Detection of Resistance to Amphotericin B in Candida Isolates by Using Iso-Sensitest Broth

A major limitation of the National Committee for Clinical Laboratory Standards M27-A methodology is reliable detection of amphotericin B (AMB) resistance. The results obtained by using Iso-Sensitest, a synthetic medium, to detect AMB resistance were analyzed and compared with those obtained with RPMI and antibiotic medium 3 (AM3). The ability to detect AMB resistance with RPMI is not enhanced by using a higher inoculum, glucose supplementation at a final concentration of 20 g/liter, spectrophotometric reading, or 24 h of incubation time. Testing using AM3 and an inoculum of 10(3) CFU/ml detects resistance. Identification of resistant isolates is not improved by glucose supplementation, changes in reading method, or changes in incubation time. However, the use of AM3 as assay medium and an inoculum of 10(5) CFU/ml did not allow detection of AMB resistance. Testing using Iso-Sensitest medium appears to be similar to AM3 in detecting resistance. The most pronounced discrimination is achieved by testing in Iso-Sensitest supplemented with glucose and spectrophotometric reading after 24 h of incubation. The reproducibility of MIC testing was greatest for Iso-Sensitest-based procedures. Use of Iso-Sensitest produces both highly reproducible MICs and reliable identification of AMB-resistant Candida isolates.