Does in vitro susceptibility to rifabutin and ethambutol predict the response to treatment of Mycobacterium avium complex bacteremia with rifabutin, ethambutol, and clarithromycin?

The in vitro susceptibilities of baseline Mycobacterium avium complex (MAC) blood isolates from 86 patients with AIDS who were treated with clarithromycin, ethambutol, and rifabutin were determined to examine whether these results predict bacteriologic response to treatment. No patient received prior prophylaxis with clarithromycin or azithromycin. Minimum inhibitory concentrations (MICs) of clarithromycin for all isolates were less than or equal to 2 mu g/mL. The median MIC of rifabutin was between 0.25 and 0.5 mu g/mL, and all isolates were susceptible to less than or equal to 2 mu g of rifabutin/mL. The median MIC of ethambutol was 4 mu g/mL, and the MIC90 was 8 mu g/mL. There was no correlation between ethambutol susceptibility and subsequent bacteriologic clearance, At all time points through week 12, bacteriologic clearance occurred more frequently in patients with isolates for which MICs of rifabutin were lower, but this difference was statistically significant only at week 2. Susceptibility testing for baseline MAC isolates from AIDS patients not previously treated with clarithromycin or azithromycin does not appear to be useful in guiding therapy.