Comparison of phenotypic and genotypic methods for the detection of clarithromycin resistance in Mycobacterium avium.

The MICs of clarithromycin for 10 clinical isolates of Mycobacterium avium were determined using three methods: Bactec 460-TB, broth microdilution and Etest. The results were compared with the presence of resistance mutations in the 23S rRNA gene. Isolates were obtained from five AIDS patients who were treated with clarithromycin. Five isolates were recovered before and five during treatment. MICs were reproducible and comparable between the three methods. They were < or = 4 mg/L for pre-treatment isolates and > or = 128 mg/L for strains recovered during treatment. An MIC > or = 128 mg/L was associated with the presence of mutations in the 23S rRNA gene that were absent in the isolates exhibiting MIC < ro = 4 mg/L.