Methods: We conducted a retrospective analysis to compare accuracy/reliability of dAST using agar-disk diffusion method with a commercially available automated broth microdilution method (MicroScan). We compared categorical agreement (CA) on 776 isolates from positive blood cultures at a university hospital from February 2014-January 2015. A total of 5,346 bug-drug combinations were analyzed to assess the rates of CA, very major errors (VME), major errors (ME), and minor errors. Errors were further stratified based on type of organism and specific drug.
Results: The total CA for all bug-drug combinations was 90.4% with 1.8% VME, 1.9% ME, and 5.8% minor errors. CA for Staphylococcus aureus overall was 94% with excellent CA for penicillin (98.9%), cefoxitin/oxacillin (98.4%), and erythromycin (94.9%). However, clindamycin performed poorly for S. aureus with a CA of only 83.3% due to a high rate of VME. For Enterococcus faecalis, CA was high for penicillin (92.8%) and ampicillin (97.6%) but was only 66.6% for vancomycin due to frequent minor errors and major errors . CA for Enterococcus faecium, however, was excellent for penicillin (100%), ampicillin (100%), and vancomycin (94.1%). Among the Enterobacteriaceae, CA was excellent for ampicillin (95.2%), levofloxacin (97.7%), aminoglycosides (94.1%-98.3%), trimethoprim-sulfamethoxazole (94.1%), and third generation cephalosporins (92.1-96.3%). However, CA for ampicillin-sulbactam (71.7%) and cefazolin were poor (64.8%), largely due to minor errors. For Pseudomonas aeruginosa, there was excellent CA for levofloxacin (95.6%), gentamicin (95.6%), and tobramycin (100%) but not for aztreonam (78.2%) due to increased minor errors.
Conclusion: The accuracy of dAST is comparable to sAST methods for common organisms and provides results 24 hours earlier. Our results suggest that Clindamycin and Vancomycin should not be reported for S. aureus and E. faecalis, respectively. Cefazolin and ampicillin-sulbactam should not be reported for Enterobacteriaceae.
M. Weinstein, None
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