Multicenter evaluation of an automated system using selected bacteria thatharbor challenging and clinically relevant mechanisms of resistance to antibiotics

A multicenter study was carried out to evaluate the performance of a new co mmercial automated system in comparison with that of the reference agar dil ution method. Ten clinical microbiology laboratories tested a collection of 61 strains of grain-negative bacilli (49 Enterobacteriaceae and 12 Pseudom onas aeruginosa), and 6 other laboratories tested a collection of 55 strain s of gram-positive cocci (10 enterococci and 45 staphylococci) against 10-2 0 antimicrobial agents. The strains were selected on the basis that they ha rbored challenging and characterized mechanisms of resistance. In compariso n with the agar reference method, the automated system gave an overall esse ntial agreement (+/-1 dilution) of 94.5%, 93.5%, and 97% for the gramnegati ve bacilli, enterococci. and staphylococci, respectively. According to the interpretive standards of the National Committee for Clinical Laboratory St andards, the category agreement ranged from 96 to 96.4% for the three sets of organisms. The accuracy of the automated system, as determined by the ka ppa test, ranged from 0.80 to 0.88, reflecting an almost perfect agreement with the reference technique. Very major. major, and minor errors obtained with the automated system were 0.3%. 2.9%, and 6.6% for gram-negative bacil li, 3.4%, 0%, and 5% for enterococci. and 1%, 1.6%, and 2.7% for staphyloco cci. respectively. The high rate of very major errors in enterococci was mo stly due to a single strain of multidrug-resistant Enterococcus faecium, wh ich was found susceptible to several antibiotics in a majority of participa nt laboratories. The use of a heavy inoculum and of a broth test medium by the automated system might account for a better expression of certain resis tance mechanisms, including beta -lactamases, as compared to the agar dilut ion reference method. The interlaboratory reproducibility was acceptable. a s shown by the narrow dispersion of MICs and by the results of quality cont rol.