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 Vitek automated susceptibility testing system with a modified gram-posi tive susceptibility (GPS) 106 card (bioMerieux Vitek, Inc., Hazelwood. Mo.) and a rapid slide latex agglutination test (MRSA-Screen test; Denka Seiken Co., Ltd., Tokyo, Japan) were evaluated for their abilities to detect oxac illin resistance in coagulase-negative staphylococci (CoNS). The reference broth microdilution method and the detection of the mecA gene by PCR ("gold standard" reference result) were used to compare the results obtained with the commercial products. A total of 123 clinical isolates consisting of ei ght species were selected from U.S. surveillance collections. Among the mec A-positive isolates (95 strains), 30 isolates were initially negative on th e MRSA-Screen test read at 3 min. When the agglutination reaction was exten ded for 10 min, 26 of the 30 isolates became positive. For a different four isolates, the oxacillin MIC was less than or equal to0.25 pg/ml on the Vit ek GPS 106 card. Among the mecA-negative isolates (28 strains), for two Sta phylococcus warneri, two S. lugdunensis, and two S. saprophyticus strains M ICs were greater than or equal to0.5 mug/ml by the reference broth microdil ution method. Four of these isolates were also categorized as resistant wit h the Vitek GPS 106 card and two isolates were positive by the MRSA-Screen test. Overall, the MRSA-Screen test, GPS 106 card, and reference broth micr odilution method had sensitivities of 95.7 (result at 10 min), 95.7, and 10 0%, respectively, and specificities of 92.8, 85.7, and 78.5%, respectively. Although the MRSA-Screen test required a slight procedural modification, b oth commercial methods achieved a sensitivity and specificity at detecting oxacillin resistance in CoNS at a level that was acceptable for clinical la boratory use.