Comparison of the Vitek Gram-Positive Susceptibility 106 Card and the MRSA-Screen Latex Agglutination Test for Determining Oxacillin Resistance in Clinical Bloodstream Isolates of Staphylococcus aureus

Abstract

The Vitek automated susceptibility testing system with a modified Gram-Positive Susceptibility (GPS) 106 Card (bioMerieux Vitek, Inc., Hazelwood, Mo.) and a rapid slide latex agglutination test (MRSA-Screen; Denka Seiken Co., Ltd., Tokyo, Japan) were evaluated for their ability to detect oxacillin resistance in Staphylococcus aureus. The oxacillin-salt agar screen (OS) test, the reference broth microdilution method, and the detection of the mecA gene by PCR were compared with the commercial products. A total of 200 contemporary (1999) bloodstream infection isolates were collected from the SENTRY Antimicrobial Surveillance Program, representing diverse geographic areas throughout the world. Among the 99 mecA-positive isolates, 3 isolates were found negative by the MRSA-Screen. Another two isolates did not grow on OS plates and had MICs of 0.5 and 2 g/ml with the Vitek GPS card. All 101 mecA-negative isolates were also found negative by the MRSA-Screen and were categorized as susceptible by the GPS card. Overall, the MRSA-Screen, GPS card, and OS test had sensitivities of 96.9, 98.0, and 98.0% and specificities of 100.0, 100.0, and 98.0%, respectively. MRSA-Screen was a rapid (<15 min) and simple test to perform, and the GPS card provided results in <8 h. Both methods were sensitive and specific for detecting staphylococcal oxacillin resistance in the clinical microbiology laboratory.Methicillin-resistant Staphylococcus aureus (MRSA) is a significant pathogen that has steadily emerged over the last four decades to now cause both nosocomial and community-acquired infections (2). The optimal phenotypic method for detecting (methicillin) oxacillin resistance in S. aureus remains controversial. Moreover, errors in the detection of oxacillin resistance can lead to important adverse clinical consequences. False-susceptible results may contribute to treatment failure as well as to the unchecked spread of MRSA due to a failure to apply appropriate infection control measures. False-resistant results contribute to increased health care costs related to unnecessary isolation precautions, in addition to the consequences of overuse of glycopeptide antimicrobials agents, with associated resistance risk (8). The definitive genotypic test for oxacillin resistance in S. aureus (mecA gene detection by PCR) is not practical for routine performance in clinical laboratories. Therefore, there is an urgent need for a rapid, sensitive, and specific test for MRSA that can be easily performed in clinical microbiology laboratories.This study was performed to compare two newly available tests: (i) the Vitek automated susceptibility testing system with the recently modified Gram-Positive Susceptibility (GPS) 106 Card (bioMerieux Vitek, Inc., Hazelwood, Mo.) and (ii) a rapid slide latex agglutination test (16) for the detection of oxacillin resistance in recent (1999) blood culture isolates of S. aureus obtained from patients worldwide.

Comparison of the Vitek Gram-Positive Susceptibility 106 Card, the MRSA-Screen Latex Agglutination Test, and mecA Analysis for Detecting Oxacillin Resistance in a Geographically Diverse Collection of Clinical Isolates of Coagulase-Negative Staphylococci

“…The weakly positive reactions by some isolates at 3 min became stronger by extension of the test interval. Similar results have been reported with methicillinresistant S. aureus (19,20), and the manufacturer recently changed its package insert recommendations, indicating the need for an extended time (10 min) for agglutination.…”

“…The MRSA-Screen test was very rapid (Ͻ1 h) and easy to perform. It has been extensively evaluated mainly for S. aureus and has been found to be very sensitive and specific for the detection of oxacillin resistance (9,19,20). Hussain et al (7) described its reliability in a variety of species of CoNS.…”

Rapid Detection of Antimicrobial-Resistant Organism Carriage: an Unmet Clinical Need

“…Many approaches to the more rapid detection of MRSA and VRE are described in the literature, including several commercial assays (1,38,42,45). However, most of these approaches still require bacterial growth in culture prior to detection and therefore require 24 h or more to complete.…”

Sensitivity and Specificity of a Rapid rRNA Gene Probe Assay for Simultaneous Identification of Staphylococcus aureus and Detection of mecA

“…Alternatively, the Oxoid PBP2Ј latex agglutination (OLA) assay (Oxoid, United Kingdom) is a rapid test for differentiating MRSA from methicillin-sensitive Staphylococcus aureus (MSSA) on agar plates (2,11,14,23,24,25,26,27). Unfortunately, reliable testing of CoNS with the OLA assay requires induction with oxacillin (Ox), and a heavier inoculum size and longer lysis are critical for accurate results (11,14,26).…”