Stability of cefazolin sodium and metronidazole at 8 degrees C for use as an i.v. admixture

The stability of cefazolin 1 g in metronidazole 500 mg/100 mL at 8 degrees C was studied for use as an IV admixture. The commercially available injection of cefazolin sodium 1 g vial was diluted to 5 mL with 0.9% sodium chloride injection and added to metronidazole 500 mg/100 mL. Following dilution of 2 mL to 100 mL with water, 1-mL aliquots were transferred to glass vials, refrigerated at 8 degrees, and assayed for cefazolin and metronidazole concentration at 0, 1, 2, 4, 8, 12, 24, 36, 48, and 72 hours after preparation. The concentration of cefazolin and metronidazole was determined by a stability-indicating high-performance liquid chromatographic method. The range of concentration was determined to be within 5% of the 0-hour mean concentration. Over the 72-hour period, the mean concentration of cefazolin at all assay times was within 98.4 to 101.0% of the initial concentration. The mean concentration of metronidazole at each assay time was 96.9 to 104.9% of the initial concentration. Cefazolin sodium 10 mg/mL and metronidazole 5 mg/mL, prepared by adding reconstituted cefazolin to a glass bottle of metronidazole ready-to-use solution, were stable for 72 hours when stored at 8 degrees C.