Methicillin-resistant Staphylococcus Aureus PCR Research Articles

513 Predictive Values of MRSA Nasal Swabs for Pneumonia in Burn ICU Patients

Abstract Introduction Sepsis in burn-injured patients remains a leading cause of mortality in those who survive initial resuscitation. Methicillin-resistant Staphylococcus aureus (MRSA) is a common causative pathogen for pneumonia in burn-injured patients. Studies evaluating the predictive value of MRSA nasal swabs for pneumonia have largely excluded the burn-injured population. Methods Patients 18 years or older admitted to the Burn ICU at a tertiary medical center from 2016 to 2021 were included if they had any burns, a pneumonia ICD-10 code, an MRSA nasal swab obtained during admission, and any respiratory cultures associated with at least five consecutive days of antibiotics. Results There were 267 instances of pneumonia across 136 patients included. MRSA nasal swabs had an overall sensitivity of 39%, specificity of 98.7%, positive predictive value (PPV) of 84.2%, and negative predictive value (NPV) of 89.9%. MRSA nasal swabs obtained less than seven days from antibiotic initiation had a specificity of 98.6% and NPV of 98.6%; meanwhile, swabs obtained at least seven days from antibiotic initiation had a specificity of 98.7% and NPV of 86.4%. Conclusions The high specificity and NPV indicate that negative MRSA nasal swabs obtained less than seven days from antibiotic initiation may be used to de-escalate anti-MRSA antibiotics in clinically stable burn-injured patients with suspicion of pneumonia. The decrease in NPV suggests that it may be beneficial to obtain a repeat swab periodically or upon suspicion of pneumonia. Applicability of Research to Practice This study is the first to measure the sensitivity, specificity, PPV, and NPV of the MRSA PCR nasal swab in a cohort comprised only of patients admitted to a burn intensive care unit. The findings support the use of the MRSA nasal swab as a tool to aid in the decision to initiate or de-escalate anti-MRSA antimicrobials for pneumonia.

Evaluation of the reliability of MRSA screens in patients undergoing universal decolonization.

Colonization of methicillin-resistant Staphylococcus aureus (MRSA) can be detected via nasal screens. Evidence indicates that negative MRSA nasal screens may be used to de-escalate anti-MRSA antibiotics in pulmonary infections. In the ICU, universal decolonization with intranasal mupirocin is implemented to reduce MRSA infection risk. This study aimed to determine whether mupirocin administration affects the reliability of MRSA PCR nasal screens. This retrospective study divided subjects based on timing of intranasal mupirocin administration-before and after MRSA screen. Subjects with confirmed pulmonary infection that received vancomycin, blood/respiratory cultures, and had MRSA PCR screen collected were included. Subjects with concurrent infection requiring vancomycin or MRSA infection in prior 30 days were excluded. Primary outcome of this non-inferiority study was the negative predictive value (NPV) of the screen. Secondary outcomes included the positive predictive value (PPV), sensitivity, and specificity of the screen and duration of vancomycin. Ultimately, 125 subjects were included in each group. The NPV in the group receiving mupirocin before screen was 95.2%, whereas the NPV in the group receiving mupirocin after screen was 99%. The difference between groups was -3.8% (90% CI -7.8%-0.2%; p=0.31), which failed to meet non-inferiority criteria. The secondary outcomes of PPV, sensitivity and specificity of the screen were similar in both groups. The duration of vancomycin was significantly longer in subjects receiving mupirocin before screen (3 days vs. 2 days; p<0.05). Intranasal mupirocin prior to the screen may reduce NPV in pulmonary infections. Approach de-escalation of vancomycin based on screen results with caution.

Impact of Hand Hygiene Simulation Driven by Neonatal Intensive Care Unit Multidisciplinary Team on Hospital Associated Methicillin-resistant Staphylococcus Aureus Screens

