Direct Disk Diffusion Susceptibility Testing for Staphylococcus aureus from Blood Cultures: Diagnostic Accuracy and Impact on Antimicrobial Stewardship

BACKGROUND: In order to detect multidrug resistant strains of bacteria, our laboratory routinely performs direct susceptibility (DS), in addition to standardized susceptibilities (SS), testing from positive blood cultures. We conducted a prospective study to determine the accuracy, reporting time (RT), and antimicrobial stewardship impact of DS testing for Staphylococcus aureus positive blood cultures. METHODS: From March–December 2016, first time positive blood cultures for S. aureus were included. Broth from positive blood culture bottles was inoculated to standard media, as well as to Mueller–Hinton agar with a cefoxitin disk. CLSI breakpoints for S. aureus were used to guide interpretation. If DS results were to be reported to clinicians, a penicillin-binding protein 2a (PBP2a) Alere™ test was performed. When the PBP2a result was concordant with the CLSI interpretation, the isolate was reported as either Methicillin-susceptible S. aureus (MSSA) or Methicillin-resistant S. aureus (MRSA). Antibiotic therapy changes made based on reporting of DS results were recorded. In order to determine RT, the following time points were recorded: blood culture positivity, reading of DS, and reporting of SS RESULTS: Of the 100 patients with S. aureus bacteremia, 97 showed pure growth of MSSA or MRSA; 3 patients had mixed infections, all of which were only detected using the DS plates. Average RT was 23 hours and 36h for DS and SS, respectively. There were 32 MRSA isolates, with a cefoxitin zone size range of 6–15 mm (median = 8 mm). PBP2a was performed on 11 isolates; all were positive. Of the 69 MSSA isolates, the cefoxitin range was 22 to 32 mm (median = 27 mm). PBP2a was performed on 26 isolates; all were negative. Direct susceptibility results were reported on 31 patients. Of the 21 patients with MSSA bacteremia, 15 changed therapy from vancomycin/daptomycin to cloxacillin/cefazolin. These results were reported an average of 23 hours prior to SS. CONCLUSION: DS testing is an accurate and rapid method to determine whether isolates are MSSA or MRSA. We had no major or minor errors. PBP2a testing was concordant for all isolates tested. DS also has the added benefit of detecting mixed S. aureus infections. Clinicians acted on the reported results of DS testing, with 15/21 (71%) of our patients narrowed to a cloxacillin/cefazolin 23 hours before the availability of SS. DISCLOSURES: All authors: No reported disclosures.