Making a case for MRSA screening

By John Osiecki, Ph.D., Manager, Medical and Scientific Affairs Roche Diagnostics Corporation

The incidence of hospital-associated methicillin-resistant Staphylococcus aureus (MRSA) is still on the rise around the globe, and the Centers for Disease Control and Prevention estimates that in 2005, 20% of patients infected with MRSA died from their infection. The disease has serious implications for patients, physicians and hospitals: Nosocomial infections caused by MRSA are associated with significant adverse outcomes and increased healthcare costs, such as prolonged hospital stays, additional procedures and litigation.

“Some studies suggest that the benefits of molecular's faster turnaround time may more than offset the higher cost when viewed in the context of a facility's overall infection control budget.”
-Roche Medical and Scientific Affairs Manager John Osiecki

The argument for active surveillance

Currently, only about one out of every three hospitals has an active screening program to identify MRSA-colonized patients. Yet several recent studies provide evidence for the clinical and economic benefits of active MRSA surveillance: (1) identifying infected cases alone misses 85% of carriers; (2) carriers are 20% more likely to infect themselves; and (3) carriers are 16 times more likely to transmit MRSA to others for each day that they spend outside contact isolation.

Of course, surveillance alone is not the total solution; a comprehensive infection prevention and control program is needed. But the vast majority of guidelines available today support active surveillance as a critical component of comprehensive infection control measures - and an important step in protecting your facility from potential liability.

Organizations that want to take that step face another question:  what kind of active surveillance?

Plate-based culture screening

Determining the best type of active MRSA surveillance for your facility requires a balanced assessment of your relative needs for turnaround time, performance (sensitivity and specificity), ease of use, and cost. Currently, most testing to identify MRSA colonization is conducted in clinical microbiology laboratories using plate-based culture methods, including newer Chromagar methods. Direct plating onto solid media is the most commonly used approach.

In culture screening tests, a specimen is collected by nasal swab and is transferred to the surface of a special nutrient medium, incubated, and then examined for the growth of characteristic MRSA colonies. The same procedure can also be used with a swab collected from a wound site or skin lesion of a person who has been previously treated for an MRSA infection. The test, which can take from 24 to 72 hours, confirms the presence of the resistant bacteria and allows organisms to be further characterized.

The pros and cons of culture

Traditional culture-based MRSA screening techniques have proven to be relatively inexpensive and simple to use, and to offer acceptable sensitivity, especially if samples are collected from several body sites. The major drawback is the turnaround time for results. Culture methods can be labor-intensive and time-consuming, typically requiring 24 to 48 hours after sample collection to exclude MRSA and sometimes an additional one to two days to confirm positives. During this waiting period, a hospital may apply infection control measures unnecessarily. Or, if it doesn't apply them, the hospital may be allowing unidentified MRSA carriers to remain a hidden reservoir for cross-infection. Obtaining a faster negative result would potentially allow more effective use of hospital isolation resources, and a faster positive result would help facilities reduce the spread of the infection and MRSA infection rates.

PCR-based molecular screening

In the past few years, a variety of DNA-based tests have been developed to detect MRSA carriers more quickly. Most of these molecular methods, which are also used on nasal swab specimens, target the integration site of the SCCmec cassette into the S. aureus chromosome. Molecular tests for MRSA screening have the potential to detect nasal or wound colonization within hours, in contrast to the days required by culture. Also, several studies suggest that polymerase chain reaction (PCR)-based MRSA tests may offer greater sensitivity than culture-based tests. 

The pros and cons of molecular

Compared to culture-based methods, the most significant advantage molecular tests offer is faster turnaround time - on average, two hours for molecular vs. 24 or more for culture. On the other hand, molecular tests have a higher operator skill requirement and can cost significantly more - typically two to four times as much on a per-test basis.

But some studies suggest that the benefits of molecular's faster turnaround time may more than offset the higher cost when viewed in the context of a facility's overall infection control budget. In one recent study, for example, a 1,200-bed hospital in England reduced the rate of MRSA transmission in a critical care unit by 65% when it replaced culture-based universal admission screening with rapid PCR-based testing.

In theory, faster results allow a facility to allocate infection control resources more effectively. This can include more timely implementation of appropriate contact precaution measures for MRSA-positive patients and the elimination of unnecessary cost and personnel demands for pre-emptive isolation of MRSA-negative patients.

Finally, the improvement in sensitivity with PCR that has been noted in several studies could potentially reduce the transmission of MRSA. In one study, for example, patients tested with culture were 1.49 times more likely to acquire MRSA compared to those diagnosed with PCR.

The economic assessment

While controlling infection is a top priority for hospitals, so is controlling costs. The impact of adopting different screening methods varies according to the hospital setting and local MRSA prevalence, but several published studies support the economic viability of moving from culture to molecular MRSA screening methods. For example, at a University College Hospital facility in London, switching from culture-based to PCR-based screening resulted in a net savings of £545,486 for the management of bacteraemia and wound infections over the previous year.

One solution for active screening is the Roche LightCycler® MRSA Advanced Test, a qualitative in-vitro diagnostic test performed with nasal swab specimens from patients suspected of MRSA colonization. The molecular test delivers results within two hours and offers a simple, flexible and reliable way to incorporate MRSA surveillance into your hospital's infection control program.

Regardless of which method you choose, simply implementing an active MRSA screening program has the potential to make a significant impact on your infection control program and patient care costs. But recent evidence suggests that adopting a rapid PCR-based molecular screening program may enable a hospital to implement measures more quickly. When combined with systematic on-admission screening and pre-emptive isolation of newly admitted patients, the ability to act faster can pay off in improved infection control and a better bottom line.

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