Stability of cefazolin sodium admixtures in plastic bags after thawing by microwave radiation

The effect on antibiotic stability of thawing, with microwave radiation, cefazolin sodium admixtures frozen in polyvinyl chloride minibags was studied. Two brands of cefazolin sodium (Ancef and Kefzol) were reconstituted and placed in 50-, 100- and 250-ml polyvinyl chloride minibags of 5% dextrose in water or 0.9% sodium chloride. The resulting solutions were assayed for antibiotic stability, using an agar disk diffusion technique, and for pH. The solutions were then stored at -20 degrees C for 48 hours, thawed to room temperature in a microwave oven, and kept at room temperature for four hours, after which they were reassayed for potency and pH. The results indicated that after the freeze-thaw process, the cefazolin sodium minibag admixtures retained at least 90% of their initial antimicrobial activity. The minimal pH changes could not be related to changes in antimicrobial activity, and no color changes could be detected visually. Using a microwave oven can greatly reduce thawing time of antibiotic admixtures. To maintain solution stability and prevent accidents, it is important to calibrate the oven, avoid solution overheating, and observe full precautions in oven operation.