Rapid radiometric method for determining drug susceptibility of Mycobacterium avium-intracellulare

A rapid radiometric method for susceptibility testing of Mycobacterium avium-intracellulare to eight chemotherapeutic agents was compared with a conventional method. Results were available within 72 h by radiometric testing in contrast to 21 days by the conventional method. The radiometric and conventional methods agreed to 61% of the tests, but growth inhibition of greater than or equal to 50% was detectable only by radiometric testing in an additional 36.5% of the tests. In only 2.5% of the tests was the radiometric method unable to detect complete inhibition shown by the conventional method. Quantifiable increases in inhibition with increasing concentration of isoniazid were more frequently detectable by the radiometric method than by conventional testing. The radiometric method is a simple, rapid, and quantitative test for drug susceptibility of mycobacteria and warrants further investigation.