The abilities of commercial MIC, automated, and reference methods for in vitro detection of methicillinresistant Staphylococcus aureus were determined on 49 strains from eight hospitals. Micro-Media, MicroScan, Sensititre, Sceptor, API Uniscept KB, Abbott MS-2, Vitek AMS, Autobac MTS, NCCLS disk diffusion, and broth microdilution antimicrobial susceptibility testing procedures were evaluated. All testing was performed by using manufacturers' or reference procedures, and results were determined at no later than 24 h of incubation at 35°C. With NCCLS disk diffusion, all strains were resistant to oxacillin (1 ,ug), 47 (96%) were resistant to methicillin (4 jig), and 48 (98%) were resistant to nafcillin (1 ,ug). The percentages of strains resistant to methicillin (>8 ,ug/mI) were 98% with API Uniscept KB, 86% with Sceptor, MicroScan, and Autobac MTS, 84% with Sensititre, 71% with Micro-Media, and 70% with NCCLS MIC. Abbott MS-2 detected 86% of strains resistant to methicillin (>5,ug/ml). With oxacillin (>2 ,ug/mI), 90% were detected with Vitek AMS and 70% were detected with NCCLS MIC. With nafcillin (>2 ,Ig), 82% were resistant with Micro-Media, 57% were resistant by NCCLS MIC, and 50% (three of six) were resistant by MicroScan. Two strains from one hospital and one strain from another gave susceptible results with all automated and commercial methods. All strains from three centers were detected by all methods. Variability also occurred among the systems with cephalothin, clindamycin, gentamicin, chloramphenicol, and trimethoprim sulfamethoxazole.
Susceptibility test results from 100 clinical isolates, using the AMS, MS-2, Autobac MTS, Micro-Media system, and Sensititre, were compared with results from the proposed National Committee for Clinical Laboratory Standards reference microdilution method for minimum inhibitory concentrations and with Bauer-Kirby results. Isolates were tested concurrently by each method on consecutive days to obtain duplicate results. The data were computer analyzed, using National Committee for Clinical Laboratory Standards guidelines for break point interpretation. Analysis was centered on drug-organism combinations and not on overall percent correlation. Data were analyzed for comparability to the reference methods and for reproducibility within each system. Commercial system results were very reproducible. Results from 4to 8-h tests (AMS, MS-2, MTS) gave more very major discrepancies when compared with either reference method than did results from 15to 18-h systems (Micro-Media, Sensititre). The past decade has brought tremendous progress towards standardizing susceptibility testing in clinical microbiology laboratories. Through the efforts and cooperation of investigators from the clinical, research, industrial, professional accrediting, and governmental sectors, disk diffusion and minimum inhibitory concentration (MIC) methods have been, or are being, standardized (8-10). Although the standardized disk agar diffusion method remains the primary choice in clinical microbiology laboratories, many alternate approaches for susceptibility testing are now commercially available. Recently, automated, semiautomated, and commercial microdilution systems have received approval for clinical use. Since physicians frequently treat patients at more than one hospital, they may face interpreting susceptibility results obtained by these different methods. This may present a problem. Historically, qualitative interpretation of susceptibility tests, susceptible, intermediate, or resistant, have been well accepted because of the widespread use of the standardized disk diffusion test (8). The proposed reference method for broth dilution MIC suggests a four-category interpretive reporting system (9). These categories are based on dosage of the antibiotic as well as on its distribution in certain body sites. Of the systems evaluated in this study, the AMS and MS-2 use the susceptible, intermediate, or resistant system; Autobac, the four
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