<h3>Background</h3> Hospital-associated infections (HAI) such as methicillin-resistant Staphylococcus aureus (MRSA) can have devastating consequences. The World Health Organization created the 5 moments for hand hygiene. It was identified in the neonatal intensive care unit (NICU) pod setting that health care workers did not always recognize opportunities due to workflow complexities. It was hypothesized that introducing hand hygiene simulations would increase hand hygiene awareness which would decrease prevalence of HAI MRSA screen positive NICU infants. <h3>Methods</h3> This was a pre/post study performed in a level 4 NICU with 54 beds from February 2018 through November 2019. Participants included 146 nurses, 30 physicians, 18 respiratory therapists, and 5 additional therapists. In January 2019, each discipline created a scenario which guided the participant through a clinical situation and monitored opportunities for hand hygiene. Simulations were carried out in empty pods in the NICU to closely mimic actual working conditions. MRSA screening was performed via nasal MRSA PCR or culture with testing of infants born outside the hospital on admission and routine testing of all infants every two weeks. The rate of MRSA-positive infants' pre-intervention and post-intervention was obtained, and a chi-squared test was utilized to compare the rates and determine statistical significance (p-value <.05). <h3>Results</h3> In the 11 months prior to the intervention, there were 13 infants with positive HAI MRSA screens across 11,010 patient days (1 per 847). In the subsequent 11 months, there were 6 infants with positive HAI MRSA screens across 12,191 patient days (1 per 2,032) When comparing these two rates, the p-value is 0.067. <h3>Conclusions</h3> We observed a decrease in the rate of infants with positive HAI MRSA screens following our introduction of hand hygiene simulations although that difference did not reach statistical significance. The fact that the scenarios were created by frontline care providers was one of the keys to success.

1999. Does Pharmacist-Driven Methicillin-Resistant Staphylococcus aureus PCR Nasal Screening Decrease Time to De-Escalation of MRSA Coverage in Patients with Pneumonia?

BackgroundVancomycin and linezolid are antibiotics used in cases where methicillin-resistant Staphylococcus aureus (MRSA) is suspected, including in cases where MRSA is suspected to be the cause of pneumonia. MRSA nasal PCR has been shown to have a high negative predictive value when used to rule out MRSA pneumonia. The purpose of the current study was to determine whether a pharmacist-driven MRSA PCR nasal screening protocol would decrease the time to de-escalation or discontinuation of anti-MRSA therapy when utilized for pneumonia.MethodsPatients were analyzed in two cohorts, those who received vancomycin or linezolid therapy from October 2012 to February 2013 (before pharmacist-driven MRSA nasal PCR protocol; n = 88) and those who received vancomycin from October 2016 to February 2017 (pharmacist-driven MRSA nasal PCR protocol; n = 105). During the study period, pharmacists were given the authority, via protocol to order an MRSA nasal PCR when vancomycin or linezolid was ordered for the indication of pneumonia. Subsequently, after a negative MRSA nasal PCR, pharmacists would contact the prescriber, and let the prescriber know that the MRSA PCR was negative, and then discontinue anti-MRSA therapy. The primary outcome was duration in hours of active anti-MRSA therapy. Secondary outcomes evaluated were the number of anti-MRSA antibiotic doses ordered, and the number of vancomycin troughs ordered.ResultsPatients in the pre-pharmacist driven cohort received vancomycin or linezolid for a median of 44.19 hours, whereas patients in the pharmacist-driven MRSA PCR protocol period received anti-MRSA therapy for a median of 19.1 hours (P < 0.0001). Additionally, prior to the initiation of the pharmacist-driven MRSA nasal PCR protocol, patients received 349 doses of anti-MRSA therapy, compared with 283 doses in the pharmacist MRSA nasal swab protocol group (P < 0.0001). There were also fewer vancomycin troughs ordered in the pharmacist MRSA nasal PCR protocol group (76 vs. 48, P < 0.0009).ConclusionA pharmacist-driven protocol for ordering MRSA nasal PCR led to a statistically significant decrease in the time to discontinuation of vancomycin or linezolid for suspected MRSA pneumonia when the MRSA nasal PCR was negative.Disclosures All authors: No reported disclosures.

Nasal Methicillin-Resistant Staphylococcus aureus (MRSA) PCR Testing Reduces the Duration of MRSA-Targeted Therapy in Patients with Suspected MRSA Pneumonia.

The objective of this study was to evaluate the impact of pharmacist-ordered methicillin-resistant Staphylococcus aureus (MRSA) PCR testing on the duration of empirical MRSA-targeted antibiotic therapy in patients with suspected pneumonia. This is a retrospective analysis of patients who received vancomycin or linezolid for suspected pneumonia before and after the implementation of a pharmacist-driven protocol for nasal MRSA PCR testing. Patients were included if they were adults of >18 years of age and initiated on vancomycin or linezolid for suspected MRSA pneumonia. The primary endpoint was the duration of vancomycin or linezolid therapy. After screening 368 patients, 57 patients met inclusion criteria (27 pre-PCR and 30 post-PCR). Baseline characteristics were similar between the two groups, with the majority of patients classified as having health care-associated pneumonia (68.4%). The use of the nasal MRSA PCR test reduced the mean duration of MRSA-targeted therapy by 46.6 h (74.0 ± 48.9 h versus 27.4 ± 18.7 h; 95% confidence interval [CI], 27.3 to 65.8 h; P < 0.0001). Fewer patients in the post-PCR group required vancomycin serum levels and dose adjustment (48.1% versus 16.7%; P = 0.02). There were no significant differences between the pre- and post-PCR groups regarding days to clinical improvement (1.78 ± 2.52 versus 2.27 ± 3.34; P = 0.54), length of hospital stay (11.04 ± 9.5 versus 8.2 ± 7.8; P = 0.22), or hospital mortality (14.8% versus 6.7%; P = 0.41). The use of nasal MRSA PCR testing in patients with suspected MRSA pneumonia reduced the duration of empirical MRSA-targeted therapy by approximately 2 days without increasing adverse clinical outcomes.

