In Vitro Antifungal Susceptibility Testing and Measurement of Levels of Antifungal Agents in Body Fluids

Interpretation of in vitro susceptibility data for antifungal drugs is hindered by the absence of standardized test criteria. Thus, it is extremely difficult to identify a clear relation between minimal inhibitory concentrations and clinical outcome. The situation appears more readily resolvable for yeast-like than for filamentous fungi since the former are more easily quantified by standardized microbiologic techniques. Accordingly the National Committee for Clinical Laboratory Standards has initiated the process of developing standards for yeast susceptibility testing. A related issue concerns the measurement of antifungal agents in body fluids. Whereas there may be little value in measuring concentrations of amphotericin B (because of its predictable pharmacokinetics), there is value to measuring levels of flucytosine (serum concentrations may relate to bone marrow suppression and/or the development of drug resistance) and ketoconazole (which may be absorbed unpredictably from the gut). Laboratory standards for these measurements have not been established.