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Journal of Clinical Microbiology, January 1999, p. 141-145, Vol. 37, No. 1
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Influence of Incubation Time, Inoculum Size, and Glucose Concentrations on Spectrophotometric Endpoint Determinations for Amphotericin B, Fluconazole, and Itraconazole

M. Hong Nguyen* and Christine Y. Yu

Department of Medicine, University of Florida College of Medicine, and Gainesville VA Medical Center, Gainesville, Florida

Received 24 April 1998/Returned for modification 30 June 1998/Accepted 15 October 1998

We addressed the influence of the incubation time (24 h versus 48 h), starting inoculum size (standard inoculum size, ~103 CFU/ml, versus large inoculum size, ~104 CFU/ml), and supplementation with 2% glucose of RPMI 1640 medium on the spectrophotometric determination of the MICs of amphotericin B, fluconazole, and itraconazole. We compared the MICs determined spectrophotometrically with those determined by the standard broth macrodilution method (National Committee for Clinical Laboratory Standards approved guideline M27-A). The agreement between the results of the spectrophotometric and standard methods for amphotericin B testing was 100%; this agreement was independent of the inoculum size and incubation time. On the other hand, the agreement for the results for fluconazole testing and itraconazole testing was dependent on the inoculum size and incubation time. With large inoculum size, excellent agreement can be achieved at 24 h. With standard inoculum size, acceptable agreement can be achieved only at 48 h. In contrast to previous observations, the addition of 2% glucose did not have any significant impact on the growth density at 24 h, nor did it improve the agreement with the standard method. Furthermore, supplemental glucose might falsely elevate the MIC at 48 h.

* Corresponding author. Mailing address: University of Florida College of Medicine, Department of Medicine, Section of Infectious Diseases, P.O. Box 100277, JHMHC, Gainesville, FL 32610. Phone: (352) 379-4027. Fax: (352) 379-4015. E-mail: nguyemt{at}medicine.ufl.edu.

Journal of Clinical Microbiology, January 1999, p. 141-145, Vol. 37, No. 1
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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