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Journal of Clinical Microbiology, March 1999, p. 544-547, Vol. 37, No. 3
0095-1137/99/$00.00+0

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

Christine D. Steward,^1,* Sheila A. Stocker,¹ Jana M. Swenson,¹ Caroline M. O'Hara,¹ Jonathan R. Edwards,¹ Robert P. Gaynes,¹ John E. McGowan Jr.,² and Fred C. Tenover¹

Hospital Infections Program, Centers for Disease Control and Prevention, Atlanta, Georgia 30333,¹ and Rollins School of Public Health, Emory University, Atlanta, Georgia 30322²

Received 16 July 1998/Returned for modification 13 October 1998/Accepted 1 December 1998

Fluoroquinolone resistance appears to be increasing in many species of bacteria, particularly in those causing nosocomial infections. However, the accuracy of some antimicrobial susceptibility testing methods for detecting fluoroquinolone resistance remains uncertain. Therefore, we compared the accuracy of the results of agar dilution, disk diffusion, MicroScan Walk Away Neg Combo 15 conventional panels, and Vitek GNS-F7 cards to the accuracy of the results of the broth microdilution reference method for detection of ciprofloxacin and ofloxacin resistance in 195 clinical isolates of the family Enterobacteriaceae collected from six U.S. hospitals for a national surveillance project (Project ICARE [Intensive Care Antimicrobial Resistance Epidemiology]). For ciprofloxacin, very major error rates were 0% (disk diffusion and MicroScan), 0.9% (agar dilution), and 2.7% (Vitek), while major error rates ranged from 0% (agar dilution) to 3.7% (MicroScan and Vitek). Minor error rates ranged from 12.3% (agar dilution) to 20.5% (MicroScan). For ofloxacin, no very major errors were observed, and major errors were noted only 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 were substantially 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.

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^* Corresponding author. Mailing address: Nosocomial Pathogens Laboratory Branch (G08), Centers for Disease Control and Prevention, 1600 Clifton Rd. NE, Atlanta, GA 30333. Phone: (404) 639-2825. Fax: (404) 639-1381. E-mail: cks7{at}cdc.gov.

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Journal of Clinical Microbiology, March 1999, p. 544-547, Vol. 37, No. 3
0095-1137/99/$00.00+0

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