Rapid diagnostics speed identification of MRSA

April 1, 2006

Geneva ? Interest in rapid diagnostics for methicillin-resistant Staphylococcus aureus (MRSA) is driven by the troubling increase in incidence and a growing understanding that hospital-acquired and community-acquired strains can have different patterns of resistance and susceptibility to drugs.

The Geneva University Hospitals, Switzerland, used nasal swabs and an experimental polymerase chain reactions (PCR) assay to screen all medical and surgical ICU patients for MRSA upon entry as a tool to more effectively guide a program of identification, quarantine and decontamination, and to reduce initial transmission of MRSA.

The leader of the program, Stephan Harbarth, M.D., tells Dermatology Times the hospitals were able to reduce the median time from ICU admission to notification of tests result from 87 hours to 21 hours in the surgical ICU and from 106 to 23 hours in the medical ICU. More importantly, they identified 71 carriers of MRSA, 55 of whom had previously not been identified as being at risk for being carriers of the infection and were identified only because of the screening program.

Status of U.S. testing

Two years ago, the Food and Drug Administration (FDA) approved the first and, so far, only commercially available PCR assay for diagnosing MRSA in the United States. It can yield results in as little time as one hour, compared to the two to four days required for standard cultures.

David H. Persing, M.D., Ph.D., is executive vice president and chief medical and technology officer for Cepheid, the company that manufactures the diagnostic equipment. He says the existing technology has its limitations. It is approved for nasal swabs - being nasal colony-positive for MRSA does not necessarily mean that the soft tissue wound also is MRSA positive - but, more importantly, blood or abscess samples can throw off the results.

The test is being used by a growing number of hospitals for internal investigations of MRSA outbreaks and by "a small number of hospitals that are in border-patrol mode" in an effort to maintain existing low levels of MRSA infection at their facilities.

Dr. Persing also notes an interesting development: Some hospitals are starting to market themselves as "clean." It is an outgrowth of the CMS Hospital Quality Initiative that requires hospitals to report their rates of complications for various procedures. Dr. Persing believes that control of infection will become an increasingly important factor as hospitals try to differentiate themselves to potential customers.

Next generation

What Dr. Persing is really excited about is the next-generation diagnostics that have "a lot more PCR horsepower." The assay identifies the presence of staph; the highly mobile SSCmec gene element that confers drug resistance; orfX, which is the integration point for that gene; and whether the sample contains mixtures of staph that are sensitive and not sensitive to specific drugs. It correctly differentiates MRSA from strains that have lost their resistance. More importantly, it works equally well with nasal, blood and abscess samples.

Dr. Persing says a number of factors contribute to achieving that speed and accuracy. The GeneXpert (Cepheid) system is completely self-contained and automatic, so there is no operator lag time between steps. It runs at high temperatures to facilitate the PCR reactions, and because each test is run individually, there is no waiting to accrue a number of samples to run in a batch.