Rifampin Stability in 7H9 Broth and Lowenstein-Jensen Medium

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 chromatography (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 4 degrees C or -20 degrees C; RIF in 7H9 broth and L-J medium was almost 50% decayed after 1 week of storage at 37 degrees C, and rifampin could not be detected in 7H9 or L-J medium after 3 weeks or 6 weeks of storage at 37 degrees C. Approximately half of the drug was decomposed after 4 months of storage at 4 degrees C for both media, and after 6 months of storage at 4 degrees C, RIF in L-J medium was undetectable, while 38% of RIF remained in 7H9 medium. Approximately 21, 24, 29, and 35% RIF degradations were detected when the L-J medium was coagulated at 75 degrees C, 80 degrees C, 85 degrees C, and 90 degrees C, 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 RIF 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.