Direct detection of methicillin-resistant Staphylococcus aureus in sputum specimens from patients with hospital-associated pneumonia using a novel multilocus PCR assay.

Methicillin-resistant Staphylococcus aureus (MRSA) is a significant cause of hospital-associated pneumonia (HAP). The rapid identification of MRSA would be beneficial for early diagnosis. The study aimed to evaluate a multilocus, fluorescence-based PCR assay based on the detection of mecA and nuc genes for identification of S. aureusin lower respiratory tract (LRT) specimens. Sensitivity and specificity of the PCR assay were analyzed. Clinical evaluation for the assay was performed using LRT specimens from patients with HAP, and the sensitivity, specificity, positive and negative predictive values (PPV and NPV) were evaluated in comparison with semi-quantitative culture methods. The result showed the assay provided positive identification of all MRSA reference strains with a limit of detection for MRSA of 4 × 103 CFU/mL. Compared with semi-quantitative culture, the sensitivity, specificity, PPV and NPV were 100%, 89.6%, 75.0%, and 100%, respectively. A positive correlation between MRSA bacterial colonies and PCR copy number was found. The specificity and PPV reached 96.6% and 89.7% respectively, if the PCR copy number reached a definite positive threshold of 5.96 × 105. It suggested that this novel multilocus, fluorescence-based PCR assay proved to be a fast, sensitive and specific tool for direct detection of MRSA from LRT specimens.

Rapid Detection of Methicillin-Resistant Staphylococcus Aureus using Bubble-Free Microfluidic PCR

Methicillin-resistant Staphylococcus aureus (MRSA) is the most common hospital-acquired infection and resistant to certain drugs; therefore, reliable PCR detection of MRSA is critical for early prevention of disease spread and the effective treatment of infections. The major problems of microfluidic PCR are water evaporation, loss of reagents, and inconsistent optical path-length due to random bubble generations during the thermal cycling. We report a bubble-free microfluidic PCR for the detection of MRSA. In order to avoid the bubble generation, we utilized polyethylene-based microfluidic devices since it has low gas permeability. The proposed polyethylene microfluidic PCR also offers a uniform heat distribution without the amplification inhibition by a temperature drop in the aqueous nucleic acid sample. The polyethylene layer was spin-coated over PDMS channel for effective hybrid integration of two different functions of polymeric materials (i.e. degasing pump for PDMS and polyethylene for uniform PCR and optical path-length). Consequently overall sample loss such as PCR reagent and nucleic acid was almost eliminated by bubble-free condition. The detection limit of mecA which is methicillin-resistance gene was found to be 6.4 x 102 copies of MRSA genomic DNA by the end-point fluorescence analysis for approximately 30 min reaction. The bubble-free multiplexed microfluidic MRSA PCR array will benefit for rapid detection and drug resistance determination effectively.

Predictive Value of Methicillin-Resistant Staphylococcus aureus (MRSA) Nasal Swab PCR Assay for MRSA Pneumonia

Pneumonia due to methicillin-resistant Staphylococcus aureus (MRSA) is associated with poor outcomes and frequently merits empirical antibiotic consideration despite its relatively low incidence. Nasal colonization with MRSA is associated with clinical MRSA infection and can be reliably detected using the nasal swab PCR assay. In this study, we evaluated the performance of the nasal swab MRSA PCR in predicting MRSA pneumonia. A retrospective cohort study was performed in a tertiary care center from January 2009 to July 2011. All patients with confirmed pneumonia who had both a nasal swab MRSA PCR test and a bacterial culture within predefined time intervals were included in the study. These data were used to calculate sensitivity, specificity, positive predictive value, and negative predictive value for clinically confirmed MRSA pneumonia. Four hundred thirty-five patients met inclusion criteria. The majority of cases were classified as either health care-associated (HCAP) (54.7%) or community-acquired (CAP) (34%) pneumonia. MRSA nasal PCR was positive in 62 (14.3%) cases. MRSA pneumonia was confirmed by culture in 25 (5.7%) cases. The MRSA PCR assay demonstrated 88.0% sensitivity and 90.1% specificity, with a positive predictive value of 35.4% and a negative predictive value of 99.2%. In patients with pneumonia, the MRSA PCR nasal swab has a poor positive predictive value but an excellent negative predictive value for MRSA pneumonia in populations with low MRSA pneumonia incidence. In cases of culture-negative pneumonia where initial empirical antibiotics include an MRSA-active agent, a negative MRSA PCR swab can be reasonably used to guide antibiotic de-escalation.

