The objectives of this study were to monitor the stability of rifampin (RIF) in Lowenstein-Jensen medium (L-J medium) and 7H9 broth, which are the media commonly used for drug susceptibility testing (DST) of Mycobacterium tuberculosis. Rifampin degradation in stock solution, 7H9 broth, and L-J medium and during the inspissation process for L-J medium preparation was serially monitored by high-performance liquid chroma-tography (HPLC). L-J medium-based DST was conducted to examine the effect of L-J medium storage on the DST outcome. The RIF stock solution was stable for at least 3 months when kept at either 4degC or -20degC; RIF in 7H9 broth and L-J medium was almost 50 decayed after 1 week of storage at 37degC, and rifampin could not be detected in 7H9 or L-J medium after 3 weeks or 6 weeks of storage at 37degC. Approximately half of the drug was decomposed after 4 months of storage at 4degC for both media, and after 6 months of storage at 4degC, RIF in L-J medium was undetectable, while 38 of RIF remained in 7H9 medium. Approximately 21, 24, 29, and 35 RTF degradations were detected when the L-J medium was coagulated at 75degC, 80degC, 85degC, and 90H, respectively. The DST outcomes when using L-J medium stored for different periods of time were consistent with the RIF concentration monitoring data. Rifampin in stock solution is stable for at least 3 months at a reduced storage temperature. Media containing RTF should be prepared strictly according to validated standard operating procedures. RIF degradation is a possible reason for false resistance categorizations of M. tuberculosis isolates in the clinical laboratory.
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