bWe evaluated the performance of the BD Max StaphSR assay for the direct detection of Staphylococcus aureus from blood culture medium. In a two-center trial, 155 blood cultures from the BD Bactec FX system and 212 from the bioMérieux BacT/Alert system were tested; 170 bottles yielded S. aureus, and all were identified correctly by the BD Max StaphSR assay. The assay required approximately 2.5 h, thus allowing rapid identification of blood cultures flagged positive.
The BD Max StaphSR assay (BD, Quebec, Canada) is a fully automated, qualitative, in vitro diagnostic test for the direct detection of Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) in patients at risk for nasal colonization. The test uses real-time PCR for the amplification of three DNA targets, i.e., SCCmec right-extremity junction (MREJ), thermostable nuclease (nuc), and methicillin resistance (mecA and mecC). Positivity for MREJ and mecA/mecC is required for the result "MRSA," and detection of nuc or MREJ without mecA/mecC is interpreted as "positive for S. aureus." The BD Max StaphSR assay was shown to have excellent sensitivity (96.4%) and specificity (93.6%), compared to combined direct and enriched cultures, for detection of S. aureus from anterior nares samples (1). In this study, we tested the performance of the StaphSR assay with a different sample type, namely, positive blood culture medium with Gram-positive cocci in clusters.Bloodstream infections often result in the need for intensive care and are associated with high mortality rates and large economic burdens (2). S. aureus is one of the main causes of bloodstream infections (3, 4). Rapid identification of positive blood cultures is a prerequisite for timely targeted treatment of patients with sepsis (5-7). Quick differentiation of S. aureus from coagulase-negative Staphylococcus (CoNS) can reduce the costs of patient care and decrease hospitalization times (8). The use of molecular assays for direct detection of S. aureus in blood culture bottles has been shown to allow rapid diagnosis of S. aureus septicemia, which might improve patient treatment (6, 9, 10). The recently introduced BD Max system allows fully automated realtime PCR, but the currently available BD Max StaphSR assay is CE marked and FDA cleared only for use with nasal swabs (11).In a first stage, we tested the use of the StaphSR assay with blood culture bottles (Bactec Plus Aerobic/F or Bactec Plus Anaerobic/F) that had been artificially inoculated with 1,000 CFU of MRSA, methicillin-susceptible S. aureus (MSSA), or methicillinresistant coagulase-negative Staphylococcus (MR-CoNS). After bottles were flagged positive, aliquots from those bottles were tested by inoculating the sample buffer tube (SBT) of the assay.The blood culture bottles were tested again after an additional 18 to 24 h of incubation, to mimic delayed analysis and to ensure that high DNA levels did not result in competition with any of the PCR target amplifications. We tested three volumes, i.e., 15, 50, and 100 l. All spiked blood cultu...