Comparison of two commercial systems (Vitek and MicroScan-WalkAway) for antimicrobial susceptibility testing of Pseudomonas aeruginosa isolates from cystic fibrosis patients

Antimicrobial susceptibility testing of cystic fibrosis (CF) isolates of Ps eudomonas aeruginosa is difficult because the organisms are often mucoid an d slow-growing. This study of 498 CF strains examined the correlation of re sults derived from two commonly used commercial systems (Vitek, MicroScan-W alkAway) with a reference method for 10 antimicrobials. Correlation to refe rence results was unacceptably low for all agents and both commercial syste ms had a high rate of very major (false-susceptible) errors. Although mucoi d strains produced a 4.8% greater intermethod error, it was not markedly di fferent than non-mucoid strains for the Vitek System. Overall, these tested commercial systems performed poorly for CF isolates in contrast to earlier reported, high correlations with the reference methods (broth microdilutio n frozen panels and agar dilution) of the National Committee fur Clinical L aboratory Standards, the standardized disk diffusion test, and the Etest (A B BIODISK, Solna, Sweden). (C) 2001 Elsevier Science Inc. All rights reserv ed.