A Prospective Study of the Outcome of Incident Nasal Colonization with MRSA

Background: Most studies of the risk of infection among patients colonized with S. aureus  examine colonization detected at the time of hospital admission. There are few studies of the outcome of incident colonization during hospitalization. In a prospective study 25% of inpatients with incident methicillin-resistant S. aureus  (MRSA) colonization developed invasive infection during their hospital stay. In 2007 the DVAMC-Denver began screening all medical and surgical admissions for MRSA colonization at regular intervals, using polymerase chain reaction (PCR) for the majority of screening.

Objective: To describe the outcome of hospital inpatients who convert from a negative to a positive nasal screening test for MRSA in an active screening and isolation program.

Methods: All patients admitted to DVAMC-Denver were screened for colonization with MRSA. Screening was repeated each time the patient was moved between nursing units, remained hospitalized on a nursing unit for seven days or longer, and at discharge. Cepheid polymerase chain reaction (PCR) was used for admission, transfer, and weekly screening; Chromagar culture was used for discharge screening. A database was maintained for each patient, including each screening PCR and culture. If a culture obtained for clinical reasons was positive for MRSA, it was entered into the database. We reviewed the records of each patient who converted from a negative MRSA screen to a positive MRSA screen during hospitalization. GraphPad InStat v3.0 was used to perform chi square analysis of categorical data.

Results: Between Oct 1, 2007 and Sep 30, 2009 there were 11,751 patient days in intensive care units (ICU), and 39,118 patient days on other acute care units. There were 27 conversions in ICU, for an incidence density of 0.23/1000 patient days; and 58 conversions on non-icu nursing units, for an incidence density of 0.15/1000 patient days (relative risk for nares conversion in icu 1.375, 95% CI 1.006-1.879). 38 conversions were detected by Chromagar; 47 conversions  were detected by PCR. There were five cases that subsequently had a positive culture of a clinical specimen for MRSA. Review of these cases revealed three cases of invasive MRSA infection, one colonization of lung and one colonization of devitalized soft tissue. All of the MRSA infections were pneumonia; two of the three nares conversions that led to MRSA pneumonia occurred in patients who were in the ICU receiving mechanical ventilation. The incidence of MRSA infection among nares converters in ICU was 2/27 (7.4%); for acute care converters the incidence of MRSA infection was 1/58 (1.7%), P=0.25.

Conclusions: The burden of MRSA disease following conversion of nares screening tests falls disproportionately on patients in ICU.

The risk for MRSA conversion leading to invasive MRSA disease in this population that was screened predominantly with PCR was lower than previously reported studies conducted using screening cultures.