Etest for susceptibility testing of Mycobacterium tuberculosis and Mycobacterium avium-intracellulare.

Methods currently used for susceptibility testing of Mycobacterium, including agar dilution and the Bactec radiometric method, are based on the proportion technique using a single critical concentration of antibiotic. Preliminary studies were conducted with Mycobacterium species by using the Etest method, a gradient minimum inhibitory concentration technique, well described for susceptibility testing of other fastidious and slow-growing organisms. Excellent correlation was demonstrated between Etest and agar dilution with rifampin-susceptible and -resistant isolates of M. tuberculosis and ciprofloxacin-resistant and -susceptible isolates of Mycobacterium avium-intracellulare.