Do in vitro susceptibility data predict the microbiologic response to amphotericin B? Results of a prospective study of patients with Candida fungemia

Outcome for 105 patients with candidemia treated with amphotericin B was correlated with amphotericin B in vitro susceptibility results. Thirty-three patients had microbiologic failure, which was defined as persistence of Candida in the bloodstream despite ≤3 days of amphotericin B. Amphotericin B minimum inhibitory concentrations (MICs) were determined by the National Committee for Clinical Laboratory Standards methodology. After determination of MICs, the minimal lethal concentrations (MLCs) were determined. The isolates tested yielded a narrow range of amphotericin B MICs (0.06-2 μg/mL); only 5% (5/105) exhibited MICs ≤1 μg/mL. The MLC range, on the other hand, was significantly broader (0.125 to > 16 μg/mL); 24% (25/105) exhibited MLCs ≤1 μg/mL. The strongest predictor for microbiologic failure was 48-h MLC (P < .001), followed by 24-h MLC (P = .03) and 48-h MIC (P = .11). A resistant break point for amphotericin B of >1 μg/mL for MLC and ≤ 1 μg/mL for MIC could be inferred from this study.