Testing of Mycobacterium tuberculosis susceptibility to ethambutol, isoniazid, rifampin, and streptomycin by using Etest

Etest (AB BIODISK, Solna, Sweden) is a precise MIC method and the practical method of choice for the susceptibility testing of many fastidious organisms, including rapidly growing mycobacteria. Methods recommended by the National Committee for Clinical Laboratory Standards for the susceptibility testing of Mycobacterium tuberculosis include the Bactec (Becton Dickinson, Sparks, Md.) broth and agar proportion methods. A comparison of Etest with the Bactec broth method for testing the susceptibility of M. tuberculosis to four first-line antituberculous agents demonstrated equivalent interpretive results for 100% of the isolates tested, Agreements with agar proportion MICs, within +/- 2 log(2) dilutions, were 90, 93, 100, and 93% for ethambutol, isoniazid, rifampin, and streptomycin, respectively. Etest MICs were easily read within 5 to 10 days of inoculation. Preparation of the inoculum with a turbidity equivalent to a McFarland 3.0 standard prepared from growth on an agar surface and with a broth with a Bactec growth index of >999 yielded equivalent results, Clinical isolates for which the MICs were reproducible were also identified as possible quality control strains. The Etest method appears to be an alternative method for testing the susceptibility of M. tuberculosis isolates to the four most commonly used therapeutic agents.