BD GeneOhm Methicillin-resistant Staphylococcus Aureus Research Articles

Isolation of Methicillin-resistant Staphylococcus aureus (MRSA) using a flocked swab elution transport system; with identification by PCR

Admission screening is an essential part of the control of methicillin-resistant Staphylococcus aureus (MRSA). Using collection systems that maintain viability and have high organism recovery, combined with accurate and rapid laboratory testing is critical for the success of a screening program. This review evaluates the performance of PCR testing, particularly the Roche LightCycler MRSA test, in the detection of MRSA. In addition, the clinical performance of the Copan Elution swab (ESwab) is reviewed. Several commercial molecular tests developed for use in detecting MRSA colonisation have been evaluated against culture methods, and all appear to provide high sensitivity, good specificity, and the potential for rapid reporting of MRSA colonisation. The new LightCycler MRSA advanced test is another of these assays with rapid detection time and high sensitivity. Compared with the BD GeneOhm MRSA assay, this new test also appears to have improved test specificity. The ESwab has an equal or even better recovery of bacteria, including MRSA, compared with other swab systems, via culture, and PCR. The advantages of using the ESwab are: the use of a liquid Amies medium allowed multiple uses (both culture, and PCR can be performed with one collected sample); and, specimens from multiple sites can be pooled into the same liquid transport media which limits the test costs and broadens the screening capabilities. However, there are limited publications on the clinical performance of the ESwab and more research is required on this new swab transport system.

Admission Screening of Methicillin-Resistant Staphylococcus aureus with Rapid Molecular Detection in Intensive Care Unit: A Three-Year Single-Centre Experience in Hong Kong

Background. The admission screening of methicillin-resistant Staphylococcus aureus (MRSA) by rapid molecular assay is considered to be an effective method in reducing the transmission of MRSA in intensive care unit (ICU). Method. The admission screening on patients from ICU once on their admissions by BD GeneOhm MRSA assay has been introduced to Prince of Wales Hospital, Hong Kong, since 2008. The assay was performed on weekdays and reported on the day of testing. Patients pending for results were under standard precautions until the negative screening results were notified, while contact precautions were implemented for MRSA-positive patients. In this study, we compared the MRSA transmission rate in molecular screening periods (2008 to 2010) with the historical culture periods (2006 to 2007) as control. Results. A total of 4679 samples were tested; the average carriage rate of MRSA on admission was 4.45%. By comparing with the historical culture periods, the mean incidence ICU-acquired MRSA infection was reduced from 3.67 to 1.73 per 1000 patient bed days. Conclusion. The implementation of admission screening of MRSA with molecular method in intensive care unit could reduce the MRSA transmission, especially in the area with high MRSA prevalence situation in Hong Kong.

Analysis of Staphylococcal Cassette Chromosome mec in BD GeneOhm MRSA Assay-Negative Strains

The BD GeneOhm MRSA assay could identify methicillin-resistant Staphylococcus aureus (MRSA) strains at a high ratio (97.8%). Analysis of 11 assay-negative MRSA strains suggested that insertion of non-mec staphylococcal cassette chromosome elements (SCCs) downstream of orfX, and carriage of SCCmecs with a left extremity that cannot be detected by the kit, might lead to their being given an incorrect negative status.

The Screening of Methicillin-Resistant Staphylococcus aureus in Vascular Surgery Patients: A Comparison of Molecular Testing and Broth-Enriched Culture

