An International, Prospective, Multicenter Evaluation of the Combination of AdvanDx Staphylococcus Quick FISH BC with mecA Xpress FISH for Detection of Methicillin-Resistant Staphylococcus aureus Isolates from Positive Blood Cultures

Sepsis caused by Staphylococcus aureus is 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 Staphylococcus Quick FISH BC and the new mecA Xpress FISH assay (both AdvanDx, Woburn, MA, USA) for the detection of methicillin-resistant S. aureus in 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 Quick FISH, a 20-min assay. If only S. aureus was detected, mecA Xpress FISH testing followed. The recovered S. aureus isolates were tested by cefoxitin disk diffusion as the reference method. The Quick FISH 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. aureus organisms (sensitivity, 99.4%; specificity, 99.6%). Approximately 60% of the samples (730) contained coagulase-negative staphylococci or nonstaphylococci as assessed by the Quick FISH assay and were not tested further. The 458 compliant samples positive exclusively for S. aureus by the Quick FISH assay were tested by the mecA Xpress FISH assay, which detected 209 of 211 methicillin-resistant S. aureus organisms (sensitivity, 99.1%; specificity, 99.6%). The mecA Xpress FISH 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 Staphylococcus Quick FISH BC and mecA Xpress FISH assays is sensitive, specific, and reproducible for the detection of methicillin-resistant S. aureus and yields complete results in 2 h after the blood culture turns positive.