Evaluation of the AutoMicrobic system for susceptibility testing of aminoglycosides and gram-negative bacilli.

The AutoMicrobic system (AMS; Vitek Systems, Inc., Hazelwood, Mo.) was compared with a reference broth microdilution MIC method to determine the accuracy and reproducibility of aminoglycoside susceptibility testing of gram-negative bacilli. Stock clinical isolates (n = 176) which demonstrated resistance to at least one aminoglycoside, extended-spectrum penicillin, or broad-spectrum cephalosporin (or a combination) were selected for this study. Isolates with moderate susceptibility to the aminoglycosides were also included. Of these isolates, 116 were either resistant or moderately susceptible to one or more of amikacin, gentamicin, netilmicin, and tobramycin. When AMS MIC results for 704 antimicrobial agent-organism combinations were compared with parallel microdilution MIC results, exact agreement (AMS MIC = reference MIC) rates were: amikacin, 71.6%; gentamicin, 71.6%; netilmicin, 83.0%; and tobramycin, 69.3%. Agreement rates within +/- 1 log2 dilution were: amikacin, 96.0%; gentamicin, 93.8%; netilmicin, 97.2%; and tobramycin, 96.0%. When National Committee for Clinical Laboratory Standards criteria were used to qualitatively evaluate performance, the overall agreement rates were: amikacin, 100.0%; gentamicin, 99.4%; netilmicin, 98.9%; and tobramycin, 99.4%. There were only four very major discrepancies, which represented 0.6% of the tests performed, and there were no major discrepancies. The percentages of minor discrepancies were: amikacin, 9.6%; gentamicin, 14.2%; netilmicin, 11.9%; and tobramycin, 10.8%. Of the overall average of 11.6% minor discrepancies, 9.7% occurred even though the AMS MIC was within +/- 1 log2 dilution of the reference MIC. The intralaboratory reproducibility ranged from 93.3 to 100% for the four drugs examined. With this challenge group of gram-negative bacilli, the AMS generated aminoglycoside MIC results that were comparable to those obtained by a reference broth microdilution method.