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is a major global health care-associated pathogen. This study sought to examine the prevalence of MRSA in patients who were admitted to a vascular surgery ward during a 3-month period. Methods: MRSA screening was accomplished through the acquisition of nasal, throat and perineal swabs. These swabs were placed in tryptic soy broth that had been supplemented with 6.5% NaCl and incubated for 24 h. The resulting isolates were subcultured on agar plates containing 5% sheep blood. The BD GeneOhm MRSA assay for screening swabs was performed in accordance with the manufacturer’s instructions. Results: A total of 58 patients were included in the study and swabs from 232 sites were obtained during the sampling period. MRSA was detected in 33 samples of 12 patients during the study period; thus, there was a 20.6% prevalence of patients who were recognized as MRSA carriers. There were discrepancies between the results of classical bacteriological screening and molecular MRSA detection methods in 8 of the patients. Conclusions: Nasal, throat and perineal MRSA screening can detect the carriage of this pathogen and allow for the timely use of appropriate infection control measures. The choice of screening techniques poses a challenge; it has been demonstrated that molecular detection methods should be performed with great sensitivity, specificity and, most importantly, speed. The cost of the PCR screening method is the only disadvantage of this approach.

Development of a simple, low-density array to detect methicillin-resistant Staphylococcus aureus and mecA dropouts in nasal swabs

Detection of methicillin-resistant Staphylococcus aureus (MRSA) is important for prevention and control of MRSA infections, but the discovery of mecA dropouts and SCCmec junction sequences with homology to coagulase-negative staphylococci (CoNS) has challenged several real-time PCR tests. The objective of this study was to develop a user-friendly, gel element microarray test for MRSA detection, to estimate the analytical performance characteristics of the test on bacterial isolates, and to perform an initial evaluation of the test on nasopharyngeal swabs from patients known to have a high prevalence of S. aureus containing mecA dropouts. The assay limit of detection for the test was 250fg (or less) of genomic DNA per amplification reaction (approximately 80 cell equivalents) and MRSA was consistently detected at a ratio of 1:12,000 in a non-target background. Of 87 bacterial isolates, the test accurately classified 86 (98.8%) overall, and correctly identified 14 mecA dropout specimens that were falsely positive in the BD GeneOhm MRSA test or BD GeneOhm StaphSR test. A retrospective analysis of 246 nasal swab samples acquired from a high-risk patient population (overall prevalence=10.8% by culture) resulted in 80.5% sensitivity (95% CI=68.4%, 92.6%) and 96.6% specificity. Of these 246 samples, 174 (71%) were positive for mecA, 86 (35%) were positive for S. aureus tufA and 46 (19%) were positive for a SCCmec junction sequence. To estimate method repeatability, 48 samples representing the full range of phenotypes, genotypes and microarray probe SNR values were tested in triplicate, with three discordant results for a concordance rate of 97.9% (141/144 tests). These data demonstrate that a very simple microarray test can identify mecA dropouts with high specificity in either cultured isolates or nasal swabs from a high-prevalence, high-risk patient population. However, the clinical sensitivity of the test will likely depend on local microbial ecology and the prevalence of mecA positive CoNS in any given patient population.

Multicenter Evaluation of the LightCycler MRSA Advanced Test, the Xpert MRSA Assay, and MRSASelect Directly Plated Culture with Simulated Workflow Comparison for the Detection of Methicillin-Resistant Staphylococcus aureus in Nasal Swabs

Rapid detection of nasal colonization with methicillin-resistant Staphylococcus aureus (MRSA) followed by appropriate infection control procedures reduces MRSA infection and transmission. We compared the performance and workflow of two Food and Drug Administration-approved nucleic acid amplification assays, the LightCycler MRSA Advanced Test and the Xpert MRSA test, with those of directly plated culture (MRSASelect) using 1202 nasal swabs collected at three U.S. sites. The sensitivity of the LightCycler test (95.2%; 95% CI, 89.1% to 98.4%) and Xpert assay (99%; 95% CI, 94.8% to 100%) did not differ compared with that of culture; the specificity of the two assays was identical (95.5%; 95% CI, 94.1% to 96.7%) compared with culture. However, sequencing performed on 71 samples with discordant results among the three methods confirmed the presence of MRSA in 40% of samples that were positive by both molecular methods but negative by culture. Workflow analysis from all sites including batch runs revealed average hands-on sample preparation times of 1.40, 2.35, and 1.44 minutes per sample for the LightCycler, Xpert, and MRSASelect methods, respectively. Discrete event simulation analysis of workflow efficiencies revealed that the LightCycler test used less hands-on time for the assay when greater than eight batched samples were run. The high sensitivity and specificity, low hands-on time, and efficiency gains using batching capabilities make the LightCycler test suitable for rapid batch screening of MRSA colonization.

