Evaluation of an agar diffusion method for in vitro antifungal susceptibility testing

The NeoSensitabs agar diffusion method for in vitro antifungal susceptibility testing was evaluated by comparison with a microdiluton method performed in modified Shadomy liquid medium (YNBg), high resolution medium (HB) and RPMI 1640. The effects of time of incubation and type of reading were also studied in this comparison. Thirty-three reference strains, including ATCC and NCPF pathogenic yeasts, were used in triplicate. Antifungal agents tested included amphotericin B, 5-flucytosine, ketoconazole, tioconazole, fluconazole, itraconazole and nystatin. Results showed that the best correlation between both methods, for nystatin, amphotericin B and itraconazole, was achieved in HB medium (correlation coefficients -0.53, -0.20 and -0.33). For fluconazole the best correlation was obtained in RPMI 1640 (-0.89). A statistical difference (p < 0.05) was found between two readings made 24 h apart. As the incubation period was increased, the correlation between both readings declined, with the coefficients ranging from 0.66 for amphotericin B to 0.70 for tioconazole. Similarly, an increase in resistance to all antifungal agents was associated with a prolonged period of incubation. We would like to emphasize that making readings after incubation periods which are as short as possible with respect to growth controls is important to avoid the appearance of false resistance; turbidity readings are also useful for azoles. The NeoSensitabs agar diffusion method showed a good correlation with the microdilution technique for new triazoles such as fluconazole, which was higher than that obtained for the other antifungals.