USE OF A COLORIMETRIC SYSTEM FOR YEAST SUSCEPTIBILITY TESTING

We examined the reliability and accuracy of a colorimetric assay using Alamar Blue reagent in the performance of susceptibility tests for Ca ndida albicans. We compared the broth macrodilution method recommended by the National Committee for Clinical Laboratory Standards (NCCLS) w ith a macrodilution method modified with the Alamar reagent and a micr odilution method modified with the Alamar reagent. The MICs of flucona zole and itraconazole for 97 isolates of C. albicans and 3 control iso lates were tested. For fluconazole, the Alamar-modified broth macrodil ution method yielded 94% (91 of 97) concordance within 2 dilutions com pared with the NCCLS method, while the microdilution method yielded 95 % (92 of 97) concordance. With Alamar-modified methods for itraconazol e, broth macrodilution yielded 97% (94 of 97) concordance within 2 dil utions. MICs obtained by the microdilution method, although tightly ne sted, were shifted to a higher value when compared with those obtained by the NCCLS method; there was only 77% (75 of 97) concordance within 2 dilutions but 97% concordance (94 of 97) within 3 dilutions. Tests by all methods with quality control strains showed excellent reproduci bilities. For fluconazole, the methods modified with the Alamar reagen t yielded clear endpoints and excellent correlation for the broth macr odilution and microdilution methods. For itraconazole, the methods mod ified with the Alamar reagent yielded clear endpoints and were reprodu cible, but higher MICs were obtained by the microdilution methods comp ared with those obtained by the NCCLS methods.