Stability of Biological Activity of Frozen β-lactams over Time as Assessed by Time-Lapsed Broth Microdilutions

To evaluate the antimicrobial agent's stability stored at -80 °C, six β-lactams (meropenem, ertapenem, imipenem, ceftriaxone, ceftazidime, and piperacillin-tazobactam) were studied using the broth microdilution (BMD). The minimum inhibitory concentration (MIC) remained accurate with the same amount of frozen drug for at least six months. Thereafter, there was a diminishing drug concentration due to instability. At this temperature, most β-lactams can be frozen as a stock concentration for up to six months without a significant loss in antibiotic activity.