Comparison of MRSASelect Agar, CHROMagar Methicillin-Resistant Staphylococcus aureus (MRSA) Medium, and Xpert MRSA PCR for detection of MRSA in Nares: diagnostic accuracy for surveillance samples with various bacterial densities

Southern Arizona VA Health Care System, Tucson, Arizona, USA.
Journal of clinical microbiology (Impact Factor: 3.99). 10/2009; 47(12):3933-6. DOI: 10.1128/JCM.00601-09
Source: PubMed


Rapid laboratory methods provide optimal support for active surveillance efforts to screen for methicillin-resistant Staphylococcus aureus (MRSA). Most laboratories struggle to determine the optimal use of resources, considering options to balance cost, speed, and diagnostic accuracy. To assess the performance of common methods, the first comparison of MRSASelect agar (MS) and CHROMagar MRSA (CA), with and without broth enrichment followed by a 24-h subculture to MS, was performed. Results were compared to those of the Xpert MRSA assay. For direct culture methods, the agreement between MS and CA was 98.8%. At 18 h, direct MS identified 93% of all positive samples from direct culture and 84% of those identified by the Xpert MRSA. For Trypticase soy broth-enriched MS culture, incubated overnight and then subcultured for an additional 24 h, the agreement with Xpert MRSA was 96%. The agreement between direct MS and Xpert MRSA was 100% when semiquantitative culture revealed a bacterial density of 2+ or greater; however, discrepancies between culture and Xpert MRSA arose for MRSA bacterial densities of 1+ or less, indicating low density as a common cause of false-negative culture results. Since 1+ or less was established as the most common MRSA carrier state, broth enrichment or PCR may be critical for the identification of all MRSA carriers who may be reservoirs for transmission. In this active-surveillance convenience sample, the use of broth enrichment followed by subculture to MS offered a low-cost but sensitive method for MRSA screening, with performance similar to that of Xpert MRSA PCR.