Genotypic and Phenotypic Characterization of Methicillin-Susceptible Staphylococcus aureus Isolates Misidentified as Methicillin-Resistant Staphylococcus aureus by the BD GeneOhm MRSA Assay

Twenty-three nasal swab samples that tested positive for methicillin-resistant Staphylococcus aureus (MRSA) on initial testing by the BD GeneOhm MRSA assay (BD-MRSA PCR; BD GeneOhm, San Diego, CA) were culture positive only for methicillin-susceptible S. aureus (MSSA) from an enrichment broth. The 23 recovered isolates were confirmed as MSSA by a variety of phenotypic methods, including the BD Phoenix automated microbiology system (BD Diagnostics, Sparks, MD), oxacillin screening agar (BD Diagnostics), BBL CHROMagar MRSA (BD Diagnostics), and a PBP2' assay (Denka Seiken Co., Tokyo, Japan); susceptibilities were determined by using Mueller-Hinton agar with oxacillin. All were positive by nuc PCR, specific for S. aureus, but negative for mecA with one exception. Isolates were characterized by using multiplex PCR methodology to determine structural types and variants (SCCmec typing); additional PCRs were performed for the detection of the ccr and mec complexes, the junkyard regions as well as the Panton-Valentine leukocidin. Pulsed-field gel electrophoresis was used to determine clonality. One phenotypic MSSA isolate contained an intact SCCmec. Twelve MSSA isolates tested positive for MRSA by the BD-MRSA PCR because of amplification of the mec priming site flanking the SCC insertion point, although these isolates lacked mecA. The 10 remaining isolates were not MRSA and tested as MSSA by phenotypic and genotypic assays. In our patient population, diagnostic and surveillance testing and subsequent infection control practices may be impacted by the frequency of these excision events when using the BD-MRSA PCR for MRSA detection.

Rapid screening of methicillin-resistant Staphylococcus aureus using PCR and chromogenic agar: a prospective study to evaluate costs and effects

Pre-emptive isolation of suspected methicillin-resistant Staphylococcus aureus (MRSA) carriers is considered essential for controlling the spread of MRSA, but noncolonized patients will be isolated unnecessarily as a result of a delay in diagnosis of 3–5 days with conventional cultures. We determined costs per isolation day avoided, and incremental costs of rapid MRSA screening tests when added to conventional screening, but with decisions on isolation measures based on PCR results. A prospective multicentre study evaluating BD GeneOhm MRSA PCR (‘IDI’) (BD Diagnostics, San Diego, CA, USA), Xpert MRSA (‘GeneXpert’) (Cepheid, Sunnyvale, CA, USA) and chromogenic agar (MRSA-ID) (bioMérieux, Marcy-l’Etoile, France) was performed in 14 Dutch hospitals. Among 1764 patients at risk, MRSA prevalence was 3.3% (n = 59). Duration of isolation was 19.7 and 16.1 h with IDI and GeneXpert, respectively, and would have been 30.0 and 76.2 h when based on chromogenic agar and conventional cultures, respectively. Negative predictive values (at a patient level) were 99.5%, 99.1% and 99.5% for IDI, GeneXpert and chromogenic agar, respectively. Numbers of isolation days were reduced by 60% and 47% with PCR-based and chromogenic agar-based screening, respectively. The cost per test was €56.22 for IDI, €69.62 for GeneXpert and €2.08 for chromogenic agar, and additional costs per extra isolation day were €26.34. Costs per isolation day avoided were €95.77 (IDI) and €125.43 (GeneXpert). PCR-based decision-making added €153.64 (IDI) and €193.84 (GeneXpert) per patient to overall costs and chromogenic testing would have saved €30.79 per patient. Rapid diagnostic testing safely reduces the number of unnecessary isolation days, but only chromogenic screening, and not PCR-based screening, can be considered as cost saving.

