Impact of endpoint definition on the outcome of antifungal susceptibility tests with Candida species: 24- versus 48-h incubation and 50 versus 80% reduction in growth

The growth inhibition patterns of 764 clinical yeast isolates, in response to amphotericin B, flucytosine, itraconazole and fluconazole, were studied in order to determine the frequency of trailing growth and any impact this, as well as 24- or 48-h incubation periods, may have on minimum inhibitory concentration (MIC) results. A broth microdilution method following National Committee for Clinical Laboratory Standard No. M27A recommendations was used. Trailing growth was observed mainly with azoles. Furthermore, over 98% of isolates exhibiting a trailing effect at 24 h with fluconazole and itraconazole were either Candida albicans or Candida tropicalis. When comparing 24- and 48-h IC50 values, discrepancies were observed with itraconazole and fluconazole, respectively, in 18 and 11% of C. albicans and 24 and 30% of C. tropicalis. When comparing IC50 and IC80 values at 24 h, discrepancies were again essentially seen with itraconazole and fluconazole, respectively, in 11 and 10% of C. albicans, and 17 and 27% of C. tropicalis. In susceptibility tests performed with a microdilution method and read spectrophotometrically, 48-h IC80 values result in an unlikely high number of resistant isolates, indicating that a 24-h incubation and a 50% reduction in optical density may correlate better with clinical outcome.