Patient-associated Risk Factors for Acquisition of Methicillin Resistant Staphylococcus aureus (MRSA) in a Tertiary Hospital Setting

Background: MRSA is a dominant hospital pathogen because of increasing incidence, cost of treatment, antibiotic resistance, limited antimicrobial armamentarium, and associated increased mortality.  Determining risk factors for MRSA acquisition in hospital settings helps define targets for infection control.  UPMC Presbyterian University Hospital (PUH) is a 766-bed tertiary care facility with 160 ICU beds and a 2.29 case mix index. It employs an MRSA prevention bundle involving MRSA screening and contact isolation for colonized/infected patients.

Objective: To determine patient-associated risk factors (RFs) for MRSA acquisition at PUH. It was hypothesized that RFs for MRSA acquisition could be identified and used to enhance or tailor infection control strategies. 

Methods: A retrospective matched case-control study was conducted. Case subjects who acquired MRSA during hospitalization and 2 matched controls were selected among patients admitted to high risk units where MRSA screening was routinely performed from 2001 to 2008. The odds of exposure to potential patient-associated RFs for MRSA acquisition were compared between cases and controls, using matched univariate conditional logistic regression. A single multivariate conditional logistic regression model was generated.  

Results: 475 cases and 880 matched controls were identified. The final model included 15 independently significant variables. Seven variables were positively associated with MRSA acquisition and 8 variables were protective (Table).

Conclusions:

·         In general, RFs for MRSA acquisition were not modifiable during the current stay.

o        3/7 RFs were conditions present on admission.

o        Admission to a high risk unit in itself is not modifiable.

o        Pneumonia could potentially be reduced by maintaining high compliance with pneumococcal vaccine but this would need to occur prior to admission.

o        Ventricular shunting was a factor but only found in 33(7%) of cases. The need for this procedure is not modifiable.

·         Due to study design, residual confounding may have occurred, such as in the protective effect of transplant. Other protective variables may be due to the underlying health status of patients undergoing these procedures and anti-S. aureus mechanisms of the drugs, but these would not be utilized for controlling MRSA acquisition.

·         Lack of association with many specific common bedside or interventional procedures performed in high risk areas argues for intensified environmental control and strict sterile technique for all procedures performed on patients.

·         Increased risk for MRSA acquisition appears to be more associated with underlying diagnoses and exposure to overall conditions in high risk units.  

·         The MRSA prevention bundle may have helped safeguard patients undergoing procedures, as this program identifies colonized patients and requires implementation of barrier precautions.