Determination of the Critical Concentration of Rifabutin for Susceptibility Testing Using the Proportion Method with Löwenstein-Jensen Medium against Mycobacterium Tuberculosis Isolates

Abstract

AbstractObjectiveTo determine the critical concentration of rifabutin (RFB) for susceptibility testing against Mycobacterium tuberculosis (Mtb) on Löwenstein-Jensen (L-J) medium using the proportion method.MethodsWe used 47 strains were used to determine the critical concentration of RFB. The microplate antimicrobial assay (MABA) was used as a reference method. We used 160 strains to evaluate its correlation with the classification results derived from the MABA method. We performed antimicrobial susceptibility testing (AST) against RFB and rifampin (RIF) for 2748 other strains using the proportion method on L-J medium.ResultsThe determined critical concentration for RFB was 20 μg per mL. Identical classification as susceptible or resistant was observed in 93.8% and 92.5% strains for RFB and RIF, respectively, using the 2 different methods. The cross-resistance ratio between RFB and RIF was 72.7% in the 2748 Mtb strains.ConclusionsWe determined that a critical concentration of 20 μg per mL RFB was reliable for AST of Mtb.