MULTICENTER LABORATORY EVALUATION OF THE BIOMERIEUX VITEK ANTIMICROBIAL SUSCEPTIBILITY TESTING SYSTEM WITH 11 ANTIMICROBIAL AGENTS VERSUS MEMBERS OF THE FAMILY ENTEROBACTERIACEAE AND PSEUDOMONAS-AERUGINOSA

A four-center study in which a total of 1,082 recent clinical isolates of members of the family Enterobacteriaceae and Pseudomonas aeruginos n were examined versus 11 antimicrobial agents with the bioMerieux Vit ek susceptibility test system (Hazelwood, Mo.) and the GNS-F6 card was conducted. In addition, a challenge set consisting of the same 200 or ganisms was examined in each of the four participating laboratories, R esults obtained with the Vitek system were compared to MICs determined by a standardized broth microdilution method, For purposes of compari son, susceptibility categories (susceptible, intermediate, or resistan t) were assigned on the basis of the results of both methods, The resu lt of the broth microdilution test was considered definitive, The tota l category error rate with the Vitek system and the recent clinical is olates (11,902 organism-antimicrobial comparisons) was 4.5%, i.e,, 1.7 % very major errors, 0.9% major errors, and 1.9% minor errors, The tot al category error rate calculated from tests performed with the challe nge set (i.e., 8,800 organism-antimicrobial comparisons) was 5.9%, i.e ., 2.2% very major errors, 1.1% major errors, and 2.6% minor errors, V ery major error rates higher than the totals were noted with Enterobac ter cloacae versus ampicillin-sulbactam, aztreonam, ticarcillin, and t icarcillin-clavulanate and with P. aeruginosa versus mezlocillin, tica rcillin, and ticarcillin-clavulanate. Major error rates higher than th e averages were observed with Proteus mirabilis versus imipenem and wi th Klebsiella pneumoniae versus ofloxacin, Excellent overall interlabo ratory reproducibility was observed with the Vitek system, The importa nce of inoculum size as a primary determinant in the accuracy of susce ptibility test results with the Vitek system was clearly demonstrated in this study, Specifically, when an inoculum density fourfold higher than that recommended by the manufacturer was used, high rates of fals e resistance results were obtained with cell wall-active antimicrobial agents versus both the Enterobacteriaceae and P. aeruginosa.