An International, Prospective, Multicenter Evaluation of the Combination of AdvanDx StaphylococcusQuickFISH BC with mecA XpressFISH for Detection of Methicillin-Resistant Staphylococcus aureusIsolates from Positive Blood Cultures

ABSTRACTSepsis caused by Staphylococcus aureusis a major health problem worldwide. Better outcomes are achieved when rapid diagnosis and determination of methicillin susceptibility enable early optimization of antimicrobial therapy. Eight large clinical laboratories, seven from the United States and one from Scotland, evaluated the combination of the StaphylococcusQuickFISH BC and the new mecA XpressFISH assay (both AdvanDx, Woburn, MA, USA) for the detection of methicillin-resistant S. aureusin positive blood cultures. Blood cultures flagged as positive by automated blood culture instruments and demonstrating only Gram-positive cocci in clusters on Gram stain were tested by QuickFISH, a 20-min assay. If only S. aureuswas detected, mecA XpressFISH testing followed. The recovered S. aureusisolates were tested by cefoxitin disk diffusion as the reference method. The QuickFISH assay results were concordant with the routine phenotypic testing methods of the testing laboratories in 1,211/1,221 (99.1%) samples and detected 488/491 S. aureusorganisms (sensitivity, 99.4%; specificity, 99.6%). Approximately 60% of the samples (730) contained coagulase-negative staphylococci or nonstaphylococci as assessed by the QuickFISH assay and were not tested further. The 458 compliant samples positive exclusively for S. aureusby the QuickFISH assay were tested by the mecA XpressFISH assay, which detected 209 of 211 methicillin-resistant S. aureusorganisms (sensitivity, 99.1%; specificity, 99.6%). The mecA XpressFISH assay also showed high reproducibility, with 534/540 tests performed by 6 operators over 5 days achieving reproducible results (98.9% agreement). The combination of the StaphylococcusQuickFISH BC and mecA XpressFISH assays is sensitive, specific, and reproducible for the detection of methicillin-resistant S. aureusand yields complete results in 2 h after the blood culture turns positive.