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Antimicrobial Agents and Chemotherapy, August 2002, p. 2477-2481, Vol. 46, No. 8
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.8.2477-2481.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Comparison of Visual and Spectrophotometric Methods of Broth Microdilution MIC End Point Determination and Evaluation of a Sterol Quantitation Method for In Vitro Susceptibility Testing of Fluconazole and Itraconazole against Trailing and Nontrailing Candida Isolates

Beth A. Arthington-Skaggs,1* Wendy Lee-Yang,1 Meral A. Ciblak,1 Joao P. Frade,1 Mary E. Brandt,1 Rana A. Hajjeh,1 Lee H. Harrison,2,3 Andre N. Sofair,4 and David W. Warnock,1 and for the Candidemia Active Surveillance Group{dagger}

Mycotic Diseases Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia,1 Johns Hopkins University School of Hygiene and Public Health, Baltimore, Maryland,2 University of Pittsburgh Graduate School of Public Health, Pittsburgh, Pennsylvania,3 Yale University School of Medicine, New Haven, Connecticut4

Received 5 February 2002/ Returned for modification 6 March 2002/ Accepted 1 May 2002

Visual determination of MIC end points for azole antifungal agents can be complicated by the trailing growth phenomenon. To determine the incidence of trailing growth, we performed testing of in vitro susceptibility to fluconazole and itraconazole using the National Committee for Clinical Laboratory Standards broth microdilution M27-A reference procedure and 944 bloodstream isolates of seven Candida spp., obtained through active population-based surveillance between 1998 and 2000. Of 429 C. albicans isolates, 78 (18.2%) showed trailing growth at 48 h in tests with fluconazole, and 70 (16.3%) showed trailing in tests with itraconazole. Of 118 C. tropicalis isolates, 70 (59.3%) showed trailing growth in tests with fluconazole, and 35 (29.7%) showed trailing in tests with itraconazole. Trailing growth was not observed with any of the other five Candida spp. tested (C. dubliniensis, C. glabrata, C. krusei, C. lusitaniae, and C. parapsilosis). To confirm whether or not isolates that showed trailing growth in fluconazole and/or itraconazole were resistant in vitro to these agents, all isolates that showed trailing growth were retested by the sterol quantitation method, which measures cellular ergosterol content rather than growth inhibition after exposure to azoles. By this method, none of the trailing isolates was resistant in vitro to fluconazole or itraconazole. For both agents, a 24-h visual end point or a spectrophotometric end point of 50% reduction in growth relative to the growth control after 24 or 48 h of incubation correlated most closely with the result of sterol quantitation. Our results indicate that MIC results determined by either of these end point rules may be more predictive of in vivo outcome for isolates that give unclear visual end points at 48 h due to trailing growth.

* Corresponding author. Mailing address: Mycotic Diseases Branch, Centers for Disease Control and Prevention, 1600 Clifton Rd., N.E., Mailstop G-11, Atlanta, GA 30333. Phone: (404) 639-4041. Fax: (404) 639-3546. E-mail: BSkaggs{at}cdc.gov.

{dagger} Members of the CDC Candidemia Active Surveillance Group who participated in this study were as follows: Sharon Huie, Theresa Im, and Lily Yeo (Connecticut); Margaret Pass and Laurie Thomson Sanza (Maryland); and G. Marshall Lyon, Sara Mirza, Juliette Morgan, and Gabriel Ponce de Leon (Centers for Disease Control and Prevention, Atlanta, Ga.).

Antimicrobial Agents and Chemotherapy, August 2002, p. 2477-2481, Vol. 46, No. 8
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.8.2477-2481.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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