A New Triplex Real Time PCR which Distinguishes Between MRSA, MSSA, and mecA Coagulase Negative Strains by Means of Melting Point Analysis Using SYTO 9

A new screening method was developed for the detection and identification of methicillin resistant staphylococcus aureus (MRSA) from sterile sites or mixed flora samples (inguinal or nose swabs). After rapid treatment of samples, the method consists of two steps, template DNA preparation by a simple and rapid boiling procedure and a multiplex real time PCR. The triplex PCR system simultaneously detects the following targets, (i) the integration site for the open reading frame X (orfX) of the staphylococcal cassette chromosome mec type I - V (SCCmec), (ii) the mecA gene which codes for the penicillin-binding protein PBP-2a, and (iii) an internal control (IC) which can be amplified with the SCCmec primer system. A new buffer system, which contains the fluorescent dye SYTO 9, allows a reproducible real time PCR for the discrimination of the above mentioned PCR products by means of a high resolution melting point analysis (HRM). This new PCR system distinguishes between MRSA, MSSA, and mecA positive but coagulase-negative staphylococci (CoNS) strains. An internal control confirms the integrity of the PCR run. The HRM shows three melting points specific for each amplification product. 78.75 degrees C for the mecA gene, 83.15 degrees C for the SCCmec/orfX fragment and 88.25 degrees C for the internal control. This new multilocus MRSA PCR system is a fast and inexpensive alternative to commercially available test systems and costs only five to six euros.

Molecular Epidemiology of Methicillin-Resistant Staphylococcus aureus Isolates from Patients Newly Identified as Nasal Carriers

We aimed to determine whether additional molecular and microbiological evaluations of methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients newly identified as nasal carriers were useful for control strategies and whether longitudinal testing during the same or repeat hospitalization changed MRSA status. Nasal swabs from patients positive by Xpert MRSA PCR and not known to be colonized in the previous year were cultured for S. aureus. Isolates were tested for resistance to a variety of antibiotics, including high-level mupirocin resistance (HLMR) and low-level mupirocin resistance (LLMR) and the presence of genes mecA and mupA and those for Panton-Valentine leukocidin (PVL), USA300, and USA400. Repeat nasal screens during the 6-month study were tested for continued presence of MRSA. Among 130 patients, cultures revealed MRSA in 85 (65.4%), methicillin-susceptible S. aureus in 19 (14.6%), and no growth in 26 (20%). MRSA isolates were USA300 positive in 13/85 (15.3%) and LLMR in 8/85 (9.4%) patients. No isolates were HLMR or mupA positive. mecA dropout was detected in 9/130 (6.9%) patients. The rate of subsequent MRSA infections in USA300-positive versus -negative patients was not different. MRSA nasal status remained concordant in 69/70 (98.6%) patients who had follow-up testing. The findings do not support expanding MRSA surveillance to include routine detection of genes for USA300, PVL, or mupA, all of which were either of low frequency or not significantly associated with MRSA infection risk in our population of newly identified nasal carriers. Repeat nasal screening for MRSA during the same or subsequent hospitalizations over 6 months could also be deferred, reducing costs associated with screening.

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.

Costs and benefits of rapid screening of methicillin-resistant Staphylococcus aureus carriage in intensive care units: a prospective multicenter study.

