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

Abstract

Universal surveillance upon patient admission is important in reducing the transmission of methicillinresistant Staphylococcus aureus (MRSA) and associated disease in hospitals. High costs for the health care system in conjunction with MRSA have promoted the development of rapid screening methods to detect MRSA carriers. This study compared two real-time PCR methods, the BD GeneOhm MRSA assay (BDGO) and the Xpert MRSA assay, with broth-enriched culture to define their performance characteristics and rapidity in an area with low MRSA prevalence. In total, 414 swabs from the nose and 389 swabs from the groin from 425 patients were tested. Of those 425 patients, 378 had swabs from both the nose and groin in parallel. Two hundred thirty-one and 194 patients were randomly assigned to the BDGO group and the Xpert MRSA group, respectively. In general, sensitivity, specificity, and negative predictive value (NPV) were high for the BDGO (100%, 98.5%, and 100%, respectively) and the Xpert MRSA (100%, 98.2%, and 100%, respectively), irrespective of whether or not nasal and inguinal specimens were considered alone or combined. In contrast, the positive predictive value (PPV) was lower: before the resolution of discrepant results, the PPVs for nasal and inguinal specimens alone and combined were 87.5%, 86.7%, and 82.4% for the BDGO and 91.7%, 66.7%, and 92.9% for the Xpert MRSA, respectively. After the resolution of discrepant results, PPVs were 93.8%, 93.3% and 94.1% for the BDGO and 91.7%, 88.9% and 92.9% for the Xpert MRSA, respectively. With the BDGO, 4 of 16 carriers were each identified by nasal or inguinal swabs alone, whereas in the Xpert MRSA group, 4 of 13 carriers were exclusively identified by nasal swabs and 2 of 13 were identified by inguinal swabs alone. Both PCR methods showed no significant difference in the number of discrepant results (odds ratio, 0.70 [P ‫؍‬ 0.789]), but specimens from wounds and other body sites (axilla, vagina, and throat) produced discrepancies more often than nasal and groin specimens (odds ratios, 4.724 [P ‫؍‬ 0.058] and 12.163 [P < 0.001], respectively). The facts that no false-negative PCR results were detected and increased PPVs were found after the resolution of discrepant results point to PCR as the actual gold standard. Since both sensitivity and NPV were exceptionally high for PCR, backup cultures may, therefore, be unnecessary in an area with low prevalence and with a preemptive isolation strategy but may still be useful for PCR-positive specimens because of the lower PPV for both methods and the possibility of susceptibility testing. The median time for analysis, including extraction, hands-on time, and actual PCR was 2 h 20 min for the Xpert MRSA versus 5 h 40 min for the BDGO. Concerning reporting time, including administration and specimen collection, the Xpert MRSA was faster than the BDGO (7 h 50 min versus 17 h).

Evaluation of BD Max StaphSR and BD Max MRSA XT Assays Using ESwab-Collected Specimens

“…An active MRSA surveillance program that uses a laboratory test with 98% sensitivity and with a reporting time of Յ15 h can capture 85% of potential inpatient MRSA isolation days, leading to a significant (Ͼ70%) reduction in MRSA infection (28). For these reasons, tremendous amounts of effort and resources have been focused on the development of several new molecular-based rapid screening tests for MRSA (12)(13)(14)(30)(31)(32).…”

“…MRSA screening using molecular amplification methods provides faster results than culture and has been demonstrated to be more sensitive (11)(12)(13)(14)(15). However, most of the molecular commercially available rapid tests are based on the detection of a sequence indicating the integration of the staphylococcal cassette chromosome mec element (SCCmec) within the chromosome and do not specifically target the methicillin resistance gene mecA (16).…”

Comparison of Xpert MRSA/SA Nasal and MRSA/SA ELITe MGB Assays for Detection of the mecA Gene with Susceptibility Testing Methods for Determination of Methicillin Resistance in Staphylococcus aureus Isolates

“…Whether disease caused by mecA LGA251 S. aureus is identical to that caused by conventional MRSA is not clear without evidence from formal epidemiological and virulence studies. The Xpert MRSA nasal assay and the MRSA/SA ELITe MGB kit were chosen for this study because both have good classical performance parameters, such as sensitivity, specificity, PPV, and NPV, and the results are rapidly available with both kits (22)(23)(24).…”

Comparative Evaluation of Two PCR-Based Methods for Detection of Methicillin-Resistant Staphylococcus aureus (MRSA): Xpert MRSA Gen 3 and BD-Max MRSA XT

“…More recently, livestock-associated MRSA carrying variant mecC genes further complicated the epidemiology of MRSA by increasing its molecular diversity (5). Microbiology laboratories usually define MRSA phenotypically, but the underlying genetic structures responsible for this phenotype are constantly changing (6)(7)(8), and the molecular assays designed several years back are not necessarily relevant to the epidemiology observed today (9)(10)(11). A recent comparison of culture and a commercial molecular assay performed in a high-prevalence environment showed the molecular assay to be slightly less sensitive than enriched culture using ChromID MRSA agar but also showed that the results were not significantly different, although the molecular assay yielded results in hours instead of days (12).…”