MRSA Screening: Culture or Molecular?

Vol. 19 • Issue 20 • Page 42


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.1 The disease has serious implications for patients, physicians and hospitals, including potentially substantial economic consequences.

Patients colonized with MRSA serve as a reservoir for spreading infection within the healthcare environment, primarily through the hands of healthcare workers.2 Nosocomial infections caused by MRSA are associated with significant adverse outcomes and increased healthcare costs such as prolonged hospital stays, additional procedures and litigation.

The Case for Active Screening

The use of screening cultures to identify MRSA-colonized patients so that infection control measures can be implemented and prevent transmission to other patients is well established.3,4 In addition, the Society for Healthcare Epidemiology of America recently issued guidelines that emphasize the importance of identifying reservoirs of nosocomial transmission by the use of active screening methods.3,5

Currently, only about one out of every three hospitals has an active MRSA screening program. Yet, while results vary according to the hospital setting, several recent studies provide evidence for the clinical and economic benefits of identifying MRSA carriers through active surveillance. For example, studies indicate that:

• identifying infected cases alone misses 85% of carriers;6

• carriers are 20% more likely to infect themselves;7 and

• carriers are 16 times more likely to transmit MRSA to others for each day that they spend outside contact isolation.8

Of course, surveillance alone is not the total solution; a comprehensive infection prevention and control program is needed. Effective programs include multiple components designed to help eliminate MRSA from the hospital environment, prevent its transmission among patients and staff, and reduce the risk of systemic infection in colonized patients. 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. Most testing to identify MRSA colonization is conducted in clinical microbiology laboratories using plate-based culture methods, including newer Chromagar methods. This can be done with or without prior broth enrichment, which can enhance test sensitivity but extends incubation times. The significance of the increased sensitivity is not clear.3,9 Direct plating onto solid media is the most commonly used approach.

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

For patients who are colonized with bacteria (carriers), hospital infection control procedures involve isolation of patients and may include preventive decontamination. When an outbreak of MRSA is under investigation, procedures may include screening of healthcare workers, family members and close contacts to identify the source of the infection and help devise a plan to contain these infections.

Pros, Cons of Culture

Traditional culture-based MRSA screening techniques are relatively inexpensive and simple to use and offer acceptable sensitivity, especially if samples are collected from several body sites. The major drawback is the turnaround time for results. Often labor-intensive and time-consuming, culture methods typically require 24-48 hours after sample collection to exclude MRSA and sometimes an additional one to two days to confirm positives.3,9 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.2,3,9-11 Obtaining a faster negative result would potentially allow more effective use of hospital isolation resources; 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.12,13 The rapid identification or exclusion of MRSA colonization is increasingly being seen as essential for the effective control of MRSA and other antibiotic-resistant organisms in the hospital environment.

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.14-16

Pros, 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 versus 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. One recent study, for example, demonstrated that a PCR-based screening method can significantly reduce the risk of acquiring MRSA in the intensive care unit compared with culture-based methods.17 In another study, 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.18

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 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.19

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. 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.17

Of course, 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. 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.

Dr. Osiecki is manager, Medical and Scientific Affairs, Roche Diagnostics Corp., Indianapolis.


1. Centers for Disease Control and Prevention reports, 2007 and 2009.

2. Harbarth S, Masuet-Aumatell C, Schrenzel J et al. Evaluation of rapid screening and pre-emptive contact isolation for detecting and controlling methicillin-resistant Staphylococcus aureus in critical care: an interventional cohort study. Critical Care. 2006,10:R25.

3. Johnson G, Millar MR, Matthews S et al. Evaluation of BacLite Rapid MRSA, a rapid culture based screening test for the detection of ciprofloxacin and methicillin resistant S. aureus (MRSA) from screening swabs. BMC Microbiol. 2006;6:83.

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14. Creamer E, Dorrian S, Dolan A et al. When are the hands of healthcare workers positive for methicillin-resistant Staphylococcus aureus? J Hosp Infect. 2010;75(2):107-11.

15. Peterson, LR, Liesenfeld, O, Woods CW et al. Multicenter evaluation of the LightCycler methicillin-resistant staphylococcus aureus (MRSA) advanced test as a rapid method for detection of MRSA in nasal surveillance swabs. J Clin Microbiol. 2010;48(5):1661-1666.

16. van Hal SJ, Stark D, Lockwood B, Marriott D and Harkness J. Methicillin-resistant Staphylococcus aureus (MRSA) detection: comparison of two molecular methods (IDI-MRSA PCR assay and GenoType MRSA Direct PCR assay) with three selective MRSA agars (MRSA ID, MRSASelect, and CHROMagar MRSA) for use with infection-control swabs. J. Clin. Microbiol. 2007;45:2486-2490.

17. Cunningham R, Jenks P, Northwood J, Wallis M, Ferguson S, Hunt S. Effect on MRSA transmission of rapid PCR testing of patients admitted to critical care. J Hosp Infect. 2007;65:24-8.

18. Keshtgar MR, Khalili A, Coen PG et al. Impact of rapid molecular screening for methicillin-resistant Staphylococcus aureus in surgical wards. Br J Surg. 2008;95:381-6

19. Hardy K, Price C, Szczepura A et al. Reduction in the rate of methicillin-resistant Staphylococcus aureus acquisition in surgical wards by rapid screening for colonization: a prospective, cross-over study. Clin. Microbiol. Infect. 2010;16(4):333-9.