Evaluation of the BD GeneOhm Assay Using the Rotor-Gene 6000 Platform for Rapid Detection of Methicillin-Resistant Staphylococcus aureus from Pooled Screening Swabs

As health services move toward universal methicillin-resistant Staphylococcus aureus (MRSA) screening for hospital admissions, the most cost-effective approach is yet to be defined. In this study, one of the largest to date, we evaluated the performance of the BD GeneOhm MRSA assay on the Rotor-Gene 6000 thermal cycler, using samples taken directly from pooled MRSA screens. Results were compared with the same assay performed on the Smart-Cycler II platform and overnight broth culture. Samples yielding discrepant results were subjected to detailed analysis with an in-house PCR and patient note review. A total of 1,428 pooled MRSA screens were tested. Sensitivities and specificities of 85.3% and 95.8% for the Rotor-Gene and 81% and 95.7% for the Smart-Cycler were obtained, compared with broth enrichment. The sensitivity of the BD GeneOhm assay was increased to 100% when the results of in-house PCR and patient note review were taken into account. This study demonstrates that the Rotor-Gene 6000 thermal cycler is a reliable platform for use with the BD GeneOhm assay. It also proves that commercial PCR can be performed direct on pooled samples in selective broth, without the need for overnight incubation.

Performance of the BD GeneOhm Methicillin-resistant Staphylococcus aureus (MRSA) PCR Assay for Detecting MRSA Nasal Colonization in Taiwanese Adults

A rapid diagnostic method for methicillin-resistant Staphylococcus aureus (MRSA) has been implemented for surveillance of the at-risk population, but its performance in those without traditional risk factors is not clear. The objective of this study was to evaluate MRSA colonization status by comparing the performance of the BD GeneOhm MRSA polymerase chain reaction (PCR) assay with that of conventional culture during a 3-month active surveillance of Taiwanese adults in the community. From 1 October 2007 to 28 December 2007, adults (≥ 18 years old) attending a mandatory health examination arranged by their employers as a part of the workplace health promotion program at three medical centers in northern Taiwan were enrolled in the study. No healthcare workers were included. A total of 498 paired nasal swabs were prospectively obtained and used for both the BD GeneOhm MRSA PCR assay and conventional culture. Of the 498 paired nasal swabs, 14 (2.8%) were positive for MRSA by conventional culture and 34 (6.8%) were positive by the BD GeneOhm MRSA PCR assay (p < 0.005). Thirteen specimens were both culture- and PCR-positive, and 463 samples were both culture- and PCR-negative. There were two discordant results: 21 specimens were culture-negative/PCR-positive, and one was culture-positive/PCR-negative. The simple kappa coefficient for measuring the agreement between conventional culture and the MRSA PCR assay was 0.52. This study demonstrates the feasibility of using both the MRSA PCR assay and conventional culture as surveillance tools. Also, the MRSA-positive rate detected by MRSA PCR assay was significantly higher than that of conventional culture.

Reliability of the BD GeneOhm Methicillin-Resistant Staphylococcus aureus (MRSA) Assay in Detecting MRSA Isolates with a Variety of Genotypes from the United States and Taiwan

The BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) assay is a molecular screening test for detection of MRSA in nasal colonization. This assay coamplifies the extremity of staphylococcal chromosome cassette mec (SCCmec) and adjacent chromosomal DNA at the SCCmec insertion site. Increasing reports of novel SCCmec types and the diverse genetic backgrounds of MRSA strains prompted us to test the accuracy of the BD GeneOhm MRSA kit with 914 MRSA isolates with a variety of SCCmec types harbored in 21 genetic backgrounds, as determined by the multilocus sequence type (ST). The BD GeneOhm MRSA assay was performed on colony lysates; purified genomic DNA (0.2 pg/μl and 0.2 ng/μl) was tested to confirm negative results from lysates. Of 914 MRSA isolates tested, 911 tested positive (detection rate, 99.7%). The SCCmec types carried by assay-positive isolates were I, II, III, IV, V, V(5C2&5), VI, and VIII and SCCmec composite islands with mec class A and ccr complexes 2 and 4. One of the assay-negative isolates had a community-associated genotype: ST8, SCCmec type IV. However, this was an outlier among the 99.8% (434/435) ST8, SCCmec type IV-containing isolates that tested positive. The two other assay-negative isolates had a health care-associated genotype (ST5); both carried a distinct, uncommon, composite SCCmec type. In summary, the BD GeneOhm MRSA assay had a high rate of detection of MRSA isolates harboring common and uncommon SCCmec types from the United States and Taiwan.

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

Universal surveillance upon patient admission is important in reducing the transmission of methicillin-resistant 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).

Multicenter Evaluation of the LightCycler Methicillin-Resistant Staphylococcus aureus (MRSA) Advanced Test as a Rapid Method for Detection of MRSA in Nasal Surveillance Swabs

The rate of methicillin-resistant Staphylococcus aureus (MRSA) infection continues to rise in many health care settings. Rapid detection of MRSA colonization followed by appropriate isolation can reduce transmission and infection. We compared the performance of the new Roche LightCycler MRSA advanced test to that of the BD GeneOhm MRSA test and culture. Double-headed swabs were used to collect anterior nasal specimens from each subject. For both tests, DNA was extracted and real-time PCR was performed according to manufacturer's instructions. For culture, one swab of the pair was plated directly to CHROMagar MRSA. The swab paired with the BD GeneOhm MRSA test was also placed into an enrichment broth and then plated to CHROMagar MRSA. Colonies resembling staphylococci were confirmed as S. aureus by standard methods. Discrepant specimens had further testing with additional attempts to grow MRSA as well as sample amplicon sequencing. Agreement between results for the two swabs was 99.3% for those with valid results. A total of 1,402 specimens were tested using direct culture detection of MRSA as the gold standard; 187 were culture positive for MRSA. The LightCycler MRSA advanced test had relative sensitivity and specificity of 95.2% (95% confidence interval [CI]: 91.1% to 97.8%) and 96.4% (95% CI: 95.2% to 97.4%), respectively. The BD GeneOhm assay had relative sensitivity and specificity of 95.7% (95% CI: 91.7% to 98.1%) and 91.7% (95% CI: 90.0% to 93.2%), respectively. Following discrepancy analysis, the relative sensitivities of the LightCycler MRSA advanced test and the BD GeneOhm MRSA assay were 92.2 and 93.2%, respectively; relative specificities were 98.9 and 94.2%, respectively. Specificity was significantly better (P<0.001) with the LightCycler MRSA advanced test. The sensitivity of direct culture was 80.4%. The LightCycler MRSA advanced test is a useful tool for sensitive and rapid detection of MRSA nasal colonization.

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

This study compared the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) real-time PCR assay to culture by the use of BBL CHROMagar MRSA for the detection of MRSA in 627 nasal surveillance specimens collected from intensive care unit (ICU) patients. The PCR assay had a sensitivity, specificity, positive predictive value, and negative predictive value of 100%, 96.7%, 70.3%, and 100%, respectively. Nine of 19 false-positive PCR specimens grew methicillin-susceptible S. aureus (MSSA) from broth enrichment culture, of which two demonstrated evidence of mecA gene dropout. Compared to culture by the use of BBL CHROMagar MRSA, the BD GeneOhm MRSA PCR assay demonstrated sensitivity and specificity above 95% for the detection of MRSA nasal colonization and provided shorter turnaround time in generating positive and negative final results.

