Published 12/1/2009
Annie Hayashi

New method for detecting MRSA in trauma patients

New technique three times more effective than culture

“Methicillin-resistant Staphylococcus aureus (MRSA) screening protocols and control policies for elective surgical patients have decreased the MRSA infection rates up to fivefold nationally,” said John M. Fennessy, MD, at the Annual Meeting of the Orthopaedic Trauma Association.

Based on these positive results, Dr. Fennessy and his colleagues implemented a new MRSA screening technique called multiplex polymerase chain reaction to identify bacterial organisms and their antibiotic sensitivities. They also established a control protocol for trauma patients.

The investigators hypothesized that this new screening method for MRSA would identify the organisms and their antibiotic sensitivities more accurately and quickly than routine cultures.

“Screening trauma patients for MRSA on admission could provide documentation that infections were due to MRSA colonization prior to hospitalization,” said Dr. Fennessy.

Multiple MRSA-related factors examined
“We prospectively evaluated the prevalence of MRSA in our trauma population,” he explained. “We also looked at the acquisition and infection rates of MRSA throughout the patient’s hospital stay.

“We attempted to compare patients who were colonized on admission to those who were not colonized; we also wanted to conduct a retrospective comparison of those groups to a similar population before institution of the protocol. Finally, we wanted to evaluate the results of the multiplex polymerase chain reaction test versus the bacterial culture to determine which test provided a more accurate measure of MRSA colonization,” he said.

During October and November 2008, 332 patients were enrolled. Each patient had two nasal swabs. One swab was analyzed with the multiplex polymerase chain reaction test and the other swab by routine bacterial culture.

Contact isolation precautions were instituted but no intentional decolonization with chlorhexidine body washes or other such measures were taken. Treating physicians were advised of their patient’s colonization status so the physicians could adjust their empiric and prophylactic anti-biotic use.

New method is effective
At the time of admission, the multiplex polymerase chain reaction test detected MRSA in 19.9 percent of the nasal swabs. In contrast, routine bacterial culture identified MRSA in only 6 percent of the swabs (
Table 1).

“Of the patients who tested positive for MRSA at admission, 61 percent were still positive when the final repeat nasal screening was done at an average of 7.7 hospital days,” Dr. Fennessy explained.

According to Dr. Fennessy, researchers found a statistically significant reduction of clinically documented MRSA infections in trauma patients after implementation of the MRSA screening protocol.

The retrospective comparison of infection rates both before and after the protocol was instituted found a significant reduction in the number of MRSA infections—from seven total infections before the protocol to only one infection in the postprotocol period. This patient had MRSA when admitted to the hospital (Table 2).

The investigators also compared the multiplex polymerase chain reaction to the bacterial culture.

“We found a very high sensitivity (0.97) and specificity (0.84) and a negative predictive value (0.998),” Dr. Fennessy reported. “The multiplex polymerase chain reaction detected much higher numbers overall and only missed one case that the culture identified.”

The test is not only highly effective in recognizing MRSA; it can identify a host of other bacteria, including methicillin-resistant Staphylococcus epidermidis. It can also distinguish whether the MRSA is a community-acquired or hospital-acquired strain and whether it is resistant to erythromycin or vancomycin.

Lessons learned
“If we can identify a patient’s colonization status within the first 12 hours of hospital admission, we have a better chance of preventing cross-colonization among patients and preventing infections in the patients who are already colonized,” Dr. Fennessy said. “This could have significant implications in this new era of denied reimbursement for nosocomial infections.

“We saw a much higher MRSA colonization rate in our trial population than has been previously reported in a regular community setting.

“We also saw an increased detection of MRSA with the multiplex polymerase chain reaction test versus routine bacterial culture techniques,” he continued.

“By implementing this protocol, we reduced the MRSA infection rate in our trauma population. We attributed this to more appropriate antibiotic regimens. It was also probably due to better control in patient-to-patient transmission,” Dr. Fennessy concluded.

MRSA screening, surveillance, and control protocol in the trauma population

All of the authors report support from Diatherix Laboratories.

Annie Hayashi is the senior science writer for AAOS Now. She can be reached at hayashi@aaos.org