Can the Xpert MRSA/SA BC assay be used as an antimicrobial stewardship tool? A prospective assay validation and descriptive impact assessment study in a South African setting

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Abstract Background: Positive blood cultures showing Gram positive cocci in clusters can signify the presence of Staphylococcus aureus or the less-virulent coagulase-negative staphylococci. Rapid identification and methicillin susceptibility determination with the Xpert MRSA/SA BC assay has the potential to improve management of S. aureus bloodstream infection and reduce inappropriate antibiotic use, and to support antibiotic cessation in patients with coagulase-negative staphylococci who have received further workup while awaiting blood culture positivity. Methods: We prospectively evaluated the Xpert MRSA/SA BC assay in comparison with culture-based methods on samples referred to our laboratory in the Western Cape, South Africa. We interviewed attending clinicians upon culture result availability, to assess antibiotic choices and to estimate potential impact of the assay.Results: We included 231 samples, for which the error rate was 1.7%. There was 100% concordance between the Xpert MRSA/SA BC assay and culture-based methods (methicillin-resistant S. aureus 15/15, methicillin-susceptible S. aureus 42/42, coagulase-negative staphylococci 170/170). We estimated a time saving of approximately 30 hours in time to final result with the assay. Of the 177 patients with adequate antibiotic history, changes to optimise antistaphylococcal therapy could have occurred more than one day sooner in 68.9% with S. aureus bloodstream infection (31/45, 95% CI 53.2-81.4%). Six of the 11 patients with methicillin-resistant S. aureus bloodstream infection (54.5%) could have received anti-MRSA cover sooner. Fifty-three days of antibiotic therapy could have been spared in these 177 based on use of this assay, equating to 0.3 days (95% CI, 0.2-0.4) saved per patient. The most substantial reductions were for broad-spectrum beta-lactam agents (31 days, in 17.5% of the cohort), aminoglycosides (7 days) and vancomycin (5 days).Conclusion: This assay has potential for use as an antibiotic stewardship tool; further studies should assess costing and impact on clinical outcome in patients with S. aureus bloodstream infection.

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