Rapid Detection of Methicillin-Resistant Staphylococcus aureus (MRSA) in Diverse Clinical Specimens by the BD GeneOhm MRSA Assay and Comparison with Culture

The efficacy of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) assay was assessed by analyzing nasal swabs and swabs from other body sites for the presence of MRSA in a low-prevalence area. From 681 patients with a high risk for MRSA carriage, 1,601 specimens were collected and transported in Amies agar. After discordant analysis, the sensitivity, specificity, positive predictive value, and negative predictive value of the BD GeneOhm MRSA assay were 84.3%, 99.2%, 88.4%, and 98.9%, respectively, compared to culture.

Detection limits of a rapid MRSA detection assay based on multiplex real-time PCR

I wish to report the results of a study for the limits of detection of a rapid methicillin-resistant Staphylococcus aureus (MRSA) detection assay based on a multiplex realtime polymerase chain reaction (PCR) (BD GeneOhm MRSA assay, Fukushima, Japan), which was reported in the Journal of Infection and Chemotherapy, Volume 15 (No. 4), 2009 [1]. ATCC 33591 (MRSA) was recovered on Mueller Hinton agar by overnight incubation at 37 C. Colonies were picked from the agar, adjusted to Mcfarland 0.5, and diluted to 10–10 colony-forming units (CFU)/ml (corresponding to approximately 10–10 CFU/reaction) by the sample buffer (Tris-EDTA buffer) of the BD GeneOhm MRSA assay kit [1]. The BD GeneOhm MRSA assay was performed four times at about the same concentration to determine the limits of detection. Table 1 shows the result of the tests. All tests were positive with 10–10 and 10 CFU/reaction; however, only half of the tests were positive with at a reaction concentration of 10 CFU. Although the results of the study show that at least 10 CFU/reaction is necessary to detect MRSA stably by the BD GeneOhm MRSA assay, compilation of data and further study with clinical samples are needed. Reference

Real-time PCR法を用いた血液培養からのMRSA迅速診断法の検討

Methicillin-resistant Staphylococcus aureus (MRSA), a well-known causative multidrug-resistant pathogen responsible for nosocomial and community-acquired infections, particularly in blood stream infection, often proves difficult and expensive to treat. Despite the need for rapid, accurate MRSA detection for treatment and infection control, conventional testing including culture, have sensitivity and turn-around time (TAT) problems. We evaluated BD GeneOhm MRSA Detection Kit rapid detection performance directly from positive blood culture using real-time PCR. The kit recognizes, a specific part of the staphylococcal cassette chromosome mec (SCCmec) gene, not a mecA gene. Compared to conventional culture in 138 samples with gram stains showing gram-positive cocci (GPC) clusters, the kit's sensitivity was 100%, specificity 97.3%, positive predictive value 90% and negative predictive value 100%. Three of the 27 MSSA isolates found was false-positive, indicating that the kit detected SCCmec/orfX region sequences lacking mecA. Coupled with direct tube coagulase testing to rapidly differentiate MRSA from methicillin-susceptible S. aureus (MSSA) could provide optimum treatment through appropriate antibiotic use. The kit thus appears to be useful in rapidly diagnosing MRSA from blood culture, improving the prognosis and reducing medical cost.