IntroductionPre-emptive isolation of suspected methicillin-resistant Staphylococcus aureus (MRSA) carriers is a cornerstone of successful MRSA control policies. Implementation of such strategies is hampered when using conventional cultures with diagnostic delays of three to five days, as many non-carriers remain unnecessarily isolated. Rapid diagnostic testing (RDT) reduces the amount of unnecessary isolation days, but costs and benefits have not been accurately determined in intensive care units (ICUs).MethodsEmbedded in a multi-center hospital-wide study in 12 Dutch hospitals we quantified cost per isolation day avoided using RDT for MRSA, added to conventional cultures, in ICUs. BD GeneOhm™ MRSA PCR (IDI) and Xpert MRSA (GeneXpert) were subsequently used during 17 and 14 months, and their test characteristics were calculated with conventional culture results as reference. We calculated the number of pre-emptive isolation days avoided and incremental costs of adding RDT.ResultsA total of 163 patients at risk for MRSA carriage were screened and MRSA prevalence was 3.1% (n = 5). Duration of isolation was 27.6 and 21.4 hours with IDI and GeneXpert, respectively, and would have been 96.0 hours when based on conventional cultures. The negative predictive value was 100% for both tests. Numbers of isolation days were reduced by 44.3% with PCR-based screening at the additional costs of €327.84 (IDI) and €252.14 (GeneXpert) per patient screened. Costs per isolation day avoided were €136.04 (IDI) and €121.76 (GeneXpert).ConclusionsIn a low endemic setting for MRSA, RDT safely reduced the number of unnecessary isolation days on ICUs by 44%, at the costs of €121.76 to €136.04 per isolation day avoided.

Rapid diagnostic testing of methicillin-resistant Staphylococcus aureus carriage at different anatomical sites: costs and benefits of less extensive screening regimens

Multiple body site screening and pre-emptive isolation of patients at risk for methicillin-resistant Staphylococcus aureus (MRSA) carriage are considered essential for control of nosocomial spread. The relative importance of extranasal screening when using rapid diagnostic testing (RDT) is unknown. Using data from a multicentre study evaluating BD GeneOhm™ MRSA PCR (IDI), Xpert MRSA (GeneXpert) and chromogenic agar, added to conventional cultures, we determined cost-effectiveness assuming isolation measures would have been based on RDT results of different hypothetical screening regimes. Costs per isolation day avoided were calculated for regimes with single or less extensive multiple site RDT, regimes without conventional back-up cultures and when PCR would have been performed with pooling of swabs. Among 1764 patients at risk, MRSA prevalence was 3.3% (n = 59). In all scenarios the negative predictive value is above 98.4%. With back-up cultures of all sites as a reference, the costs per isolation day avoided were £15.19, £30.83 and £45.37 with ‘nares only’ screening using chromogenic agar, IDI and GeneXpert, respectively, as compared with £19.95, £95.77 and £125.43 per isolation day avoided when all body sites had been screened. Without back-up cultures costs per isolation day avoided using chromogenic agar would range from £9.24 to £76.18 when costs per false-negative RDT range from £5000 up to £50 000; costs for molecular screening methods would be higher in all scenarios evaluated. In conclusion, in a low endemic setting chromogenic agar screening added to multiple site conventional cultures is the most cost-effective MRSA screening strategy.

Preemtive Isolation for Methicillin-Resistant Staphylococcus aureus: Development of a Predictive Model in a VA Hospital

In 2007, the Department of Veterans Affairs (VA) began a nationwide initiative to combat the burden of Methicillin-resistant Staphylococcus aureus (MRSA). This included MRSA screening for all hospital admissions. Although MRSA PCR results are available within hours of screening, patients are still admitted before MRSA statuses are known. This time period allows opportunity for MRSA transmission. In addition, patients admitted to a double occupancy room who then screen positive for MRSA must be moved, resulting in additional personnel time, patient dissatisfaction and admission delays.

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.

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.

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

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.

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 Chromogenic Media to BD GeneOhm Methicillin-Resistant Staphylococcus aureus (MRSA) PCR for Detection of MRSA in Nasal Swabs

To select a method for detecting methicillin-resistant Staphylococcus aureus (MRSA) in nasal swabs, we compared BD GeneOhm MRSA PCR and various culture media (mannitol salt agar with cefoxitin, MRSASelect, CHROMagar MRSA, and Spectra MRSA). While PCR detection of MRSA was more rapid, MRSASelect and Spectra MRSA demonstrated performance equivalent to that of PCR with maximal detection at 24 h.

MVPlex Assay for Direct Detection of Methicillin-Resistant Staphylococcus aureus in Naris and Other Swab Specimens

We evaluated the MVPlex assay (Geneco Biomedical Products), which uses target-enriched multiplex PCR amplification followed by liquid array identification, for the detection of methicillin-resistant Staphylococcus aureus (MRSA) from 307 dual-swab specimens. By using a combination of culture (Trypticase soy agar-5% sheep blood agar and Columbia CNA agar-5% sheep blood) and an FDA-approved MRSA PCR assay as the "gold standard," the MVPlex MRSA assay and culture were found to have sensitivities of 97.8% and 84.4% (P = 0.002) and specificities of 95.8% and 98.6% (P < 0.05), respectively.