BacteriologyDetection of methicillin-resistant Staphylococcus aureus in a low-prevalence setting by polymerase chain reaction with a selective enrichment broth
Introduction
Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) carriage can improve patient care and, by accelerating implementation of infection control measures, decrease the risk of MRSA transmission. Furthermore, rapid identification of persons free from MRSA can significantly decrease the number of isolation days. This is especially important when using a “search and destroy” strategy; as in this latter case, patients at risk for MRSA colonization are isolated until proven negative. Furthermore, the rapid identification of carriers during an outbreak facilitates outbreak management and will stop further transmission in an earlier phase. Because of low prevalence (<1%) (Tiemersma et al., 2004), MRSA coverage is not included in standard empirical antibiotic therapy regimens in The Netherlands. Therefore, in this country, early identification of MRSA carriers is also important for optimizing therapeutic decisions.
The BD GeneOhm MRSA assay, formerly known as the IDI-MRSA assay, has been primarily used in high-prevalence settings. Most studies describe direct application of IDI assay on swabs (Bishop and Grabsch, 2006, de San and Denis, 2007, Drews and Willey, 2006, Gilpin and Tunney, 2007, Oberdorfer and Pohl, 2006, Paule and Hacek, 2007, Rossney and Herra, 2007, Warren and Liao, 2004, Wren and Carder, 2006, Zhang and Drews, 2007). The polymerase chain reaction (PCR) assay performed directly on swabs demonstrated relatively high rates of initial inhibition, ranging from 0.3% to 8.8% of specimens, which would compromise its usefulness in daily practice. We hypothesized that the introduction of a preincubation step in a selective broth would circumvent inhibition and increase the sensitivity and specificity of the assay. We also exchanged the standard DNA isolation kit for a Sigma Extract-N-Amp plant Reagent Kit (Sigma, St. Louis, MO) because this latter kit is easier to use and automate.
The present study aimed to compare the performance of a modified BD GeneOhm MRSA PCR assay on individual and pooled samples, with selective broth cultures as a screening tool in areas with low MRSA prevalence.
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Setting
The study was performed in the Erasmus University Medical Center, a 1200-bed tertiary care university teaching hospital, in Rotterdam, The Netherlands. Specimens were collected prospectively in April and May 2006 and from October 2006 through January 2007.
Specimen collection
Screening of patients and health care workers at risk for MRSA colonization was performed according to the Dutch National guidelines (http://www.wip.nl). Nasal, throat, and rectal specimens and, when indicated, swabs from other sites including
Principle of the test
The test is based on real-time PCR detection of the 3′-end-SSCmec and adjacent S. aureus-specific DNA (orfX). The probes are developed to target the direct-repeat/inverted-repeat sequence at the transition of SCCmec and SA genome. There is no information supplied by BD concerning the internal control.
Individual specimens
We modified the BD GeneOhm MRSA assay procedure by overnight (16–24 h) preincubation of swabs in a selective enrichment broth. Furthermore, we changed the DNA isolation kit provided and used a
Individual specimens
A total of 581 samples from 201 persons were taken. Eighteen of the 581 (3.2%) were MRSA culture positive, and 563 were culture negative. There were no inhibited or, otherwise, unresolved PCR results. Table 1 summarizes the results.
Pooled specimens
After overnight incubation, 510 swabs from 174 persons were combined in 106 pools (4.8 swabs per pool). Of the pooled broths, 8 (7.5%) contained 1 or more MRSA culture-positive specimens and 98 were MRSA culture negative. There were no inhibited or, otherwise,
Discussion
In this study, we found sensitivity (92%) and specificity (98%) that are comparable with that found in the literature. Also, the assay had an excellent NPV (99%) but a low PPV (63%). Therefore, negative PCR results can be relied on, but positive PCR results need to be confirmed by a culture. The preincubation step also eliminated the problem of initial inhibition of the direct PCR assay, which ranged from 0.3% to 8.8% in other studies. Pooling of samples did not significantly reduce the
Conclusion
The BD GeneOhm MRSA assay on specimens incubated overnight in a selective broth has shown to be a reliable screening test with a sensitivity of 92%, specificity of 98%, and PPV and NPV of 63% and 99%, respectively. Negative sample results are reliable so that MRSA colonization can be ruled out. It is possible to pool specimens to reduce costs without reducing sensitivity.
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