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Journal of Clinical Microbiology, January 1999, p. 141-145, Vol. 37, No. 1
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.
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
*
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.
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