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Multicenter evaluation of 375ug/ml ethambutol in BACTEC vials for susceptibility testing of Mycobacterium tuberculosis



Multicenter evaluation of 375ug/ml ethambutol in BACTEC vials for susceptibility testing of Mycobacterium tuberculosis



Abstracts of the General Meeting of the American Society for Microbiology 101: 207



In the United States, determination of resistance to EMB in Mycobacterium tuberculosis (M. tb) is most commonly performed by the radiometric detection method (BACTEC). BACTEC uses 2.5 g/ml of EMB as the test concentration, and the results should be equivalent to those obtained when testing the critical concentration (5ug/ml) in agar proportion (AP). To assess and characterize reported problems with BACTEC EMB testing, M. tb susceptibility results from 4 laboratories were evaluated. Among 1905 consecutive patient isolates, 169 isolates were resistant (R) to 2.5 ug/ml EMB in BACTEC. However, only 50.3% of these (85/169) were R to 5.0 ug/ml of EMB in AP. In an attempt to increase the rate of agreement between BACTEC and AP, an additional higher concentration of EMB (3.75 ug/ml) was tested in BACTEC and the results of both BACTEC tests were compared to 5.0 ug/ml in AP. 384 isolates were tested, including both consecutive patient isolates and selected EMB-R isolates. Of these, 37 were R to EMB and 347 were susceptible to EMB by AP. Among those isolates susceptible in AP, the rates of "false" resistance with 2.5 and 3.75 ug/ml EMB were 6.3% (22/347) and 4.9% (17/347), respectively. Among those isolates resistant in AP, the rates of "false" susceptibility were 2.7% (1/37) and 18.9% (7/37) for 2.5 and 3.75 ug/ml EMB, respectively. The overall agreement increased from 93.2% to 93.7%. The concentration of 3.75 ug/ml of EMB in BACTEC did not significantly improve the accuracy of testing since the decrease in "false" resistance was offset by a corresponding increase in "false" susceptibility to EMB. In this study, R to EMB detected by AP was usually accompanied by R to other drugs such as INH. In view of these findings, laboratories should not report mono-resistance to EMB based on BACTEC results alone.

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Accession: 035349763

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