Multicenter evaluation of ethambutol susceptibility testing of Mycobacterium tuberculosis by agar proportion and radiometric methods

Reproducibility of ethambutol (EMB) susceptibility test results for Mycobacterium tuberculosis has always been difficult for a variety of reasons, including the narrow range between the critical breakpoint for EMB resistance and the MIC for susceptible strains, borderline results obtained with the BACTEC 460TB method, the presence of microcolonies determined using the agar proportion (AP) method, and a lack of agreement between these two testing methods. To assess the frequency of these problems, At. tuberculosis drug susceptibility data were collected in a multicenter study involving four laboratories. Resistant, borderline, and susceptible isolates were shared among the laboratories to measure interlaboratory test agreement. Half of isolates determined by BACTEC 460TB to be resistant were determined to be susceptible by the AP method. Isolates determined to be resistant to EMB by both BACTEC 460TB and AP methods were almost always resistant to isoniazid. Results from isolates tested by the BACTEC 460TB method with an EMB concentration of 3.75 mug/ml in addition to the standard 2.5 mug/ml did not show improved agreement by the AP method. While these results do not indicate that the AP method is more accurate than the BACTEC 4601,13 method, laboratories should not report EMB monoresistance based on BACTEC 460TB results alone. Monoresistance to EMB should only be reported following confirmation by the All method. Microcolonies could not be confirmed as resistant by the BACTEC 460TB method or by repeat testing with the All method and do not appear to be indicative of resistance.