Evaluation of BD MAX Staph SR Assay for Differentiating Between Staphylococcus aureus and Coagulase-Negative Staphylococci and Determining Methicillin Resistance Directly From Positive Blood Cultures

Abstract

Background: We evaluated the performance of the BD MAX StaphSR Assay (SR assay; BD, USA) for direct detection of Staphylococcus aureus and methicillin resistance not only in S. aureus but also in coagulase-negative Staphylococci (CNS) from positive blood cultures.

Methods: From 228 blood culture bottles, 103 S. aureus [45 methicillin-resistant S. aureus (MRSA), 55 methicillin-susceptible S. aureus (MSSA), 3 mixed infections (1 MRSA+Enterococcus faecalis, 1 MSSA+MRCNS, 1 MSSA+MSCNS)], and 125 CNS (102 MRCNS, 23 MSCNS) were identified by Vitek 2. For further analysis, we obtained the cycle threshold (Ct) values from the BD MAX system software to determine an appropriate cutoff value. For discrepancy analysis, conventional mecA/mecC PCR and oxacillin minimum inhibitory concentrations (MICs) were determined.

Results: Compared to Vitek 2, the SR assay identified all 103 S. aureus isolates correctly but failed to detect methicillin resistance in three MRSA isolates. All 55 MSSA isolates were correctly identified by the SR assay. In the concordant cases, the highest Ct values for nuc, mecA, and mec right-extremity junction (MREJ) were 25.6, 22, and 22.2, respectively. Therefore, we selected Ct values from 0-27 as a range of positivity, and applying this cutoff, the sensitivity/specificity of the SR assay were 100%/100% for detecting S. aureus, and 97.9%/98.1% and 99.0%/95.8% for detecting methicillin resistance in S. aureus and CNS, respectively.

Conclusions: We propose a Ct cutoff value for nuc/mec assay without considering MREJ because mixed cultures of MSSA and MRCNS were very rare (0.4%) in the positive blood cultures.

Keywords: BD MAX; Coagulase-negative Staphylococcus; Ct value; MREJ; MRSA; Methicillin; Staph SR.

Fig. 1. Distribution of Ct values of nuc gene among the S. aureus and coagulase-negative Staphylococci (A) and mecA/mecC among methicillin-resistant and methicillin-susceptible coagulase-negative Staphylococci (B). The nuc gene assay perfectly distinguishes CNS from SA. Note that most of MRCNS reveal Ct value under 27.

Abbreviations: Ct, cycle threshold; SA, Staphylococcus aureus; CNS, coagulase-negative Staphylococcus; MSCNS, methicillin susceptible CNS; MRCNS, methicillin resistant CNS.

Fig. 2. Distribution of MREJ (A) and mecA/mecC (B) Ct values among methicillin-resistant and methicillin-susceptible S. aureus. Note that there are MREJ negative MRSAs.

Abbreviations: Ct, cycle threshold; SA, Staphylococcus aureus; MSSA, methicillin susceptible SA; MRSA, methicillin resistant SA; MREJ, mec right-extremity junction.