Comparison of agar dilution: Disk diffusion, MicroScan, and Vitek antimicrobial susceptibility testing methods to broth microdilution for detection of fluoroquinolone-resistant isolates of the family Enterobacteriaceae

Fluoroquinolone resistance appears to be increasing in many species of bact eria, particularly in those causing nosocomial infections. However, the acc uracy of some antimicrobial susceptibility testing methods fur detecting fl uoroquinolone resistance remains uncertain. Therefore, wt compared the accu racy of the results of agar dilution, disk diffusion, MicroScan Walk Away N eg Combo 15 conventional panels, and Vitek GNS-F7 cards to the accuracy of the results of the broth microdilution reference method for detection of ci profloxacin and ofloxacin resistance in 195 clinical isolates of the family Enterobacteriaceae collected from six U.S, hospitals for a national survei llance project (Project ICARE [Intensive Care Antimicrobial Resistance Epid emiology]). For ciprofloxacin, very major error rates were 0% (disk diffusi on and MicroScan), 0.9% (ag-ar dilution), and 2.7% (Vitek), while major err or rates ranged from 0% (agar dilution) to 3.7% (MicroScan and Vitek). Mino r error rates ranged from 12.3% (agar dilution) to 20.5% (MicroScan). For o floxacin, no very major errors were observed. and major errors were noted o nly with MicroScan (3.7% major error rate). Minor error rates ranged from 8 .2% (agar dilution) to 18.5% (Vitek), Minor errors for all methods Here sub stantially reduced when results with MICs within +/-1 dilution of the broth microdilution reference MIC were excluded from analysis. However, the high number of minor errors by all test systems remains a concern.