Laboratory Medicine and Nosocomial Infections: Dealing with MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is the most widespread hospital acquired infection (HAI). The August 10, 2009, issue of Archives of Internal Medicine reported that MRSA appears to be relatively common among patients discharged from the hospital into home healthcare. About one-fifth of infected patients can transmit the organism to other people in their households.

According to Carol Smith, senior product manager at Thermo Scientific Remel Products (Lenexa, KS, USA), nasal MRSA colonization at admission or during hospitalization can increase the risk of MRSA infection. Once colonized with MRSA, 30%-50% of patients become infected. Noninfected carriers can transmit the infection to other patients in the hospital, preventing effective public health control.

"Active surveillance for MRSA colonization is highly recommended for control of nosocomial transmission," Carol Smith said. "Rapid, accurate, and cost-effective screening tests for MRSA colonization are needed in order to reduce the economic burden of this pathogen."

Remel's Spectra MRSA is a selective and differential medium for detection of nasal colonization of MRSA. The test is performed with anterior nares swab specimens from patients and health care workers to screen for MRSA colonization.

An opaque medium that uses a novel chromogen yields a denim blue color after 24 hours as a result of phosphatase activity. To allow the medium to differentiate MRSA accurately, it contains a combination of antibacterial compounds designed to inhibit the growth of a wide variety of competitor organisms. Also included are compounds that encourage the production of the MRSA pathogenicity marker, ensuring expression of the phosphatase enzyme and providing enhanced sensitivity and specificity. If after the 24-hour incubation no denim blue colonies are observed, the specimen is considered negative and plates can be discarded.

Carol Smith claimed that Spectra MRSA provides the highest positive predictive value compared to all other available methods, translating into lower false positives. "With this excellent performance, hospitals can be assured that they are correctly identifying patients colonized with MRSA," she said. "This allows appropriate patient isolation and contact precautions, and avoids the unnecessary financial burden of isolating or decolonizing a patient."

The easy-to-use procedure and simple-to-read result allows for efficient batching on any shift in the microbiology lab. Results are available within 24 hours, regardless of positive or negative results. Additionally, no confirmatory testing is required for positives, which reduces ancillary testing and technician time in the microbiology lab.

BD Diagnostics of Becton, Dickerson and Co. (Franklin Lakes, NJ, USA) offers an assay for direct detection of nasal colonization by MRSA. The GeneOhm MRSA assay is a qualitative in vitro diagnostic test performed on the SmartCycler instrument with a nasal swab specimen. It utilizes PCR for the amplification of MRSA DNA and fluorogenic target-specific hybridization probes for the detection of the amplified DNA. The GeneOhm MRSA assay is not intended to diagnose MRSA infections, nor to guide or monitor treatment for MRSA infections. Concomitant cultures are necessary to recover organisms for epidemiological typing or for further susceptibility testing.

Related Links:
Thermo Scientific Remel Products
Becton, Dickerson and Co.
August 10, 2009 isssue of Archives of Internal Medicine