Stability of four carbapenem antibiotics in discs used for antimicrobial susceptibility testing

Aims In low- to middle-income countries, microbiology laboratories often use disc diffusion for antimicrobial susceptibility testing (AST). Transport and storage conditions of materials used for AST are critical and may be challenging in remote, low-resource settings where temperature cannot always be controlled. This study aimed to test the stability of four carbapenem antibiotics in discs for AST under such unfavourable conditions. Methods and results Imipenem, meropenem, ertapenem, and doripenem discs from three brands (Oxoid, Becton Dickinson, HiMedia) were included. The discs were exposed to six unfavourable conditions (i.e. at room temperature and 35°C for up to 72 h) and the recommended storage condition as control. Subsequently, disc diffusion testing following EUCAST guidelines was performed with four well-defined Escherichia coli strains. In parallel, the carbapenems' degradation was observed using a spectrophotometric method. Carbapenems in discs were generally stable in unfavourable conditions although the concentration of most decreased over time. Overall, imipenem (Oxoid and Becton Dickinson) discs were the most stable. Meropenem discs were less stable at 35°C than at room temperature. Carbapenem concentrations in HiMedia discs were higher than those in Oxoid and Becton Dickinson. Conclusions Imipenem in discs for AST showed to be the most stable carbapenem under unfavourable conditions.