Radiometric and conventional drug susceptibility testing of Mycobacterium tuberculosis

One hundred and four clinical isolates of M. tuberculosis were susceptibility tested by the radiometric method (RAD) using the BACTEC system in parallel with a conventional modified proportion method (CON). In the latter, the strains were tested against four concentrations of drugs in Lowenstein-Jensen medium (isoniazid (INH), streptomycin (SM) and ethambutol (EMB)) OR 7H10 agar medium (rifampicin (RIF)) and reported as "sensitive", "intermediate" or "resistant" from the minimum inhibitory concentrations observed. The radiometric results were classified in the same three groups in accordance with the BACTEC methodology. The overall agreement between the results obtained by the two methods was 97.4% (INH 95.2%, EMB 96.2%, SM 98.1% and RIF 100%). In addition, the agreement between RAD and each of the drug concentration steps employed in CON was examined and the results discussed in relation to the established critical concentrations of the drugs. The BACTEC technique was found to be a rapid and convenient method for routine use.