Results obtained with various antifungal susceptibility testing methods do not predict early clinical outcome in patients with cryptococcosis

Abstract

The in vitro susceptibilities of Cryptococcus neoformans isolates from consecutive human immunodeficiency virus-positive and -negative patients to the antifungal agents fluconazole, amphotericin B, and flucytosine were determined by different techniques, including the CLSI method, Etest, and broth microdilution in yeast nitrogen base (YNB) medium, during a multicenter prospective study in France. The relationship between the in vitro data and the clinical outcome 2 weeks after the initiation of antifungal therapy was assessed. In addition, the correlation between the strain serotype and the in vitro activities of the antifungals was determined, and the susceptibility results obtained with the different techniques were also compared. Thirty-seven patients received a combination of amphotericin B with flucytosine as first-line therapy, 22 were treated with amphotericin B alone, and 15 received fluconazole alone. Whatever the antifungal tested, there was no trend toward higher MICs for strains isolated from patients who failed to respond to a given therapy compared to those from patients who did not with either the CLSI method, Etest, or broth microdilution in YNB medium. The MICs obtained by the CLSI or Etest method were significantly lower for serotype D strains than for serotype A strains for both fluconazole and amphotericin B, while flucytosine MICs were not different according to serotype. These findings suggest that the in vitro antifungal susceptibility of C. neoformans, as determined with the techniques used, is not able to predict the early clinical outcome in patients with cryptococcosis.

FIG. 1.

Study design. AMB, amphotericin B; FCZ, fluconazole.

FIG. 2.

Comparison of fluconazole, amphotericin B, and flucytosine MICs for C. neoformans isolates belonging to serotypes A and D. Antifungal susceptibility testing was performed by the CLSI methodology. Bars represent the geometric mean MICs with 95% confidence intervals.

FIG. 3.

Relationship between fluconazole MICs and early outcome in 15 patients treated with fluconazole alone according to the in vitro method used: the broth microdilution test in YNB medium or the agar diffusion test (Etest) on RPMI agar. The incubation time was 48 h, and the MIC endpoints were 50 and 80% of inhibition for the microdilution tests and Etest, respectively. The bars represent the geometric mean MICs.

FIG. 4.

Relationship between amphotericin B MICs and early outcome in 56 patients treated with amphotericin B alone or in combination with flucytosine according to the in vitro method used: broth microdilution test in YNB medium or agar diffusion test (Etest) on RPMI agar. The incubation time was 48 h, and the MIC endpoints were 50 and 100% inhibition for the microdilution test and Etest, respectively. The bars represent the geometric mean MICs.