Rapid Tests Preemptively Screen Drug Resistant Bacteria

Rapid diagnostic testing (RDT) can be used to screen carriers of suspected methicillin-resistant Staphylococcus aureus (MRSA) in hospitals.

Preemptive isolation of suspected MRSA carriers is a cornerstone of successful MRSA control policies but is hampered when using conventional cultures with diagnostic delays of three to five days, as many noncarriers remain unnecessarily isolated.

Scientists at the University Medical Center (Utrecht, The Netherlands) investigated whether RDT reduces the amount of unnecessary isolation days, the costs, and benefits in intensive care units (ICUs) in a multicenter hospital-wide study. The effects of RDT of MRSA was evaluated using polymerase chain reaction (PCR) assays added to screening with conventional microbiological culture methods for patients eligible for MRSA screening. Two real time PCR assays were subsequently evaluated: BD GeneOhm MRSA PCR between December 2005 and May 2007, and Xpert MRSA assay between April 2007 and June 2008.

A total of 163 patients at risk for MRSA carriage were screened and MRSA prevalence was 3.1%. Duration of isolation was 27.6 hours with BD GeneOhm MRSA PCR (previously known as IDI-MRSA, BD Diagnostics, San Diego, CA, USA) and 21.4 hours with GeneXpert (Cepheid, Sunnyvale, CA, USA), and would have been 96 hours when based on conventional cultures. The negative predictive value was 100% for both tests.

Numbers of isolation days were reduced by 44.3% with PCR-based screening at the additional costs of EUR 327.84 for the BD assay and EUR 252.14 for the GeneXpert test per patient screened. Costs per isolation day avoided were calculated to be EUR 136.04 for the BD assay and EUR 121.76 for the GeneXpert. The authors concluded therefore that in a low endemic setting for MRSA, RDT safely reduced the number of unnecessary isolation days.

The platforms used are not suited for the large volumes of multiple tests in a short period of time, as only 16 could be performed simultaneously on Cepheid's Smartcycler and four tests on the GeneXpert systems. This endorses the need for large volume testing or pooling of swabs in ICU patients to decrease unnecessary preemptive isolation time. Another option would be to use chromogenic agar-based screening, which has a slightly longer turn around time in the laboratory, but can be performed in large volumes and is easily implemented in routine laboratory practice, including weekends. The study was originally published on February 7, 2012, in the journal Critical Care.

Related Links:
University Medical Center Utrecht
BD Diagnostics
Cepheid