Use of potassium tellurite for rapid-drug susceptibility testing of Mycobacterium avium complex

Background: Mycobacterium avium complex (MAC) is a major cause of infection in immunocompromised patients. MAC possesses an enzyme that reduces potassium tellurite in less than 3 days and results in formation of a black precipitate. The objective of this study was to determine whether reduction of potassium tellurite by mycobacteria can be used as a means of testing the susceptibility of MAC to clarithromycin. Methods: Minimum inhibitory concentrations (MICs) for 104 clinical isolates of MAC were determined bg the tellurite method and compared with those tested by a recommended microdilution method. Microdilution breakpoints mere used for interpretation of susceptibility. MIC of less than 8 mug/mL was considered as susceptible, and MIC of greater than or equal to 8 mug/mL. was resistant. Results: There was agreement within a 2-fold dilution between MICs for 89% of isolates. Of the 53 isolates that had discrepant MICs by the two methods, 70% had higher MICs by the tellurite method. When the MICs were classified into interpretive categories, there was 100% agreement by the two methods. The MI6 tested by the tellurite method was available within 5 days, Conclusions: These data suggest that use of potassium tellurite is a more rapid, reliable, and inexpensive method of testing the susceptibility of MAC to clarithromycin.