In Vitro Susceptibility of Respiratory Isolates of Aspergillus species to Itraconazole and Amphotericin B. Acquired Resistance to Itraconazole

122 Aspergillus strains were isolated from respiratory specimens from 80 patients. Aspergillus fumigatus was the most common species, constituting 88% of the isolates. Susceptibility testing by the NCCLS broth macrodilution procedure revealed that the minimal inhibitory concentration for 50% of the strains (MIC50) was 0.25 mg/l for itraconazole and 0.5 mg/l for amphotericin B. The MIC90 was 1 mg/l for both drugs. To our knowledge, no cases of in vitro resistance during long-term itraconazole use in treatment of Aspergillus infection have been documented. We identified 3 patients infected with A. fumigatus strains that acquired in vitro resistance to itraconazole during prolonged therapy. This finding supports the importance of susceptibility testing.