Rapid identification of methicillin-resistant staphylococci bacteremia among intensive care unit patients.

Abstract

Staphylococci represent the most commonly encountered blood culture isolates. With the spread of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant coagulase-negative staphylococci (MRCoNS) in hospitals, rapid and reliable methods for their detection are warranted in order to provide choice of appropriate antimicrobial therapy. This study evaluated 4 rapid methods directly from positive blood cultures in parallel with each other (on the same day) for identification of methicillin-resistant staphylococcal isolates, in addition to antimicrobial susceptibility testing (AST), to compare the workflow for each test and to reduce the turnaround time (TAT) in order to be presented as practical applications in our microbiology laboratory. A total of 56 bacteremic patients' blood cultures with Gram stains showing gram-positive cocci (GPC) in clusters were included. The following direct assays were evaluated: direct tube coagulase (DTC) test, analytical profile index (API)-Staph kit for species identification coupled with antimicrobial susceptibility testing (AST), latex agglutination for detection of PBP2a (PBP2a LA Assay), and cefoxitin disk diffusion assay. The direct results were compared with results obtained with isolated colonies using standard methods as well as detection of the mecA gene by PCR. DTC and API-staph exhibited sensitivities of 96% and 96.8% and specificity of 100% for direct identification of staphylococcal isolates. Both PBP2a LA and cefoxitin DD assays exhibited sensitivity of 100% for detection of both MRSA and MRCoNS and specificities of 100% and 75% (PBP2a assay) and 90% and 100% (cefoxitin DD) for identification of methicillin-sensitive isolates, respectively. For direct antimicrobial susceptibility testing (DAST), the overall error rate was 1.11%. In conclusion, direct identification and susceptibility testing by any of these assays yielded acceptable performance and timely results - 24 hours earlier than routine subculture - and can be easily incorporated into routine processing of positive blood cultures to improve the outcomes for the patient and the costs to hospitals. Therefore, it is recommended to use the method with high sensitivity and the shortest TAT.