TESTING OF SUSCEPTIBILITY OF MYCOBACTERIUM-TUBERCULOSIS TO ISONIAZID AND RIFAMPIN BY MYCOBACTERIUM GROWTH INDICATOR TUBE METHOD

We tested isolates of Mycobacterium tuberculosis recovered from 117 pa tients for their susceptibilities to isoniazid (INH) and rifampin (RIF ) by the Centers for Disease Control and Prevention's disk modificatio n of the indirect method of proportions (MOP) test and a three-tube my cobacteria growth indicator tube (MGIT; BBL) antimycobacterial suscept ibility test (AST). Sixty-seven of the M. tuberculosis isolates were r ecovered from Lowenstein-Jensen (BBL) subcultures, and 50 of the isola tes were recovered from MGIT cultures of samples from various body sit es. For the MGIT AST method, 0.5 mi of test organism suspension was in oculated into an MGIT with 0.1 mu g of INH per ml, an MGIT with 1.0 mu g of RIF per mi, and growth control MGIT. The tubes were incubated at 37 degrees C and were examined daily. The MGIT AST results were inter preted as follows: susceptible if the tubes containing INH or RIF did not fluoresce within 2 days of the time that the positive growth contr ol fluoresced and resistant if the tubes containing INH or RIF did flu oresce, within 2 days of the time that the positive growth control flu oresced. The mean time fluorescence for the positive growth control wa s 5.5 days. The two methods were in agreement for 114 of the 117 isola tes from patients, while for 3 isolates there were minor discordant re sults.