Evaluation of four anti-microbic susceptibility testing systems for gram-negative bacilli

A total of 200 clinical isolates were assayed by five anti-microbic susceptibility testing systems. Two frozen minimal inhibitory concentration (MIC) systems (MicroScan and Pasco), an automated MIC system (AMS, Vitek Systems), and the standard disk diffusion were compared with a reference broth dilution method. Organisms tested included 100 resistant clinical stock strains and 100 fresh random clinical isolates. Overall, there were 1,600 anti-microbic-organism combinations analyzed. The Pasco and MicroScan systems had no major discrepancies, the AMS system had seven, and the disk diffusion two. The number of very major discrepancies were as follows: AMS, 11; disk diffusion, 9; MicroScan, 5; Pasco, 2. Of the total 36 major or very major discrepancies in the study, 33% (12 of 36) were with an aminoglycoside and 44% (16 of 36) occurred with a second-generation cephalosporin, of which 10 of 16 were with cefamandole. Overall, there was a greater than 98.8% essential agreement with all systems compared with the reference method.