Comparison of the Cobas-Bact five-hour susceptibility testing system with the NCCLS agar diffusion and dilution methods.

The results of susceptibility tests performed by the Cobas-Bact system were compared with those of the NCCLS agar diffusion (Kirby-Bauer) and NCCLS agar dilution methods. A total of 998 clinical isolates were tested against 10 to 18 antimicrobial agents. Essential agreement (comprising full agreement and minor discrepancies) varied from 90.5% to 99.2% on comparison of Cobas-Bact with Kirby-Bauer results, depending on the bacterial group (mean for all 998 strains tested 95.7%). These figures ranged from 91% to 99.2% (mean 96.3%) for the Cobas-Bact/MIC comparison and from 95.2% to 99.7% (mean 98.7%) for the Kirby-Bauer/MIC comparison. The best results were found for Enterobacteriaceae and Staphylococcus aureus, whereas for enterococci and coagulase-negative staphylococci there was a lower rate of essential agreement in all three comparisons. In the case of Pseudomonas aeruginosa there was a good rate of essential agreement but many minor discrepancies, resulting in a disappointing rate of full agreement of between 67.5% and 78.9% in the three comparisons. The Cobas-Bact system would appear to provide satisfactory susceptibility test results in most cases, however there are still some major problems in the system which should be resolved.