Variation in the abilities of automated, commercial, and reference methods to detect methicillin-resistant (heteroresistant) Staphylococcus aureus

The abilities of commercial MIC, automated, and reference methods for in vitro detection of methicillin-resistant 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 degrees C. With NCCLS disk diffusion, all strains were resistant to oxacillin (1 microgram), 47 (96%) were resistant to methicillin (4 micrograms), and 48 (98%) were resistant to nafcillin (1 microgram). The percentages of strains resistant to methicillin (greater than 8 micrograms/ml) 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 (greater than 5 micrograms/ml). With oxacillin (greater than 2 micrograms/ml), 90% were detected with Vitek AMS and 70% were detected with NCCLS MIC. With nafcillin (greater than 2 micrograms), 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.