Introduction Traditionally, single critical concentrations of drugs are utilized for Mycobacterium tuberculosis (Mtb) drug susceptibility testing (DST); however, the level of drug resistance can impact treatment choices and outcomes. Mutations at the katG gene are the major genetic mutations in multidrug resistant (MDR) Mtb and usually associated with high level resistance. We assessed the minimum inhibitory concentrations (MICs) of MDR or rifampin resistant (RR) and isoniazid (INH) resistant Mtb isolates to determine the quantification of drug resistance among key anti-tuberculosis drugs. Methods The study was conducted on stored Mtb isolates collected as part of a national drug resistance survey in Ethiopia. MIC values were determined using Sensititre™ MYCOTB plates. A line probe assay (MTBDRplus) was also performed to identify genetic determinants of resistance for all isolates. Results MIC testing was performed on 74 Mtb isolates including 46 MDR, 2 RR and 26 INH phenotypically resistant isolates as determined by the Löwenstein Jensen (LJ) method. Four (15%) INH resistant Mtb isolates were detected as borderline rifampin resistance (MIC = 1 μg/ml) using MYCOTB MIC plates and no rifampin resistance mutations were detected by LPA. Among the 48 MDR/RR TB cases, 9 (19%) were rifabutin susceptible (MIC was between ≤0.25 and 0.5μg/ml). Additionally, the MIC for isoniazid was between 2–4 μg/ml (moderate resistance) for 58% of MDR TB isolates and 95.6% (n = 25) of the isolates had mutations at the katG gene. Conclusion Our findings suggest a role for rifabutin treatment in a subset of RR TB patients, thus potentially preserving an important drug class. The high proportion of moderate level INH resistant among MDR Mtb isolates indicates the potential benefit of high dose isoniazid treatment in a high proportion of katG gene harboring MDR Mtb isolates.
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