Simplified bioassay method for measurement of flucytosine or ketoconazole

A simple agar-well diffusion bioassay suitable for measurement of flucytosine or ketoconazole was developed by using Candida pseudotropicalis ATCC 46764 as the assay organism. A test medium composed of (per liter) 7 g of Trypticase peptone, 7 g of YNB (yeast-nitrogen base), 15 g of glucose, and 15 g of agar was seeded with an inoculum which had been grown to no. 2 McFarland turbidity after 4 to 6 h in YNB-glucose broth. Determinations of flucytosine or ketoconazole were performed without necessity of heating or diluting of serum samples to alleviate amphotericin B interference. A linear relationship between zone diameters and log10 concentration of the drugs was observed over the pharmacologically relevant ranges of 25 to 160 micrograms/ml for flucytosine and 0.5 to 20 micrograms/ml for ketoconazole. The mean coefficient of variability for samples measured on 5 separate days was 2.4% for flucytosin and 4.0% for ketoconazole. This assay represents a significant improvement over previous bioassay methods in that a single test system may be used for measurement of either flucytosine or ketoconazole, no serum dilution or pretreatment is required, inoculum preparation is accomplished entirely on the day of the assay, and sharp, clearly defined zones of inhibition are obtained with both drugs.