Comparison of MRSA Select 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

Abstract

Rapid laboratory methods provide optimal support for active surveillance efforts to screen for methicillinresistant 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.The spread of methicillin-resistant Staphylococcus aureus (MRSA) is a major concern in healthcare settings because human "carriers" can spread MRSA to others, resulting in increased morbidity, mortality, and costs (13,14,17). One strategy to help prevent and control MRSA infections is the use of active surveillance cultures to screen patients for nasal carriage of MRSA, a practice coupled with appropriate barrier precautions for colonized or infected patients. Active surveillance programs are growing in number in the United States (10,20,23,24), despite differences in opinions about the practice and reported gaps in the literature (19). Therefore, clinical microbiology laboratories must respond to provide support for active surveillance screening methods.There is continued debate about the practicality and cost benefit of different MRSA detection methods. Typically, agar culture or PCR methods are used (4); however, additional workload and costs cause laboratories to struggle with resource allocation issues, which often depend on the interplay between assay accuracy, turnaround time, cost per test, and workforce availability. While PCR results can be available in as little as 2 h and are known to provide excellent sensitivity and specificity for MRSA screening (4, 25, 28), PCR methods are more costly than culture methods (6). Alternately, selective and differential chromogenic agars have gained popularity because of lower cost and...

Comparison of the BD GeneOhm Methicillin-Resistant Staphylococcus aureus (MRSA) PCR Assay to Culture by Use of BBL CHROMagar MRSA for Detection of MRSA in Nasal Surveillance Cultures from Intensive Care Unit Patients

“…Broth enrichment is frequently mentioned as an adjunct to plated medium and molecular methods to enhance the detection of MRSA in patient specimens. Many publications have reported increases in the recovery of MRSA and calculated statistical performance achieved at the expense of marginally increased recoveries of MRSA, increased work load, increased effort, longer time to detection and the impediment to expedient enforcement of hospital infection control practices (17,20,24,25,34), in addition to the associated increased costs. Broth enrichment methods need to be standardized as to optimal growth medium, incubation time and temperature, the influence of sampling schedules, single versus multiple body sites, and the correlation of MRSA bioburden levels from body sites other than nares, and the correlation of such MRSA bio-burden loads in broth enrichment only to the actual transmission rates of MRSA to others.…”

Cepheid Xpert MRSA Cycle Threshold in Discordant Colonization Results and as a Quantitative Measure of Nasal Colonization Burden

“…In 2009, Wolk et al demonstrated that a lower MRSA colonization burden is associated with discordant screening tests (positive PCR test and negative agar test), and the MRSA cycle threshold (C T ) values for the concordant samples in the Xpert MRSA assay (Cepheid, Sunnyvale, CA) were statistically lower than those for discordant samples (21). In our study, we examined the variation between PCR and direct culture screening methods for nasal MRSA colonization among hospitalized veterans, compared the C T values for patients with discordant screening results at admission and discharge, and evaluated the quantitative abilities of the Xpert MRSA PCR assay.…”

Detection of Methicillin-Resistant Staphylococcus aureus (MRSA) in Specimens from Various Body Sites: Performance Characteristics of the BD GeneOhm MRSA Assay, the Xpert MRSA Assay, and Broth-Enriched Culture in an Area with a Low Prevalence of MRSA Infections

“…Several recent studies have compared the BDGO and Xpert MRSA assays in countries with a high prevalence of MRSA (22,29,(34)(35)(36). In our study, we not only compared classical performance parameters like sensitivity, specificity, PPV, and NPV in an area with a low prevalence but also analyzed the likelihood of discrepant PCR and culture results for specimens from body sites other than the nares.…”