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Available from: Donna M. Wolk
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    • "While PCR testing by non-laboratory personnel also may have impacted study results, all personnel were tested for competency, prior to performing study testing, and performance did not vary substantially from other published results (Wolk et al., 2009b). That being said, we believe this scenario was an accurate representation of how molecular testing would occur in an ED setting. "
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    ABSTRACT: Extranasal sites are common reservoirs of Staphylococcus aureus colonization, and may be relevant for methicillin-resistant S. aureus (MRSA) screening and infection control strategies. The objective here was to determine whether inguinal specimens could also be screened using Xpert SA Nasal Complete assay for MRSA. Results were compared to broth enrichment culture. Among 162 consented adults seeking care in the Emergency Department for cutaneous abscesses, inguinal specimens were found positive for MRSA more often than nares specimens; 24% and 26% by PCR or culture, respectively compared to 19% each by PCR or culture. Overall, 6% of adults colonized with MRSA would have been missed by nares screening alone. Compared to culture, Xpert SA Nasal Complete assay demonstrated sensitivity and specificity of 89% and 97%, respectively for detecting nares and/or inguinal MRSA colonization. In conclusion, inguinal specimens were a more common reservoir for MRSA than nares specimens in this population of patients.
    Full-text · Article · Jun 2014 · Diagnostic microbiology and infectious disease
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    • "ASC were performed routinely on admission and discharge from wards by swabbing nares, axilla and groin and using direct culture on chromogenic media. Estimated sensitivity of direct culture on chromogenic media is approximately 84% when compared to PCR detection, and would also be dependent on sampling technique [21]. This could have resulted in some false negative results from patients who were prevalent carriers but swab negative on admission. "
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    ABSTRACT: Patients newly colonised with methicillin-resistant Staphylococcus aureus (MRSA) are at higher risk of clinical MRSA infection. At present, there are limited data on the duration or magnitude of this risk in a hospital population with a known time of MRSA acquisition. A retrospective cohort study of 909 adult patients known to have newly identified MRSA colonisation during admission to National University Hospital, Singapore between 1 July 2007 and 30 June 2011 was undertaken. Patients were excluded if they had history of previous MRSA colonisation or infection, or if they had been a hospital inpatient in the preceding 12 months. Data were collected on the development of MRSA infection requiring hospitalisation up to 30 June 2012. Of 840 patients newly colonised with MRSA as identified on active surveillance and not clinical specimens, 546 were men (65.0%) and the median age was 65 years (range 18--103 years). Median follow up was 24 months (range 0 --64 months, 85.1% followed >6 months). Clinical infection occurred in 121 patients (14.4%) with median time to infection of 22 days (95% CI 14--31). Overall 71.9% (87/121) of infected patients developed infection within 60 days of the date MRSA colonisation was detected. However, 17/121 patients (14.0%) developed clinical infection more than six months after documented MRSA acquisition. The most common sites of clinical infection were skin and soft tissue (49/121, 40.5%, 95% CI 31.7-49.8), respiratory tract (37/121, 30.6%, 95% CI 22.5-39.6) and bone and joint infections (14/121, 11.6%, 95% CI 6.5-18.7). Thirteen patients (13/121, 10.7%, 95% CI 5.8-17.7) had bacteraemias, of which six (5.0% 95% CI 1.8-10.5) were primary and seven (5.7%, 95% CI 2.3-11.6) were secondary to infection at other sites. Crude mortality at 30 days and six months was higher in patients with MRSA infection than colonisation alone (aOR 5.49, 95% CI 2.75-10.95, p<0.001 and aOR 2.94, 95% CI 1.78-4.85, p<0.001 respectively). Risk of clinical infection is highest soon after MRSA acquisition. Prevention of MRSA acquisition in hospital will have significant impact on morbidity and mortality for patients.
    Full-text · Article · Oct 2013 · BMC Infectious Diseases
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    • "Currently, various selective and differential chromogenic media are available that allow direct colony color-based identification of the pathogen from the primary culture 18-24 hr after incubation. However, broth enrichment and incubation for 48 hr are necessary to increase the detection sensitivity, especially in cases with low bacterial density [9, 10]. "
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    ABSTRACT: We evaluated the performance of three chromogenic media (Brilliance agar I [Oxoid, UK], Brilliance agar II [Oxoid], and ChromID MRSA [Biomérieux, France]) combined with broth enrichment and the Xpert MRSA assay for screening of methicillin-resistant Staphylococcus aureus (MRSA). We obtained 401 pairs of duplicate nasal swabs from 321 patients. One swab was suspended overnight in tryptic soy broth; 50-µL aliquots of suspension were inoculated on the three chromogenic media. Brilliance agar I and II were examined after 24 hr, and ChromID MRSA, after 24 and 48 hr. The paired swab was processed directly using real-time PCR-based Xpert MRSA assay. True positives, designated as MRSA growth in any of the culture media, were detected with the prevalence of 17% in our institution. We report the sensitivity, specificity, positive predictive value, and negative predictive value of MRSA growth as follows: 92.3%, 94.0%, 75.9%, and 98.4% in Brilliance agar I (24 hr); 92.7%, 97.9%, 90.0%, and 98.5% in Brilliance agar II (24 hr); 95.6%, 95.8%, 82.3%, and 99.1% in ChromID MRSA (24 hr); 100%, 92.5%, 73.1%, and 100% in ChromID MRSA (48 hr); 92.6%, 96.7%, 85.1%, and 98.5% in Xpert MRSA assay. The agreement between the enriched culture and Xpert MRSA assay was 96.0%. Three chromogenic culture media combined with enrichment and Xpert MRSA assay demonstrated similar capabilities in MRSA detection. The Xpert MRSA assay yielded results comparable to those of culture methods, saving 48-72 hr, thus facilitating earlier detection of MRSA in healthcare settings.
    Full-text · Article · Jul 2013 · Annals of Laboratory Medicine
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