRE carriers on intensive care unit admission. The mean age was 61 years and 58% were male. All VRE and MRSA carriers occupied single rooms.

Fourteen percent of the patients stayed in a room where the prior occupant was MRSA positive while 13% stayed in a room where the previous occupant was infected with VRE.

The unadjusted risk of a new MRSA infection was significantly higher at 3.9% for patients in a room previously occupied by an MRSA carrier compared with 2.9% (crude OR 1.4, 95% confidence interval 1.0 to 1.9, P=0.03). The unadjusted risk of a new VRE infection was also significantly higher for patients staying in a room previously occupied by a VRE carrier (4.5% versus 2.8%, crude OR 1.6, 95% CI 1.2 to 2.2, P=.001).

Comorbidities were not significantly different between exposed and unexposed groups when accounting for clustering by ICU. There was also no difference in time to antibiotic-resistant infection acquisition between patients in a room where the previous occupant was infected or not infected, which likely reflects protocols on timing of surveillance cultures, the investigators noted.

The main analyses excluded patients who were known to be carriers or in whom infection was identified within two days of admission in keeping with CDC guidelines.

In an analysis that included cases detected within two days of admission to the ICU, there was no difference in transmission risk for those staying in a room where an MRSA-infected person had been. However, there was a significant difference for rooms where the previous occupant was a VRE carrier versus non-carrier (5.6% versus 4.1%, P=0.008).

"This excess risk occurred despite our hospital's room cleaning procedures at discharge, which exceed the Centers for Disease Control and Prevention and Healthcare Infection Control Practices Advisory Committee 2003 national guideline," Dr. Huang and colleagues wrote.

They acknowledged that the study may have underestimated the rate somewhat because surveillance cultures were only performed on admission and weekly. Also they did not perform environmental sampling to test the level of residual contamination under which transmission occurred.

The researchers noted that their findings may not be generalizable to other hospitals or non-intensive care unit settings because of differences in patient populations or end-of-stay room cleaning procedures.

Since their study "strongly" suggested a role for environmental contamination in transmission of MRSA and VRE in the hospital, the authors said that the small fraction infected in this manner "could account for a substantial number of transmission events if the prevalence of these organisms continues to rise."

The study was funded by the CDC and the NIH. One of the authors reported research support from GlaxoSmith Kline, Pfizer, Sanofi-Aventis, and TAP Pharmaceuticals.