Performance Evaluation of BD GeneOhm MRSA PCR Assay for Detection of Nasal Colonization of Methicillin-Resistant Staphylococcus aureus at Endemic Intensive Care Units

The BD GeneOhm MRSA PCR assay (Becton Dickinson, USA) is a qualitative real-time PCR test for rapid detection of nasal colonization of methicillin-resistant Staphylococcus aureus (MRSA). We evaluated the performance of BD GeneOhm MRSA PCR assay versus MRSASelect (Bio-Rad, France) and broth enrichment cultures for detection of MRSA from nasal swabs. From August 2008 to January 2009, 295 nasal swabs were taken from patients in intensive care units and transported to the laboratory with BD CultureSwab Liquid Stuart Single Swab (Becton Dickinson, USA). The swabs were inoculated onto MRSASelect first and then suspended into GeneOhm sample buffer: 100 microL of the suspension was inoculated into 6.5% NaCl-tryptic soy broth (Becton Dickinson, USA), which was subcultured on MRSASelect after overnight incubation (TSBS). Performances of GeneOhm MRSA and MRSASelect were compared to TSBS. With GeneOhm MRSA, 125 swabs (44.6%) were positive for MRSA, 13 (4.4%) were unresolved, and 2 were not determined. With MRSASelect and TSBS 86 (29.4%) and 106 swabs (36.2%), respectively, were positive. The sensitivity, specificity, and positive and negative predictive value of GeneOhm MRSA were 85.8%, 77.5%, and 72.8% and 93.5%, respectively, and corresponding values for MRSASelect were 78.3%, 94.8%, and 96.5% and 88.9%. Of the 33 patients whose 34 specimens were found false positive in GeneOhm MRSA, 23 patients were MRSA-positive either previously or subsequently to this study. All of the 10 patients with false-negative specimens in GeneOhm MRSA PCR assay were previously MRSA or methicilln-resistant coagulase negative staphylococci (MRCNS)-positive and were treated for MRSA, but they became MRSA-positive after 1 to 4 negative surveillance cultures. GeneOhm MRSA PCR assay showed a relatively high negative predictive value. However, its low specificity and frequent occurrence of unresolved results would be problematic in the endemic areas with a high prevalence of MRSA.

Comparison of the BD™ GeneOhm MRSA assay, broth enrichment culture and chromID™ MRSA for detection of methicillin-resistant Staphylococcus aureus from inter-hospital intensive care transfer patients

The objective of the present study was to compare the current chromogenic methicillin-resistant Staphylococcus aureus (MRSA) agar plate culture method with broth enrichment and the BD GeneOhm MRSA assay. Another key objective was to determine whether two admission surveillance swabs collected 1 h apart would reliably detect MRSA colonisation in patients transferred to a tertiary referral intensive care unit from other hospitals compared with the current routine method of two samples collected 24 h apart. In total, 593 swabs from 102 consecutive patients transferred from another hospital to the Princess Alexandra Hospital ICU were screened for the MRSA using nose and groin swabs collected on admission, 1 h and 24 h after admission. The non-selective broth enrichment step produced results in complete concordance with the existing method without an increase in sensitivity. The GeneOhm MRSA assay demonstrated 100% sensitivity, 97% specificity and 100% negative predictive values. The initial positive predictive value of this assay, however, was only 28%, largely due to the low prevalence of MRSA. Despite this low positive predictive value and because of the demonstrated 100% negative predictive value, the use of this assay could significantly improve turn-around times of surveillance screens in our laboratory by obviating the need for culture of over 90% of MRSA screening swabs. Positive polymerase chain reaction results require confirmation by culture, given that phenotypical characterisation of isolates is required for infection prevention and control. Comparison of the two screening collection timings in determining MRSA carriage could not be answered, due to insufficient positive results in the study group.

Evaluation of a commercial MRSA assay when multiple MRSA strains are causing epidemics

Rapid and reliable diagnostic methods are needed to control methicillin-resistant Staphylococcus aureus (MRSA) transmission. We studied the BD GeneOhm MRSA Assay which is based on one specific amplification product at the junction of the right extremity sequence of the staphylococcal cassette chromosome mec (SCCmec) and the chromosomal sequence of orfX of S. aureus. The test was applied on 95 clinical isolates in Finland: 83% were positive. The isolates giving negative results represented several pulsed-field gel electrophoresis (PFGE) types and harboured SCCmec types IV, V, VI or were new types with different combinations of ccr genes.