Rifampicin stability in 7 H 9 broth and Löwenstein - Jensen medium 1

18 Objectives: To monitor the stability of rifampicin (RIF) in Löwenstein-Jensen medium (L-J 19 medium) and 7H9 broth, which are the commonly used media for drug susceptibility testing (DST) 20 for Mycobacterium tuberculosis. Methods: Rifampicin degradation in stock solution, 7H9 broth, 21 L-J medium, and during the inspissation process for L-J medium preparation was serially 22 monitored by high performance liquid chromatography (HPLC). L-J medium based DST was 23 conducted to examine the effect of L-J medium storage on DST outcome. Results: RIF stock 24 solution was stable for at least 3 months when kept at either 4°C or -20°C; RIF in 7H9 broth and 25 L-J medium was almost 50% decayed after one weeks’ storage at 37°C and rifampicin could not 26 be detected from 7H9 or L-J medium after 3 weeks or 6 weeks storage at 37°C. Approximately 27 half of the drug was decomposed after 4 months storage at 4°C for both media, and after 6 months 28 storage at 4°C, RIF in L-J medium was undetectable, while 38% of RIF remained in 7H9 medium. 29 Approximately 21, 24, 29 and 35% RIF degradation was detected when the L-J media was 30 coagulated at 75,80,85,90°C respectively. The DST outcomes when using L-J medium stored for 31 different period of time were consistent with the RIF concentration monitoring data. Conclusions: 32 Rifampicin in stock solution is stable for at least three months at reduced storage temperature. 33 Media containing RIF should be prepared strictly according to validated standard operating 34 procedures. RIF degradation is a possible reason for false resistance categorization of M. 35 tuberculosis in the clinical laboratory. 36